Tools & consumables (minimum)
- Full metric hand tool set (ratchets, 6–24 mm sockets, extensions), combination wrenches.
- Torque wrench (0–200 ft·lb / 0–270 N·m range) and smaller 0–150 ft·lb for lower-torque fasteners.
- Transmission jack (or heavy-duty floor jack + wide-block support).
- Jack stands and wheel chocks.
- Engine support bar or engine hoist (to support engine when trans removed).
- Breaker bar, impact wrench (use carefully for removal only).
- Pry bars / large screwdrivers (plastic prying tools recommended for connectors).
- Clutch alignment tool (specific to the DCT clutch pack / diameter adapter for the input shaft).
- Snap-ring pliers, circlip pliers.
- Press or hydraulic bearing puller (for bearings / races if replacing).
- Slide hammer or axle puller for CV axles.
- Punches and drift set.
- Dial indicator and magnetic base (for runout and backlash checks).
- Vernier caliper / micrometer for plate thickness measurements.
- Feeler gauges.
- Clean solvent (brake parts cleaner) and lint-free shop rags.
- New gaskets, O-rings, oil seals, crush washers as required (always replace).
- OEM-specified DCT fluid and fluid pump/suction tool for filling (or vacuum fill kit).
- Threadlocker (medium strength) and anti-seize where specified.
- New clutch friction plates / steel plates / pressure plates / release actuators / bearings — as required.
- Replacement bolts for any one-time-use fasteners (flywheel/drive plate bolts often torque-to-yield).
- Diagnostic scan tool capable of DCT adaptation/calibration and clearing codes.
- PPE: safety glasses, nitrile gloves.

Safety precautions
- Work on a level surface. Use wheel chocks and rated jack stands under manufacturer's lift points.
- Disconnect negative battery terminal and keep it isolated during the job.
- Support the engine with an engine support bar or hoist before removing the transmission. Do not rely on the transmission to carry engine load.
- Use a transmission jack to raise/lower the transmission; never let it hang by wiring or hoses.
- Cleanliness: clutch friction surfaces and hydraulic/mechatronic components must remain oil- and grease-free. Work in a clean area and use lint-free rags.
- Mark and bag electrical connectors and fasteners for correct reassembly.
- Follow lift capacity limits for tools and stands.

High-level procedure (step-by-step)
Note: Mazda3 DCT design and exact fastener locations vary by year/market. Use the factory service manual for bolt sizes, sequences and torque specs. This is a general complete-repair flow.

1) Preliminary checks & diagnosis
- Scan for transmission codes and record symptoms (slippage, harsh shifts, inability to engage gears, leaks).
- Verify whether the unit is a dry or wet DCT (this determines clutch pack replacement parts and fluid). Dry units have separate dry friction packs and usually no wet pump/bath; wet are oil-cooled and require specific fluids.
- Order correct parts: clutch kit (friction plates steels), pressure plates, seals, output/input shaft bearings, gear selector solenoids or mechatronic module (if faulty), new bolts, and OEM DCT fluid.

2) Prepare vehicle
- Park on level surface, chock rear wheels.
- Loosen front wheel lug nuts, raise vehicle, secure on jack stands.
- Remove front wheels, splash shields, and any engine covers blocking access.
- Remove negative battery cable and store away.

3) Drain transmission fluid
- Place drain pan under transmission.
- Remove the fill/check plug first (if present) then the drain plug to ensure it will refill later. Drain fluid fully and dispose/label.
- If wet DCT, be prepared for more fluid (engine and cooler lines may also retain fluid). Disconnect cooler lines and catch fluid.

4) Remove intake/ancillaries / wiring
- Remove airbox, intake piping, heat shields, battery tray, and any harnesses, sensors or lines attached to the transmission housing.
- Label connectors. Unclip and move aside harnesses rather than disconnecting every clip.

5) Remove drive shafts / CV axles
- Remove axle nut(s), detach lower control arms or hub as needed to free CV axles.
- Use an axle puller or slide hammer to remove half-shafts from trans splines; be careful not to damage boots.
- Secure axles out of the way.

6) Remove starter and bellhousing peripherals
- Unbolt and remove starter motor.
- Remove speed sensors, range sensors, selector linkage or shift cables, and any wiring brackets bolted to the trans.

7) Support engine and remove transmission mounts
- Support the engine with an engine support bar or hoist.
- Place transmission jack under the trans and secure with straps.
- Unbolt transmission mount(s) and crossmember(s). Lower supports carefully.

8) Separate transmission from engine
- Loosen bellhousing bolts in a crisscross pattern and remove them.
- Carefully pry the trans rearward (use a transmission alignment tool or pry at the seam with thin screwdriver) until input shaft clears pilot/bearing, and lower the transmission straight down on the transmission jack. Keep it level and controlled.

9) Disassemble transmission case and access clutch packs / mechatronics
- Place the transmission on a clean bench.
- Photograph/label orientation of internal components as you disassemble.
- Remove outer covers (bellhousing / clutch pack cover) to access the clutch units or internal clutch packs.
- If the unit has a mechatronics (valve body + electronic control unit) accessible externally, remove it per manual — greatly care with seals and bolts. If the mechatronics is suspected faulty, it often must be replaced or bench-tested by specialists.

10) Inspect and measure
- Inspect clutch friction surfaces for glazing, contamination, uneven wear and heat spots.
- Measure friction plate thickness and steel plate thickness with caliper/micrometer; compare to service limits. Measure radial and axial runout with dial indicator.
- Inspect bearings, input shaft splines, fork contact points, and the mating surface on the engine (flywheel or dual-mass flywheel) for wear or scoring.
- Check oil pump, seals, and solenoids for metal debris or clogged passages.

11) Remove & replace worn components
- Replace friction plates, steel separator plates, pressure plate springs and any wear items beyond spec. For wet DCTs, replace bearings and seals and any dampers.
- Replace all internal O-rings, gaskets, and oil seals. Replace snap rings and circlips if deformed.
- If mechatronic/solenoid faults were present, replace the mechatronics module or individual solenoids per part availability. For many DCTs, mechatronic unit replacement is common when requiring major repair.
- If you remove bearings or shafts, use press and drift to replace races/bearings. Ensure correct orientation and use service-specified torque and seating depths.

12) Reassembly notes & tool use
- Use clutch alignment tool to center the clutch pack on the input shaft precisely; this prevents misalignment and difficulty mating transmission to engine.
- Use a torque wrench and specified torque sequence when tightening pack retaining bolts and bellhousing bolts.
- Use threadlocker on any fasteners where OEM specifies. Replace torque-to-yield bolts.
- Use press to seat bearings evenly; use a driver sized to the outer race — do not press on inner race unless specified.

13) Reinstall transmission
- Clean mating surfaces on engine and transmission; install new pilot bearing/seal if called for.
- Using transmission jack, raise trans into place, align input shaft with clutch pack using alignment tool if necessary, and push forward until bellhousing faces meet. Start bellhousing bolts by hand, then torque in the specified sequence.
- Reinstall transmission mount(s), crossmember, starter, selector linkage, speed sensors and wiring.
- Reinstall CV axles into hub and transmission, torque axle nuts to spec.
- Reinstall any removed components: intake, battery tray, wheel arch liners, and wheels.

14) Fill with correct DCT fluid and initial checks
- Refill with OEM-specified DCT fluid to the correct level and method. Many DCTs require filling through specific port with engine warm, level on flat with fluid at specified temperature, or running a bleed cycle with scan tool.
- For wet DCTs, there can be specific bleeding procedures — follow manual exactly.
- Reconnect battery.

15) Programming / adaptation & test
- Use a factory-level scan tool to clear codes, perform DCT adaptation/calibration and run automatic fill/bleed procedures if required.
- Cycle ignition and run the engine in Park/Neutral to check for leaks and fluid level.
- Road test: start with gentle shifts, then perform a series of engagements across gears to check for smoothness, slippage or harsh engagement.
- Re-scan for codes after road test and correct any remaining issues.

Common pitfalls and how to avoid them
- Misidentifying DCT type: don’t assume dry vs wet. Inspect and order correct parts before teardown.
- Contaminated friction surfaces: never touch clutch/friction surfaces with oily hands or solvent; use clean nitrile gloves.
- Incorrect fluid: using the wrong fluid will damage clutch friction materials and mechatronics. Use OEM-spec DCT fluid only.
- Improper alignment: not using the clutch alignment tool causes input shaft bearing/hub damage and makes mating transmission difficult.
- Not replacing seals/gaskets: reusing old seals often leads to immediate leaks and contamination.
- Reusing torque-to-yield bolts: these must be replaced. Always follow torque sequence and values.
- Damaging electrical connectors or wiring harness: label and disconnect carefully; damaged wiring causes intermittent faults.
- Skipping adaptation: many DCTs require electronic adaptation after build; skipping this yields improper clutch engagement and erratic shift quality.
- Ignoring cleanliness when handling mechatronic: tiny metal chips or dirt in the mechatronic valve body cause erratic behavior — always clean and use lint-free towels and filtered solvent.
- Not supporting engine: failure to support engine when removing trans can cause engine to drop or cause alignment issues.

When replacement parts are commonly required
- Clutch friction plates and steels (usually always when opening the clutch pack).
- Pressure plates, springs, release bearing (or equivalent dual-clutch release actuators).
- Input shaft and output shaft seals and O-rings.
- Pilot bearing or input shaft bearing if worn.
- Mechatronics unit or solenoids (if electronic/pressure errors present).
- Gaskets and filter elements (if present).
- Transmission mount, bolts labeled as single-use, CV axle splines or boots if damaged.
- Fluid: full refill with OEM DCT fluid (quantity varies by model).

Final checks & verification
- Verify no fluid leaks at cooler lines, pan, seals and axle boots.
- Confirm engine and trans mounts are torqued and aligned.
- Run DTC scan and clear codes, then perform adaptation routine per scan tool/manual.
- Road test with progressive load, confirming normal temperature rise and shift firmness.

Notes & final advice (no fluff)
- Get the factory service manual or OEM repair guide for exact bolt torques, sequences, and fluid specifications for the specific Mazda3 year and transmission code.
- If the mechatronic module is suspected, consider bench testing or professional reconditioning — it can be costly but is often the correct fix for erratic DCT behavior.
- If uncertain about replacement of internal gears, shafts, or mechatronic repairs, consult a transmission specialist — DCT internals require precision clearances and calibrated solenoid operation.

Done.
rteeqp73

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Short, direct explanation first: gearbox (transmission/transaxle) seals stop gear oil from leaking where rotating shafts exit the housing. Replace them when you see oil leaks, smell/transmission slipping, or contamination of clutch. If you ignore a bad seal, the gearbox runs low on lubricant, bearings and gears wear or overheat, and surrounding parts (clutch, CV joints, wheel bearings) get contaminated and fail.

Theory / how the system works (simple, beginner level)
- The gearbox/transaxle is a sealed metal housing containing gears, shafts, bearings and oil. In a FWD Mazda3 the gearbox is a transaxle: gearbox + differential in one case.
- Shafts (input/mainshaft and axle/output shafts) pass through the case. Where a rotating shaft leaves the metal housing there must be a flexible seal — usually a lip seal made of rubber/elastomer with a metal case and a spring that holds the lip against the shaft.
- The lip of the seal rides on the shaft surface, keeping oil inside and keeping dirt out. Think of the seal lip like a snug rubber o-ring on a moving rod — it lets the rod spin but keeps fluid in.
- If the seal lip hardens, tears, the shaft surface is scored/corroded, or the seal was installed wrong, oil will leak past that lip.
- Oil level too low => poor lubrication and overheating; oil on clutch (input seal leak) => clutch slips and contaminates friction surfaces; oil on driveshaft boots causes CV joint contamination.

What can go wrong (failure modes)
- Rubber deterioration (age, heat, oil attack)
- Physical cuts/tears from sharp edges or debris
- Bad installation (misalignment, driven too deep, hammering the seal lip)
- Shaft surface damage (scoring, pitting, rust) causing secondary leaks even with a new seal
- Wrong seal selected (ID/OD/thickness/material)
- Overfilling gearbox (excess pressure can force oil past seals), or blocked vent causing pressure

Components you will see and what they do (every relevant piece)
- Transmission/Transaxle housing: metal case holding gears and oil.
- Input/Mainshaft: shaft from the clutch/flywheel into the gearbox, turns the gearbox internals.
- Input seal (mainshaft seal): seals around the input shaft where it enters the gearbox, often located at the bellhousing.
- Output/Axle shafts (CV shafts): shafts that transfer torque to the wheels; each passes through the transaxle case at an axle seal.
- Axle seal / output seal: seals around each axle where it leaves the case.
- CV joint & boot: constant-velocity joint transfers rotation into wheel stub; boot keeps grease in CV joint (don’t confuse with gearbox seal).
- Hub/wheel bearing assembly: holds wheel and connects to CV shaft at the axle splines.
- Snap rings / circlips: retain half-shaft in the differential/gearbox on some designs.
- Axle nut: secures the CV shaft to the hub.
- Seal retainer / carrier (on some designs): bracket/cover that holds the seal.
- Seal spring (inside the seal): helps keep sealing lip contact against shaft.
- Gasket / paper or RTV: seals covers or bellhousing to housing.
- Gear oil (transmission fluid): lubricant inside.
- Drain/fill plugs: used to drain and refill oil; fill plug also indicates correct oil level (fill to bottom of fill hole).

Tools & consumables (get good quality)
- Jack and jack stands (or automotive lift). Do NOT rely on just the jack.
- Wheel chocks.
- Socket set, ratchet, extensions.
- Breaker bar for axle nut.
- Pry bar / pry tools.
- Torque wrench (calibrated).
- Seal puller (hook style) or screwdrivers to remove old seal carefully.
- Seal driver / socket set to install new seal evenly (or a short pipe of correct diameter).
- Rubber mallet or dead blow.
- Snap ring pliers (if axle retained by circlip).
- Drain pan for fluid.
- Brake cleaner, rags.
- New seal(s) OEM or high-quality aftermarket; new axle nut and any required washers/circlips.
- Gear oil as specified in factory manual.
- Transmission jack (for input seal job you’ll remove the trans).
- CV joint grease (if you disturb boots).
- Threadlocker (if required), RTV gasket maker if needed.
- Protective gloves, safety glasses.

Two common cases and step-by-step procedures
Note: Mazda3 models vary (engine/transmission combos). The axle/output seal procedure is simpler and often can be done without removing the gearbox. Input (mainshaft) seal typically requires removing the transmission. Always consult the factory service manual for exact model-year specifics, torque values, and special steps.

Procedure A — Axle (output) seal replacement (typical, beginner-friendly)
What this fixes: oil leaking at the outer axle where CV shaft exits the transaxle.

1) Safety & prep
- Park level, chock rear wheels. Disconnect negative battery terminal if you’ll be working near electrical components.
- Raise front of car with jack, support on sturdy jack stands under recommended lift points.
- Remove wheel for that side to access hub & axle.

2) Remove hub/axle connection
- Loosen and remove axle nut (often large: 32–36 mm depending). You may need to unseat the hub from the CV shaft: back off caliper/hub to free up movement.
- Remove brake caliper and rotor as needed and hang caliper so brake hose isn’t stressed.
- If the CV shaft is bolted into the hub, remove any bolts securing hub to knuckle; if pressed, you’ll need to pry or use a hub puller. Use careful striking on hub (not on axle) if needed to unseat.
- Some vehicles require removing lower ball joint or tie rod end to swing the hub/knuckle outward — don’t force components. If you separate ball joint, support control arm and know alignment will be affected—plan alignment after.

3) Pull the half shaft out of the transaxle
- For many Mazda3s the CV shaft slides out of the transaxle. Pry gently between the shaft and housing using a pry bar; be careful not to gouge the transaxle bore. Catch any fluid that drips.
- If there’s a snap ring inside the transaxle that holds the axle, you may need to compress the shaft and pop the snap ring with a flat tool while pulling.

4) Remove old seal
- With shaft out, you can access the seal from outside. Use a seal puller or pick to pull the old seal straight out. Work around the seal evenly to avoid deforming the bore.
- Inspect the shaft surface where the seal lips ride. If scored/pitted, the new seal will likely leak; the shaft must be replaced or repaired (light polish only; no deep damage).

5) Clean and prepare
- Clean the seal bore and mating surfaces with brake cleaner. Remove any burrs, corrosion, or debris. Lightly coat the sealing lip area of the shaft with clean gear oil.

6) Install new seal
- Lubricate new seal lip with gear oil. Position seal squarely and press in by hand until flush.
- Use a seal driver or a socket with the same OD as the seal to drive it evenly into the bore. Tap gently with a mallet until fully seated. Ensure it’s flush and even; do not distort the lip.
- Do not hammer on the rubber face; use the metal case or housing contact area.

7) Reinstall axle and reassemble hub
- Slide CV shaft back into the transaxle until it seats (you may feel/hear the snap ring engage).
- Reassemble hub, rotor, caliper. Torque axle nut and all fasteners to factory torque values (look up exact specs).
- Refill transaxle fluid if you lost oil — top to the fill plug level using correct oil.
- Lower car, torque wheel lugs to spec, road test and check for leaks.

Procedure B — Input (mainshaft) seal replacement (requires transmission removal)
What this fixes: oil leaking from bellhousing area or oil on clutch/flywheel.

1) Safety & prep
- Same safety as above. You’ll need a transmission jack and possibly a helper.

2) Remove components to access transmission
- Disconnect battery negative.
- Remove intake components, battery, airbox, shifter cables (for manual), driveshafts (if front CV shafts won’t allow trans to drop), starter motor, exhaust crosspipe if it blocks access, and any sensors/wiring on the transmission.
- If manual: remove clutch release fork/throwout bearing and disconnect clutch slave cylinder (or unbolt slave and hang it — don’t disconnect fluid lines if possible). For hydraulic systems, carefully support to avoid introducing air.
- Support gearbox with a transmission jack.

3) Remove transmission
- Remove bellhousing bolts connecting gearbox to engine block and slide the transmission straight back. You may need to separate the clutch/flywheel from the transmission input; support both. Keep alignment — don’t force.
- On some cars you must remove the clutch/flywheel assembly to reach the seal; others allow access with the clutch in place once the trans is off the engine. Inspect clutch and throwout bearing for oil contamination — if oil-soaked, replace clutch components.

4) Remove old input seal
- With transmission removed and input shaft exposed, pry or pull the old seal out from the bellhousing side. Use a seal puller carefully to avoid damaging the bore or shaft splines.
- Inspect input shaft surface for wear. If shaft lip area is badly scored replace shaft or entire assembly.

5) Clean & prepare
- Clean the bore, remove nicks and burrs gently. Lightly oil the shaft and inside lip area of new seal.

6) Install new seal
- Use the correct seal (OD/ID/thickness & material). Square it onto the bore and drive in evenly with a seal driver until seated flush with housing.
- For input seals, sometimes a thin film of grease on the lip helps initial run-in, but do not over-grease (use manufacturer recommendation).

7) Reinstall transmission
- Refit transmission to engine: align input shaft with clutch disc and spline; use an alignment tool to center the clutch disc if you removed it. Slide gearbox in until bellhousing mates with engine block; reinstall bolts and torque to spec.
- Reconnect everything removed (starter, driveshafts, coolant/exhaust pieces, sensors, electrical, battery). Refill transaxle with correct fluid to proper level.

8) Test & check
- Start engine, check for leaks while engine idles and while driving. Re-check torque on axle nut and all fasteners after a short test drive.

Important do’s and don’ts / tips (no-nonsense)
- Do not reuse old seals.
- Do not pry on the lip surface or use screwdriver edge against the bore hard enough to gouge it.
- If the shaft is scored more than a light surface mark, replacing the shaft or the carrier may be required — a new seal on a bad shaft will fail quickly.
- Drive the seal squarely and evenly. Driving crooked damages the lip.
- Cleanliness matters. Dirt inside the seal area will wear the lip quickly.
- Keep the seal lip facing the fluid (the spring side usually faces the fluid). Check orientation of old seal before you remove it.
- Use the exact fluid grade and quantity specified by the factory.
- Always use jack stands; transmission support must be secure. If you’re not confident using a transmission jack, get help or take it to a shop.
- If the leak was severe enough to oil the clutch, replace the clutch disc, pressure plate, and throwout bearing — cleaning is usually inadequate.

Signs you should stop and seek a shop
- You encounter heavily corroded or damaged shafts, or the seal bore is chewed up.
- Transmission removal is beyond your tools or comfort (no transmission jack, no lift).
- You suspect internal transmission damage or cannot locate the leak source.

Final checklist before driving
- New seals installed flush and evenly.
- All bolts torqued to spec (consult service manual).
- Gear oil filled to correct level; no drips under car.
- If clutch was removed/replaced, take care with hydraulic bleeding and clutch adjustment.
- Road test, then re-check for leaks and re-torque axle nut after first short drive per factory instructions.

That’s the practical overview and step-by-step. If you follow safe procedures, use correct parts and tools, and inspect shaft surfaces, you’ll fix the leak and avoid repeat failures.
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