Summary theory (what the cat does and how it fails)
- Function: the catalytic converter (cat) contains a ceramic/metal honeycomb coated with precious metals (Pt, Pd, Rh) that convert CO, unburned HC and NOx into CO2, H2O and N2 by oxidation/reduction reactions when the substrate is hot and there is the right O2/fuel balance. It also provides a smooth exhaust flow path; excessive restriction = backpressure.
- Sensors: upstream/downstream oxygen (lambda) sensors monitor air/fuel and converter efficiency (downstream compares to upstream).
- Failure modes: clogging (sintered soot or collapsed substrate) → high backpressure, loss of power, overheating; thermal/physical damage (melted substrate, cracks) → loss of conversion, rattling; poisoning (lead, silicone, phosphorus) → loss of catalytic activity; mechanical damage or corrosion → leaks; sensor failure or upstream engine faults (misfire, oil/coolant ingestion, rich running) can cause cat damage.
- Why replacement fixes the fault: a new cat restores exhaust flow and active catalytic surface area, lowering backpressure and restoring conversion efficiency; reinstalling/confirming working O2 sensors lets the ECU control fuel trim correctly, eliminating engine drivability and emissions symptoms—but only if root causes (misfire, oil/coolant leaks, sensor faults, DPF interactions) are addressed.

Ordered workshop procedure with theory tied to each step
1) Prepare and safety
- Ensure engine/cat cooled (can be >500°C). Park on level, chock wheels, raise with jack stands or lift. Disconnect negative battery terminal to avoid ECU/sensor damage.
- Theory: hot metal and exhaust gases are hazardous; disconnecting battery prevents unintended O2 sensor heater cycling and ECU faults during work.

2) Preliminary diagnosis (do before replacing)
- Read codes with OBD-II scanner, note P0420/P0430 or sensor codes. Perform visual inspect for leaks, dents, rust, rattles. Measure differential backpressure if available or use temperature probe upstream/downstream after short drive.
- Theory: confirm cat is the cause (high backpressure, failed converter efficiency code) and identify upstream engine causes (misfire, rich condition) that would ruin a new cat if not fixed.

3) Access and support exhaust
- Support exhaust with jack/stand under mid-pipe to prevent stress on hangers when cutting/unbolting. Remove heat shields as required.
- Theory: supports prevent damage to pipes or welds and allow controlled removal.

4) Remove oxygen sensors
- Unplug sensors, remove with correct O2 socket. Note sensor locations (upstream/downstream).
- Theory: sensors must be out to avoid damage and to allow gasket replacement; upstream sensor can be reused if good, downstream often replaced/tested.

5) Disconnect cat from exhaust system
- Unbolt flange/clamps if bolted. If welded or corroded, cut pipe at recommended locations (preserve as much pipe as possible to fit new cat). Remove exhaust hangers if needed.
- Theory: clean flange/gasket surfaces and correct orientation ensure no leaks and correct backpressure/flow path.

6) Remove old catalytic converter and inspect surroundings
- Inspect substrate (rattling indicates broken substrate), piping, heat shields, O2 sensor threads, and hanger condition.
- Theory: identifying damage or source of failure (e.g., collapsed substrate causes backpressure) informs whether additional repairs are required.

7) Prepare new converter and mating surfaces
- Fit new gaskets, replace bolts/studs/clamps as needed. Apply small amount of anti-seize to O2 sensor threads (avoid contaminating sensor tip).
- Theory: new gaskets and hardware ensure sealing and correct torque; anti-seize prevents thread galling and allows future removal.

8) Install new converter
- Position converter, align flanges/hangers, tighten nuts/bolts to factory torque (or snug clamps evenly if clamp type). Reinstall heat shields.
- Theory: proper alignment and torque prevent leaks and stress which cause fatigue failure; even clamping avoids warping.

9) Reinstall oxygen sensors and reconnect electricals
- Torque sensors to spec; connect harnesses.
- Theory: working sensors feed correct signals to ECU for fuel trim and cat monitoring.

10) Final checks and test
- Lower vehicle, reconnect battery. Start engine, check for leaks, listen for unusual noises, check exhaust temperatures upstream/downstream (downstream should be warmer when cat is active), clear codes and do a drive cycle, re-scan for codes.
- Theory: leaks cause false O2 readings and emissions/fuel trim issues; verifying temperature and codes confirms converter is functioning and ECU recognizes it.

11) Address root causes and disposal
- If engine faults caused the failure (misfire, oil/coolant leaks, rich running, clogged DPF), repair those before assuming converter longevity. Dispose/recycle old cat per local regulations (precious metals recoverable).
- Theory: replacing the cat without fixing the cause will lead to premature failure and wasted expense.

Important cautions and tips (brief)
- Always work on a cold exhaust to avoid burns. Use PPE and respirator if cutting rusty pipes (avoid breathing dust from substrate).
- Do not use a wire brush or compress air on the substrate—damages and releases hazardous dust.
- Use correct-spec parts: VIN/FITMENT matters; some systems integrate DOC/DPF on diesel engines—do not substitute wrong unit.
- Torque to manufacturer specs; incorrect torque causes leaks or cracked flanges.
- After replacement, monitor fuel trims, downstream O2 sensor readings and engine codes for signs of unresolved issues.

How the repair fixes symptoms (concise)
- Removes physical restriction (resolves backpressure, restores power/idle).
- Restores catalytic surface area (reduces emissions, clears efficiency codes).
- With functioning O2 sensors and corrected engine faults, ECU can normalize fuel trims, improving fuel economy and preventing further cat damage.

End.
rteeqp73

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