Theory (short)
- The A442F transmission thermostat is a temperature-actuated valve in the ATF cooler circuit (usually in the cooler housing/tube between the transmission and radiator/aux cooler).
- When cold it restricts flow to the radiator cooler so ATF stays in the trans and warms quickly; when hot it opens and routes fluid through the radiator/aux cooler to remove heat. Most units use a wax pellet or bimetal element that moves a valve based on fluid temperature.
- Correct ATF temperature is critical for hydraulic pressures, friction material function, and shift timing/feel. A stuck-closed thermostat delays cooling (overheat); stuck-open delays warm‑up (harsh shifting, reduced clutch life).

Ordered procedure (concise, sequence only)
1. Preparation: gather replacement thermostat kit (thermostat, seals/Gasket), correct ATF, catch pan, hand tools, safety stands, torque wrench, gloves, rags. Work on a level surface and allow engine/transmission to cool to a safe touch or use heat-resistant PPE if warm. Disconnect negative battery for safety.
2. Access: raise and securely support vehicle if needed. Locate the transmission cooler plumbing/thermostat housing (the connection between transmission cooler tube and radiator/aux cooler). Place drain pan under area and under transmission pan if you expect spill.
3. Drain/relieve: loosen an ATF return hose or remove the cooler line at the thermostat housing to let ATF drain into the pan. Catch fluid—plan to reuse or dispose per local rules.
4. Remove housing: remove bolts retaining the thermostat housing/cooler tube and carefully separate it from the transmission case/line. Expect residual ATF to spill.
5. Extract thermostat: pull the thermostat assembly from its bore. Note orientation (which side faces transmission). Inspect the valve, spring/wax element, and mating seat for scoring, corrosion, debris. Also inspect screens/filters and cooler lines for contamination.
6. Clean: clean the housing bore and mating surfaces thoroughly; remove old gasket material. Flush cooler lines if contamination present. Replace any corroded/clogged hoses or clamps.
7. Install new thermostat and seals: fit the new thermostat in the same orientation, replace O-rings/gasket/seal as provided. Lightly lubricate seals with clean ATF to ease installation. Reinstall housing and tighten bolts to spec (evenly).
8. Reconnect cooler lines: attach lines/clamps and make sure connections are sealed.
9. Refill ATF: add correct type and quantity of ATF. Many trans fill procedures require adding with engine running and selector in Park, then warming to operating temperature and checking level at specified temp. Start engine, run to normal temp (or follow Toyota A442F temp-fill spec), cycle gear selector through positions, then check and correct fluid level per Toyota procedure.
10. Check for leaks and function: with engine idling and fluid at operating temp, check for leaks at housing and lines. Use a temp gun or scan tool to confirm ATF temp and observe whether the thermostat opens (drop in radiator inlet temp or change in flow). Road test and verify normal shift quality, temperature stability, and absence of overheating.
11. Final: re-torque after cool-down if required, dispose old fluid responsibly.

How the repair fixes faults (concise)
- Stuck-closed thermostat: replacing it restores flow to the external cooler when ATF reaches operating temp. That prevents ATF overheating, reduces burnt fluid, stabilizes hydraulic pressures and prevents heat-related clutch/fuel/shift failures. Result: lower steady-state ATF temp, improved longevity and shift consistency.
- Stuck-open thermostat: replacing it restores warm-up bypass so ATF reaches designed operating temp faster. That returns correct friction material coefficient and hydraulic viscosity for crisp, correct shift timing and reduced internal wear (prevents harsh/late shifts and decreased clutch life).
- Cleaning the thermostat housing and replacing seals fixes leaks that cause low fluid or air intrusion, which can cause slipping and erratic pressure. Flushing lines corrects contamination that can jam valve movement or damage solenoids/valves.

Quick tests after repair (verify)
- Infrared thermometer: compare ATF temp before and after repair at idle and under load; thermostat should close at low temp (higher trans temp) and open above the spec temp (noticeable cooling effect).
- Shift quality: warm up and road test—shifts should be consistent, no slipping, no overheating.
- No leaks, correct ATF level at specified temperature.

Safety notes (brief)
- ATF can be hot and under pressure—use gloves and eye protection. Support vehicle with jack stands. Dispose drained ATF legally.

Done.
rteeqp73

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