Check engine warning light stays off after being on for a couple weeks

I just read your column responding to a question about ignoring a check engine warning light. I have a 2004 Lexus ES330 that had a check engine warning light. After a couple of weeks I was able to take the car to a shop and they said that the diagnostic test revealed a P0741 code indicating a torque converter clutch circuit performance stuck off. It also gave a P0128 code indicating the coolant thermostat regulating not working properly. The shop recommended replacing both the transmission and the thermostat. I thought I would get a second opinion, but before I could the check engine light went off and has stayed off for over a week now. The car’s transmission and temperature gauge have both worked just fine through all of this. I’m wondering what’s going on and do I need to do anything about either of these issues.

— Jim H.

Yikes! A new thermostat might be a good idea but run, don’t walk away from anyone telling you to replace the transmission!

Your torque converter clutch (TCC) is a fuel saving device which, when activated, eliminates the torque multiplying slippage provided by the torque converter, a fluid coupling linking the engine and transmission. Locking the converter’s impeller and stator together during light load cruising speeds also reduces transmission temperature. It would be easy to not notice the clutch failing to engage as general peppiness actually improves when the torque converter is allowed to do it’s thing.

The very first statement in the P0741 diagnostic trouble code chart says if there are other DTCS, fix them first! This makes sense as engine temperature is one of several key conditions monitored in the TCC operating strategy. Let’s look at the P0128 DTC first.

P0128 indicates the powertrain control module (PCM) has determined that actual engine coolant temperature hasn’t reached the estimated value during a certain time frame. In other words the engine is slow to warm up. This could be caused by a sticky (open) thermostat, a continuously running coolant fan, or a weather related miscalculation. A scan tool’s freeze frame data would provide the offending temperature value at the time the code set, which would aid in diagnosis. Assuming your DTC was not set in error, perhaps due to odd environmental conditions, replacing the thermostat (a fairly inexpensive process) may be called for.

If the P0741 DTC returns after successful cooling system inspection/repairs, this indicates the torque converter clutch is either slipping or not working at all (the most likely). Scan tool engine RPM and input turbine RPM readings would infer which of these is occurring. Possible and most common faults could be a faulty TCC solenoid (called DSL by Toyota), a wiring connection fault, a control module fault, a sticky or dirty transmission valve body, or a failed converter clutch. All but the last are serviceable with the transmission in place. Driving with a fully off/inoperative TCC would cause no mechanical issues other than a bit higher transmission temperature. A slipping TCC should be investigated ASAP in order to try to save it.

If your symptoms return, try a different shop for diagnosis. I’m hopeful this won’t turn out as bad as it sounds!