Code P0125 - thermostat replaced

[NickR]

This is on a 1999 Tercel, mtx 100,000kms. I'm playing with my new OBD2 code reader - a Christmas present to myself.

Check engine light came on, and the only code was P0125 "insufficient coolant temperature for closed loop fuel control". I had the thermostat replaced and the codes cleared, but the code came back. I also notice that according to the temperature gauge the engine warms up nicely after a cold start, and the temperature gauge sits right in the middle, so I have extra reason to believe that the thermostat is OK, and that the coolant temperature is correct. The car drives fine.

My current theory is that the engine coolant temperature sensor is misreading the coolant temperature, and saying the coolant is colder than it really is. I plan to replace that sensor (I believe it's the 3-wire sensors just next to the thermostat, and that the 1-wire sensor on the other side of the thermostat must be for the gauge, is that right?).

But before I spend money on buying a new sensor, does anyone have any other ideas, which could perhaps be checked more cheaply?

Thanks.

[toyomoho]

This is a misleading code and may have nothing to do with the cooling system. The A/F or 02 sensor (depending on if California or non) could be open or shorted. Or a problem in the heated portion of the sensor or a bad ECM.

Suggesting doing a TN and Google search on this code to gain further info. The link below has access to Camry manuals. Not the same engine, but Gen 4 Diagnostic Section has a wealth of info on troubleshooting codes.

[NickR]

Thanks toyomoho. I did a long Google search, and found a lot of threads, including a Toyota diagnostic flowchart. The impression I get is that if P0125 appears with other codes, then one should first track down those other codes, but if P0125 appears alone (as in my case), the problem is normally either the thermostat or (if the temperature gauge reads normally) the coolant temperature sensor needs replaced. So I have decided to replace the coolant temperature sensor.

But now I have run into a small problem: locating the correct coolant temperature sensor!

Immediately to the rear of the thermostat housing there is a one-wire sensor, which is probably for the temperature gauge, right?
Immediately to the front of the thermostat housing is a 3-wire sensor. I thought that this sensor would be the one for the ecu, and the one which I should replace. But the parts guy at the Toyota dealership said that there was a also a third sensor, really a switch, which controls the radiator fan. I cannot find a third coolant temperature sensor anywhere.

Does anyone know which coolant temperature sensor controls the ecu? Is it the one just to the front of the thermostat housing? (on a 1999 Tercel). Thanks.