I got a CEL yesterday, came up P0125 "Insufficient Coolant Temperature for Closed Loop Fuel Control." Cleared it, drove some more, and the same single code returned. The coolant level is fine and it's fairly new, and the temperature of the vehicle is fine with no symptoms. Googled the code and it appeared to be a faulty Engine Coolant Temperature (ECT) sensor. Called a Toyota dealer and set an appointment, the adviser said it was that sensor but could not give me a price on Saturday.

I then called an independent Toyota certified mechanic I know. I figured he would confirm the need for the ECT sensor and give me a price. But instead, he said that my '99 RX uses old technology, that the P0125 code in all likelihood is wrong as the ECT sensor rarely fails and that more likely it's an O2 sensor or air fuel ratio sensor and the vehicle is throwing the wrong code. This surprised me.

Anyone have information on this code and the indy's opinion?

Also, the indy also told me it's fine to drive the RX for now - it's not an emergency. Agree with this? Thank you for any help.

 

Put your ohm meter on the sensor when the engine is cold like sitting over night cold and you should read over 100K ohms then warm up the car until the fans cycle on and check again it should read just a few thousand ohms. If that checks out ok then rule out the sensor itself. Check the wiring etc.

I doubt the ECT has failed but you can check it.

 

It's a very common code that piggybacks a bad air/fuel (wide band O2) sensor. I see it when the air/fuel sensor dies, but not on it's own. I think I would put some more miles on it and see if another code pops up with it.

 

Thank you for both replies. I'm satisfied it's likely not the ECT. I'll drive it some more and see if any other codes pop up. I don't own an ohm meter but I'll have the indy look it over after a few days. It sounds like he knows what he's talking about - same as both of you.

 

+1, I had that code (P0125) once many, many years ago. I cleared it, then some time passed and I got a code for an 02 sensor. Replaced it, and it hasn't appeared in ages!

 

Have you had the codes read on a TechStream reader for pending codes and previous codes? If it's an O2 sensor, you should have a couple of additional codes (1133, 1135, 1153, 1155).

 

Thanks for the replies. So it turned out a single P0125 code all week was the AF ratio sensor. Pretty interesting it never came up P1133 or P1135 - perhaps it eventually would have if I did not replace the sensor. Replaced the sensor and no more check engine. So if anyone gets this single code in a first gen RX, AF sensor is a good possibility.