P0125

First time posting here.

2000 Tacoma 2.7 4wd

To start, I have searched the forum and found that the primary cause of the P0125 is the upstream O2 Sensor, or circuit. I have replaced both the upstream and downstream sensors, Thermostat, and the ECT. When I changed the ECT a previous owner must of broke the harness plug and instead of replacing it, they soldered the wires to the sensor and filled with a silicon to keep the wires from touching, I am guessing. I installed a new plug to the harness and with my code scanner I can see that it is reading the coolant temp. I am wondering if the P0125 is being caused by the engine temp not getting warm enough, as it shows it never exceeding 170 deg and by searching the forum I found that 185-195 is what's considered normal operating temps. I had a reputable shop do the thermostat, and I did the 02 sensors.
If the temp isn't the cause, my next steps are to trace the 02 sensor and test to ensure it isn't the problem, and then possibly getting the ECM flashed. Any advice on tracing the 02 circuit? Thoughts on getting the ECM Flashed?

Thanks.

 

Any updates? I’m having the exact issue you described I replaced coolant sensor and thermostat and have the new o2 sensor I have to put on but if that doesn’t fix it I’m wondering what you did

 

Not yet. I am going to take it to a highly recommend Toyota specialist and see what he thinks.

 

A running temp of 170°F is not an issue. This assumes you have a 170° t-stat installed. Toyota does make them and some of us with a supercharger are running them.

What device are you using to measure that coolant temp? I'm pretty sure the original t-stats in these trucks have an opening temp of 86°C. If that is what you have installed, one has to wonder why you are only seeing 170°F (77°C)

 

It was a while ago now since that tstat has been replaced, but I believe they put in a standard one.
I am using a Bluetooth ODB2 scanner. The app I use shows the live data, including coolant temp. The shop that did the tstat confirmed with their scanner as well, it is wasn't a cheap off the shelf like I have. They also blocked half of the radiator (this was during the colder winter months) to see if that would help. Still didn't exceed the normal operating temperatures.

 

Any luck on this? I'm having the exact same issue you described, p0125. replaced o2 ect and thermostat as well
I hear there are two coolant temperature sensors on these- one on the back of the engine, passenger side. the other in the front. did you replace both? i'm thinking one is for the gauge, while the other is for the ecm. not sure on this though, been looking for more info here and can't find much.
did you replace with denso o2 sensors?

 

I don't see the problem with old thread revival. It shows folks are using the search tool which is always a good thing.

 

Finally a resolution for this on our Tacoma. Check your wiring very carefully. Replaced this connector and the issue hasn't came back since.

 

What sensor connector is this, please.

 

The upstream O2 sensor.