3RZ p0125

[JMunn55]

Got a p0125 code on my '99 4Runner 2.7L, did some research on here and found out it is engine coolant temperature problem. Some people say to check the thermostat but others were saying it's the A/F sensor. I don't notice any driveability issues. Is the A/F sensor part of the from O2 sensor or is it different? I have an exhaust leak at the flange right behind the cat, so I'll fix that before I do anything else. Maybe that's causing the rear O2 sensor to give a bad reading

 

[Mc1337sauce]

The problem could be either the front O2 sensor (A/F sensor) or the thermostat. I have replaced both of these parts on RZ engines, and they are relatively quick and easy jobs. My guess would be the thermostat is the culprit (replace this part with OEM if possible), but if you have more than 100K miles on the truck, I suggest replacing the front O2 sensor as well (you can get the generic Denso one from Amazon for much cheaper than the dealer, and they are the exact same item).

It is unlikely that the exhaust leak is causing the CEL, but definitely fix that as well, as it can cause a slew of other issues over time. If it is just the gasket leaking, I recommend the OEM one, as a friend replaced his with something from AutoZone, and it was burnt to pieces after about a year.

Best of luck on the repair!

 

[TheDurk]

The P0125 CAN definitely be caused by an exhaust leak. See page DI-46 in the attached. (This is for the 5VZ-FE becasue the online manual is from 2002 after the 4-banger died. I have a paper '99 manual and the 3RZ-FE P0125 chapter says the same thing. So I would absolutely fix the exhaust leak before I fixed anything else. Even though the leak is behind the cat, it uses the rear sensor to calibrate the front one so it could still throw things off.

The reason is the ECM gets confused when it doesn't get the signals it expects from the ECT and the A/F or O2 sensor. It doesn't know which is right, just that one of them is wrong. For some unknown reason (to me, anyway), it likes to set the ECT code first.

3g 4Runners used wideband A/F sensors instead of normal O2 sensors n the front in the '99 and '00 Cali. versions (if you have two cats, you have one, even if purchased in Nebraska) and all '01 and '02 models. They serve the same function as the O2 sensors but give more precise control. They require a different ECM as the signals are quite different.

 

[JMunn55]

okay, thanks guys. Oddly enough, the CEL went out on it's own and there's no code anymore. Just today. I'll be fixing the exhaust this weekend or early this week. If the code comes back after that I'll do thermostat and A/F sensor