While driving my 2000 4-Runner to work this week the Check Engine light came on and with a PO125 code. I've reset the code about 3 times and it continues show up within 5 minutes after I restart the engine. From the excellent articles on this forum it appears that the front sensor is the likely cause. The Runner has a California spec. system with 63K miles.

Is there any other tests or checks I should do before replacing the sensor? Is an aftermarket sensor acceptable or should I stick with the OEM stuff?

Thanks for the great forum.... I found it while doing a search on the code number.

Jeff

63K sounds a bit soon for the A/F sensor to go out. I'd check the wiring harness and connections. If that checks out, stay with OEM (Denso) on the replacement. I got mine at a reasonable price from sparkplugs.com.

My FSM lists the following to check in order:
1) Any other codes? Check them out first
2) Use a scan tool to read the output value of the A/F sensor
3) Check for open/short in harness and connector between A/F sensor and ECM
4) Check resistance of the heater in the A/F sensor
5) Check the air induction system
6) Check fuel pressure
7) Check injector injection
8) Check gas on exhaust system
9) Replace A/F sensor
10) Check to see if you still get P0125 -- if so, replace ECM
11) Did vehicle run out of fuel? (running out of fuel will cause P0125 to be recorded)

Seems to me like step 11 should be step one -- P0125 could be caused by lack of fuel (plugged lines or filter, weak pump, etc.)

Thanks for the feedback.

This is the only code that has shown up and it has not run out of fuel.

We'll keep checking...

Thanks
jf

Just wanted to follow up and say thanks to everyone for the insight on this problem and for the very helpful resource links. I checked the resistance across the A/F sensor heater and found it to have no resistance at all. With this data I ordered a "Denso" A/F sensor from www.Oxygensensors.com and installed it today. This seems to have fixed the problem with no check engine light after I reset everything.

Great service and communication from the guys at Oxygensensors.com!!!!

Thanks
jf
Kingsport, TN

I wish I would have come home and checked this forum first. After replacing both 02 sensors I went for a drive and after about 20 minutes the idiot light came on so I stopped in an autozone a block away. they ran the code and got the p0125 and said it was the ect sensor and needed to be replaced. I told them I had just replaced my 02 sensors and wondered if that could have caused it and they said no way. After replacing the ect sensor the light came on again, had it erased and it came on again. Look at the forums and my chiltons guide and of course it says most likely cause is 02 sensor not reading followed by possible thermostat stuck open and that a bad ect will make it run and idle rough which my vehicle doesn't do at all it just sucks gas.

Turns out I rechecked my rear 02 sensor which I had installed 1st and my crimp job was 2nd rate and one had come loose. <img src="/forums/images/graemlins/frown.gif" alt="" />

Long story short @#%# autozone and next time I'll come back to this trusty board and consult my manual before buying a part. <img src="/forums/images/graemlins/kewl.gif" alt="" />

Don't trust the parts store guys to fix your stuff. No ASE certification required, other than perhaps the ASE parts tech courses. Dem guys no be mon-kay wrenchers.

Thanks for posting back on your problem. <img src="/forums/images/graemlins/cheers.gif" alt="" />

Better than 90% of the time a p0125 code is related to O2 sensors and not truly ECT related. All it really usually means is that your ECU is not going into closed loop mode.