Completely lost on this one.....

Doc2012

Doc2012

Senior Member
Ok guys. Here's one for you.

Got a CEL. Been chasing it for a good month now.

The codes are P0125, Engine Coolant Temp, and P0135, O2 sensor bank 1, which should be directly in front of the cat.

Replace thermostat, ECT sensor, twice, one of which broke off in my block(that was fun getting out), and the O2 sensor.

Coolant level is fine, truck had NEVER run hot. Not loosing coolant. Oil is fine, no water or anything in there. Truck runs fine.

Still have the damn CEL. Anyone have any ideas what it could be? I'm completely lost.

Thanks in advance for anything you might be able to think of.
 
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So it's still throwing a code for the Temp sender? It's probably a wiring issue at this point. I had an issue on my Chevy pickup with the MAF sensor. Replaced with a new and then a used, still had the code for MAF sensor. Turned out to be a wire had rubbed through the harness and was shorting out on the frame intermittently.
 
This could have something to do with it

319454_10151952216115481_781681949_n.jpg
 
^^^ Nope. Think it has to do with the fact that the damn parts store sold me a knock off of a DENSO. Someone mixed the boxes up some how.
Going to pull one off another truck. Praying that works. If not, I give up.
 
audiness said:
well, it could be your ECU. can you find a friend with a 3rd gen 4runner to try their ECU?
Click to expand...

The problem with that is the '99 ECUs are different than the other years.

I tried a '98 harness in this and it wouldn't attach to the computer or the injectors.

It makes no damn sense to me either.
 
The P0125 CAN be the ECT, but it is frequently the O2 sensor. If you get a P0135 at the same time, it is almost certainly the O2 sensor. Do you have the correct DENSO, repeat DENSO sensor in there? If so, then it is the sensor heater circuit wiring, with the connector the prime suspect. Get out your ohmmeter.

Changing the ECT twice, even once, with a P0135 present was a bogus move, sorry. What happens is the ECU gets signals from the ECT and the O2 that it cannot reconcile, and it always accuses the ECT of being the one that is wrong--even though there is NO WAY the O2 is correct with a fail in the heater circuit. It's just a logical flaw in the ECU's. Clear the P0135 and the P0125 will go away.
 
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Nope. It sure didn't.

Got the p0135 to go away, but now the P0125 is still there. I also have the p0171, system to lean, which makes sense. The system is still running in warm up mode, so it's not delivering the right fuel to the hot air.

I'm lost guys. I guess my next step is to check out the damn wiring harness, or see if I can figure out which pin it it on the ECU and see if I can straight wire it to that.

I'm lost. Idk what else I can do.
 
TheDurk said:
The P0125 CAN be the ECT, but it is frequently the O2 sensor. If you get a P0135 at the same time, it is almost certainly the O2 sensor. Do you have the correct DENSO, repeat DENSO sensor in there? If so, then it is the sensor heater circuit wiring, with the connector the prime suspect. Get out your ohmmeter.

Changing the ECT twice, even once, with a P0135 present was a bogus move, sorry. What happens is the ECU gets signals from the ECT and the O2 that it cannot reconcile, and it always accuses the ECT of being the one that is wrong--even though there is NO WAY the O2 is correct with a fail in the heater circuit. It's just a logical flaw in the ECU's. Clear the P0135 and the P0125 will go away.
Click to expand...

Dredging up old threads yay!

I have a P0125 and P0135 :(

It's an aftermarket O2 sensor that a previous owner installed (with freaking butt connectors that I re-did with soldered joints last year). I'm replacing it with a Denso whenever it gets here.

Just wanted to say thank you TheDurk!
 
keep us updated on the finale here, as the Doc2012 didn't add a conclusion to the problem. I admit that in that case at first, after checking wires I would have thought ECU would be the culprit.
 
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