P0125 and 2k+ idle
 

posted in the 3.4 swap section originally, but hoping some extra eyes and brains might help. 1990 runner with a 96 3.4.

Originally I had a Maf issue but it seems to have turned into a coolant temp issue after replacing the MAF with the correct one. The ecu is throwing a p0125 which I believe is causing the truck to never come out of open loop causing it to idle around 2-2500 Rpms at all time.. I couldn't troubleshoot with my basic tools so I took it to a friend with a live data scanner and we saw that the O2 sensor reading is 0.0... So we checked continuity to the ecm and all wires have a reading so I figured the sensor was bad (I bought a cheap universal one off of rock auto so I figured that was my fault). So I bought a new Denso one specific to my engine, didn't fix the issue.. So I replaced the ECT sensor since it was cheap, still no dice. I'm kind of at a loss at where to look now, I can only imagine it's a wiring issue at this point. Id appreciate and advice on how to trouble shoot this. Thanks.

Update, still throwing P0125. MY high idle was cause by the wrong TPS and throttle body, found the correct one at the junkyard and that solved that issue. I still cannot get the truck to go into closed loop, the 02 sensor is working, ECT ohms out within spec, the engine is running arouns 185*, and ive got no shorts or opens in the O2 sensor harness. I'm starting to think this is an ECU issue, then i came across this-

file:///Users/admin/Downloads/CI__P0135__P0141_.pdf

and using this method, i do have voltage at the HT1 pin, 12 volts to be exact, but none at the HT2 pin. Would this point to a bad ECU? Thinking i might take this into a shop and have a professional diagnosis before i go out and buy a new ECU....

Welp, the new computer did not solve anything. Still having the 125 code... I'm really starting to think this is not gonna get fixed.lol Truck runs great, its just getting god awful gas mileage (about 160 to the tank vs the 210 the 3.0 got).

Just a recap of what ive done-

New Stat (boiled in pot of water to verify opening)
New ECT sensor
Tested ECT resistance, within spec
New 02 sensor
Burped cooling system 3 times
Replaced ECM
Verified continuity between 02 sensor and ECM
Verified 02 sensor is getting power

ideas.....anyone...lol

Did you ever solve this issue? I'm having this same exact problem with my 3.4 swap. Mine is a 98 runner m/t ecu and base engine is an 04. But engine is set up as a 98.

It's never the ECU until all other options are confirmed. They rarely break.

Check wiring continuity between the coolant sensor on the block and the ECU,
confirm connection or voltage at the coolant sensor, and
then double check your pin connections in the wiring harness as well.

I had a hell of a knock sensor problem that ended up being a wiring harness pin problem at the ECM. It was infuriating. I've got a 98 3.4 and a 96 ECM in 95 4runner body. Wiring puzzles took a full year of driving and fiddling to get it all working proper, but I also made a lot of mistakes on my swap. Good luck

Believe it or not P0125 can be caused by the front O2 sensor on the 5vz-Fe, either the wiring or upstream O2 sensor is bad or the incorrect O2 sensor part number for a Federal emissions ECU and vice versa. Only use DENSO 234-4162 for FED Emissions upstream and downstream on the 5VZ-Fe Automatic trans, Manual trans has a longer pigtail but same sensor. If you have Cali emissions you will need the upstream Wideband O2 and 2 Cats. My 99' FED Emission 4R flagged P0125, fuel trims wouldn't adjust, stuck in OPEN LOOP, swapped the O2 and back to normal.

Actually it is using great. Its multifunction's are very impressive . The multimeter is very cheering tool BR-3 OBD 2 . now it is very expensive in market. Ideas Anyone?
Lol .
I have no idea what i am talking about.

Me either! :hillbill:

That could have been a response from a rudimentary AI.