P0125 code after exhaust work

Hi guys, I need some more advice. Ive had a rough go with the exhaust system lately and had one more pop up….

I had a P0420 code. My cat and pipe was in pretty rough shape so I figured id just replace it with an aftermarket one instead of trying to chase the code down. So I replaced the cat with a new aftermarket cat and replaced the fwd o2 sensor because it had become one with the pipe.

A day of driving and I now have P0125. From reading things it seems like its almost never the coolant/fuel system and o2 sensors instead. With recent exhaust work it makes me nervous I screwed something up. My live data on the obd2 is showing good voltage but no short term fuel trim on o2 sensor 2 (which is denso and approx 1 year old)

Its a 2000 with federal emissions. Im not super up on “short term fuel trims” does this mean my o2 is shot already? Or could it be caused by an issue with the aftermarket cat?? Or could it actually be a coolant issue?
Any help or things to check would be awesome. Thanks!

 

Aftermarket cats are notorious for failures. They usually work for a while though.

Regardless, you will probably be best helped by having the factory service manual. For 2002 there is a whole section on diagnostics for "Insufficient Coolant Temp. for Closed Loop Fuel Control" (https://www.ebay.com/itm/194700467600)

Also, did you call your dealer and get the correct O2 or A/F sensor? Only the dealer was able to tell me what the correct one was for my car based on my VIN.

-Mike

 

Thanks I will call the dealer and double check I have the correct o2 sensor for the forward….Ill maybe hope its that….Id feel stupid but its easier than running down this code I think.

This is a freeze frame of the parameters for when the fault occurs. My coolant temp is 89 Celsius…so 190ish degrees. So I think its safe to assume it is not a coolant issue? And an issue in the exhaust system?

 

Denso’s website (which is what I used) shows #234-4162 as the o2 sensor for upstream and downstream for a 2000 non cali. Ill double check with the dealer but I dont think its an incorrect unit. Maybe Ill swap the 2 and see if something changes.

 

I swapped the 2 o2 sensors since Toyota confirmed they were the same…..code came back. So Im going to say its not an o2 sensor issue. And since my coolant temp is showing 190 it seems like its not a coolant issue.

So Im left trying to track something new down….any ideas?

 

Get the factory service manual and run through the diagnostic steps for this? (Seriously. Get the manual. It is invaluable if you plan on working on these.)

I agree, the O2 sensors may not be the issue. 190 degrees may be a bit on the low side for temperature. I think the engine computer may want something above 200 once warmed up. Maybe the thermostat is stuck open?

 

From what I can find 180-200 seems to be running temperature. Ive got no volts on my bank 1 o2 sensor. I swapped them and the same issue. It can spike from 0-0.05 from time to time. So Im thinking I’ve got a wiring issue, maybe a loose connection since it can get volts from time to time. Ill have to get a voltmeter on it and start from there. Im thinking the connector is my first thought of where to start. Ive got the wiring schematics….hopefully I can find it quickly because running down phantom wiring issues doesnt sound super rad.

 

Found the issue. A loose connection inside the o2 connector on the truck side seems to have been the issue. Threw a butt connecter on it to quickly see if it resolved. All looks to be fixed on the obd2 data. How an o2 sensor open wire throws a coolant code ill never understand. But glad its fixed. Seems to be not a ton of resolutions with this code. So i got lucky.

 

So looking at the factory service manual, they appear to treat the code as an Air/Fuel temperature code (not coolant). So, yeah, the code is named wrong.

The A/F (O2) sensors need to be up to temperature and responding correctly before the ECU will determine that the system is properly operating:

CIRCUIT DESCRIPTION
To obtain a high purification rate for the CO, HC and NOx components of the exhaust gas, a three-way cata-
lytic converter is used, but for the most efficient use of the three-way catalytic converter, the air-fuel ratio
must be precisely controlled so that it is always close to the stoichiometric air-fuel ratio.
The A/F sensor has the characteristic that provides output voltage* approximately proportional to the existing
air-fuel ratio. The A/F sensor output voltage* is used to provide feedback for the ECM to control the air-fuel
ratio. By the A/F sensor output, the ECM can determine the deviation amount from the stoichiometric air-fuel
ratio and control the proper injection time immediately. If the A/F sensor is malfunctioning, ECM is unable
to perform accurate air-fuel ratio control.
The A/F sensor is equipped with a heater which heats the zirconia element. The heater is controlled by the
ECM. When the intake air volume is low (the temp. of the exhaust gas is low), current flows to the heater
to heat the sensor for accurate oxygen concentration detection.