Help Needed Troubleshooting Code 25 (1 Viewer)

[Irish Reiver]

Could use some help on this one. 1994 ODB1

Problem – Stable Code 25. The initial code was Code 21 (Bank 1 O2 sensor) however after a few ECM resets and a few weeks of travel, the code changed to Code 25 (Lean) and sometime Code 26 (rich). I have never had the Code 21 since, however since that was the original code I decided to start my testing with the FSM troubleshooting tree for the O2 sensors. When I followed these steps I was measuring 0 volts at VF2 and ultimately ended up with at the “Replace the heated Oxygen Sensor”. I should point out that my O2’s are OEM with less than 15K on them. In any case @beno supplied a new set under warranty. I fitted those, reset the ECM and the code (25) came back within about 5 miles and 2 or 3 starts.

I have a Toycom data reader and I can see that Bank 2 O2 is not switching from rich to lean which lines up with me measuring 0 volts at VF2 in the DLC diagnostics box.

Today I printed the wiring diagram and set about checking for continuity across the O2 circuits. This is what I tested so far:

From the O2 engine side connectors:-

1) +B (Yellow/Red) on O2 Left and +B (Yellow/Red) on O2 Right = 0 Ohms

2) +B (Yellow/Red) on O2 Left to Chassis = 400 Ohms

+B (Yellow/Red) on O2 Right to Chassis = 400 ohms

3) OX2 (White) to OX2 in DLC = 0 ohms

OX1 (Blue/White) to OX1 in DLC = 0 ohms

4) Turned on Ignition +B on Left & Right Sensor = 11.5V

5) HT1 (O2 Harness) to HT1 (ECM) = 0 ohms

HT2 (O2 Harness) to HT2 (ECM) = 0 ohms

6) OX2+ (O2 Harness) to OX2+ (ECM) =0 ohms

OX1+ (O2 Harness) to OX1+ (ECM) =0 ohms

I think my problem is the 0volts at VF2 but maybe I am chasing a dead end with that. Vacuum hoses are all new and the VAF / Throttlebody intake pipe does not appear to have any cracks. I had a go with the starter fluid leak detection method but saw no issues.

Any help is greatly appreciated...

 

[Jgrauman]

Could use some help on this one. 1994 ODB1

Problem – Stable Code 25. The initial code was Code 21 (Bank 1 O2 sensor) however after a few ECM resets and a few weeks of travel, the code changed to Code 25 (Lean) and sometime Code 26 (rich). I have never had the Code 21 since, however since that was the original code I decided to start my testing with the FSM troubleshooting tree for the O2 sensors. When I followed these steps I was measuring 0 volts at VF2 and ultimately ended up with at the “Replace the heated Oxygen Sensor”. I should point out that my O2’s are OEM with less than 15K on them. In any case @beno supplied a new set under warranty. I fitted those, reset the ECM and the code (25) came back within about 5 miles and 2 or 3 starts.

I have a Toycom data reader and I can see that Bank 2 O2 is not switching from rich to lean which lines up with me measuring 0 volts at VF2 in the DLC diagnostics box.

Today I printed the wiring diagram and set about checking for continuity across the O2 circuits. This is what I tested so far:

From the O2 engine side connectors:-

1) +B (Yellow/Red) on O2 Left and +B (Yellow/Red) on O2 Right = 0 Ohms

2) +B (Yellow/Red) on O2 Left to Chassis = 400 Ohms

+B (Yellow/Red) on O2 Right to Chassis = 400 ohms

3) OX2 (White) to OX2 in DLC = 0 ohms

OX1 (Blue/White) to OX1 in DLC = 0 ohms

4) Turned on Ignition +B on Left & Right Sensor = 11.5V

5) HT1 (O2 Harness) to HT1 (ECM) = 0 ohms

HT2 (O2 Harness) to HT2 (ECM) = 0 ohms

6) OX2+ (O2 Harness) to OX2+ (ECM) =0 ohms

OX1+ (O2 Harness) to OX1+ (ECM) =0 ohms



View attachment 1849649

I think my problem is the 0volts at VF2 but maybe I am chasing a dead end with that. Vacuum hoses are all new and the VAF / Throttlebody intake pipe does not appear to have any cracks. I had a go with the starter fluid leak detection method but saw no issues.

Any help is greatly appreciated...

What's the rest of the exhaust system look like? Exhaust leaks before the o2s can cause bad readings and run rich and lean.

 

[Irish Reiver]

O2's are located in the exhaust manifold and I am pretty sure there are no leaks.

I was going back over the troubleshooting flowchart again today and I was wondering what the step "Disconnect the PVC hose" was testing for? I end up with 0Volts at VF2 on every test in that chart. Does anyone know what causes the ECM to apply voltage to that terminal? Am I looking at replacing the ECM?

 

[mypancreasbroke]

@Irish Reiver did you ever find a solution? I'm having symptoms of a bad o2 senor, but replaced both around 20k miles ago. I've got the new fancy toyocom obd1 reader, and it seems my bank 1 sensor doesn't like to respond until the engine is almost warmed up to operating temps (I've been letting the truck warm a while before driving). I haven't had time to go in with my fsm and start testing like you have, but hope to in the next two weeks. I've had no CEL and no codes, but have noticed a recent decrease in fuel mileage and an occasional slow throttle response before quickly catching up when driving when the truck is cold. I can see in the toyocom app that the bank 1 sensor just stays at '+20%' and lean for a while before finally responding and showing a fluctuating reading and lean/rich switching, which the bank 2 sensor does from startup.