Check engine code 25,26 (lean & rich) - 88 FJ62 (1 Viewer)

[BobbyKnobby]

My FJ62 is throwing codes 25, 26. CEL comes on after warmup and driving after 5-10 miles.

Other than the pesky CEL light, engine seems fine. I hooked a digital volt meter to Ox1 & ground, and also Ox2 and ground, and got a steady voltage swing from about 0.1 volts, to .9 volts around 1 cycle per second after a 2 minute warm-up. According to the manual, this is the expected behavior of a functional O2 sensor.

I had this same 25,26 combo when I purchased this FJ62. When I needed to smog (CA), the smog shop replaced one non-functioning O2 sensor, replaced the EGR vacuum modulator control, and replaced both cats. CEL 25,26 went away for several months…now it’s back!

Code 25+26 seems to be oxygen sensor related - or related to air is getting into the system somewhere it shouldn't, right? I’ve read a bunch of threads on this dreaded check engine light combo, and checked the basics - vacuum tubes, air intake all seem to be fine.

Any guidance is greatly appreciated!

 

[BobbyKnobby]

Update: still throwing codes 25,26…when I tested from Ox1 and Ox2 at diagnostic port after a 2 minute warmup @ 2500 rpm, the oxygen sensor voltages swing low-high-low-high as expected. I’m checking with a digital multimeter, which it’s a little sketchy, but it feels correct. I decided to test again after a 20 minute drive…sure enough one of my oxygen sensors was malfunctioning - stuck on the low voltage. Unfortunately, the broken the sensor is for cylinders 4,5,6…the bugger that is mounted upside down near the transmission, where my ogre hands do not fit! I struggled for too long to free the crusty old sensor!

…and I ordered a new denso O2 sensor ordered from Amazon! Denso DENSO Oxygen Sensor - 234-4056

https://a.co/d/3n5X6P2

While purchasing the new sensor, I also grabbed a stubby, flex-head ratchet wrench set from Amazon. This made reinstalling the awkward O2 sensor SO much easier! https://a.co/d/dMq28oO

It might seen expensive for a random speciality tool (54mm socket also comes to mind!), BUT my least favorite part of working on cars is trying to get my gorilla hands into impossible to reach places, for impossible to access bolts…maybe I’ll find other uses!

 

[BobbyKnobby]

Update on my CEL 25,26 journey: I have replaced both O2 sensors (with Densos) and replaced both catalytic converters. I have reset the computer by pulling the fuse. After a 10-30 minute drive, the check engine light comes on and it's code 25,26.

Does any one have any suggestions on where to focus next?

I have considered replacing all vacuum tubing, replacing the engine air filter, perhaps getting a valve job....
What is the next logical step?

 

[PBcuiser]

Confirm MAF meter is good, TPS adjustment/check and valve adjustment never hurt anyone... I was getting those codes on and off or a while and those maintenance items seemed to fix my issues. replacing vacuum tubing is also a pretty straight forward afternoon project.

 

[toy_tek]

TB gasket? Have you sprayed ether around your intake plumbing to see if you get an rpm increase?

I had a similar issue w a 5vze, replaced O2 sensor just like you did. Ended up being a leak at TB gasket.

 

[BobbyKnobby]

Thanks for the ideas! I checked the MAF per the service manual - all of the ohm reading were within expected ranges…so I’m thinking the MAF is okay.

After I put the MAF back together, I let the engine idle and sprayed ether all around the throttle body gasket, as well as all other air intake connection points. The engine idled steady…I could not find a place that caused it to rev.

Good to know these parts are functioning correctly, but I’m still hoping to get to a solution on why I’m getting codes 25 & 26.

Any other ideas or systems to diagnose? I’m learning all kinds of new stuff about the engine, so not all is lost!

 

[RevISK]

Thanks for the ideas! I checked the MAF per the service manual - all of the ohm reading were within expected ranges…so I’m thinking the MAF is okay.

After I put the MAF back together, I let the engine idle and sprayed ether all around the throttle body gasket, as well as all other air intake connection points. The engine idled steady…I could not find a place that caused it to rev.

Good to know these parts are functioning correctly, but I’m still hoping to get to a solution on why I’m getting codes 25 & 26.

Any other ideas or systems to diagnose? I’m learning all kinds of new stuff about the engine, so not all is lost!

View attachment 3521836

Did you ever get to a resolution?
Mine just started throwing these codes and I’m in research phase.

TIA,
Ian