Michael

I have 93 LS400 with both CEL & TRAC-OFF. Check the code to be 25 & 26. Those two code seems to lead to O2 sensor, so I went to the service manual to determine the left rear one to be the bad one. I did this by measuring the resistance for each rear O2 sensors. I bought one Denso brand O2 sensor from rockauto to replace the bad one then clear the code. Yet, both lights keep coming back. I am puzzled. Any help on this issue would be greatly appreciated.

cobalt91

Might need few drive cycles for the ecu to adjust for the new parts.

LScowboyLS

codes 25 & 26 (air fuel mixture lean malfunction and air fuel mixture rich malfunction) can point to several different issues, including:

● engine coolant temp sensor
● pre-cat O2 sensors
● pre-cat O2 sensor wiring
● engine ground connection
● fuel injectors
● ECU
● air-flow meter (AFM)
● ignition system
● jumped timing belt


the rear O2 sensors (post-cat) do not affect how the car runs, nor does the ECU make any decisions based on them - they are there to spy on your emissions compliance only!

please use the factory service manual troubleshooting section to troubleshoot those circuits using the steps shown

if the troubleshooter points to the ECM (ECU) - you may have failing ECU caps

Michael

Thanks for your advises. I've been clearing many time to find the CEL & TRAC_OFF lights repeatedly coming on. At times, the lights go away by itself for a cycle.

Those lights are annoying more than anything else. There is no loss in performance nor gas mileage as I keep track of the gas mileage on every fill. I do not even have to worry about the emission test as in Illinois, 93 model cars are past the test period.

Although I do not think I have a problem with engine/transmission doing crazy things, I am going to go ahead and replace capacitors as LScowboy has posted. It just makes sense to replace them,

Is it possible to narrow down the list or logical steps to go over, LScouwboy? As I read postings done by the LScowboy, I have a great respect for your opinion.