'94 FZJ80 - CEL Code 25

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Mar 24, 2015
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Raleigh, NC
Seeking advice - I've searched and read through all the of related threads and most CEL code 25 issues seem to have been resolved by replacing O2 sensors.

After having my headgasket done, including full cleaning of intake and injectors along with new plugs, replacing all vacuum lines, new air intake hose, disti cap, and wires, I've been throwing code 25. The engine idles fine, runs smooth, etc.

This weekend, I did the full PAIR delete and replaced my non OEM O2 sensors with OEM ones (I actually found an unused set for the OBD1) in the exhaust manifold. I drove about 25 miles after replacing the O2 sensors and thought everything was fine until this afternoon after I filled my gas tank and it threw a code 25 on the way home.

I have not done the full test in the FSM, which is likely the next step, but was am seeking any advice on what to check: VAF, something else?

I welcome any input
 
Joined
Mar 24, 2015
Messages
66
Location
Raleigh, NC
I checked the harnesses that plug into the O2 sensors - all the wiring appeared in good shape. Would a harness issue caused the CEL to trip immediately, or seemingly randomly as it does now? I regularly pull the EFI fuse to clear the code, perhaps I need to start documenting the conditions under which is trips.

Since the truck still runs great, I am tempted to just pull the damn CEL bulb; however, my OCD about the truck prevents me from doing so.

Any other advice would be welcomed. Thanks!
 

ppc

M Go Blue
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Aug 18, 2003
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Nashville, TN
Would a harness issue caused the CEL to trip immediately, or seemingly randomly as it does now?

Both, depending which wires have the problem. Failures in the heater circuit would be detected quickly, the O2 signal may take longer considering things like "two trip" logic that raises CEL codes under specific driving conditions.
 
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Cleared the CEL last night - drove 140 miles today (mostly highway) and no CEL. Probably 10 miles city and 130 miles on highway at 75 mph - this one is vexing. Any other thoughts appreciated.
 
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east coast Canada
Did you ck the O2 harness that goes behind the engine, over the front of the trans to the plug in the vicinity of the starter?
Mine had totally deteriorated harness protection on that bundle.
 
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Apr 10, 2020
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Texas
@tseck73 I just had the same CEL code 25 pop up today on my 93 FZJ80. Just want to ask if I read the thread correctly. You replaced your 02 Sensors, cleared the codes and have no problems since?
I have not replaced the 02 sensors since owning the vehicle for about a year and a half now so I am thinking of replacing those. I have seen some videos that they can be tough to remove so any advice on how you removed yours would be great.
Thank you!
 
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@TexasTom - I am still chasing an intermittent Code 25. So far I've removed the PAIR assembly and replaced the O2 sensors with OEM ones (They are NLA, but I found a set). In addition, I relocated the O2 sensors to the exhaust manifolds for easier access. I still get the random CEL code 25, but the truck runs great, idles fine, etc. I need to check the O2 sensor wiring harnesses as a next step, then the VAF. If it still occurs, I might just pull the damn light.
 
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@tseck73 - Got it, most likely going to replace my O2 sensors today if not tomorrow and see if that helps my situation. Had my VAF (aka Mass Air Flow) rebuilt a few weeks ago thinking that was the problem and it ran great for about a week then she just bogs down so much there has to be another air/fuel problem. Other than a vacuum leak which I have not been able to see I guess I'll just chase this gremlin and hope that O2 sensors at least solve the boggy running.
Thank you for the response.
 

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