this post was submitted on 18 Apr 2025
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Mechanic Advice

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'99 Corolla engine code P0125 (midwest.social)
submitted 1 week ago by [email protected] to c/[email protected]
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I'm trying to diagnose a p0125 code on my 99 corolla. Toyota forums seem to say Toyota didn't follow standards on this code and it is likely also the O2 sensor, can anyone confirm if this is the case?

I'm getting no other codes, and it will run for about 30 minutes before triggering again. Last time it triggered I saw exhaust smoke on the way home, which went away as soon as I reset the code. after clearing the code the exhaust while clear does seem to be rich smelling. Engine seems to run fine, I'm not noticing any performance issues, not signs of dipping oil/coolant amounts. coolant gauge is operating and hitting the halfway point normally.

I can't think of anything else but the O2 sensor but I expected rougher performance and don't really want to spend $125 on a part if I don't have to.

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[–] [email protected] 1 points 6 days ago (4 children)

It hasn’t been consuming oil or misfiring, so I don’t think it’s that, it is showing more signs of an air leak from what I can tell

[–] [email protected] 2 points 4 days ago (3 children)

I think you're correct, more likely to be a leaky joint or sensor out of calibration. The post was triggering because of my experience with a 99. I spent months tracking down a water leak into the passenger footwell and then the engine blew 😂

[–] [email protected] 1 points 3 days ago (2 children)

That sounds about right! And about average for every older vehicle I’ve had once they start causing these rabbit holes type issues

[–] [email protected] 2 points 1 day ago (1 children)

The skills of buying, maintaining and disposing of used vehicles are various and judging when to let go is critical. Sent my much cherished Volvo V40 to the crusher in running condition because it needed a cam chain and a catalyst that would have doubled the value of the vehicle 😅 Still unsure it was the correct call.

[–] [email protected] 2 points 1 day ago

Oh I know! I about have everything diagnosed and despite the rabbit trails the fixes are all pretty cheap. Smoke was 2 separate issues: clogged pcv hose, and dying cdi boxes causing incomplete ignition. The check engine light is the upstream O2 sensor, so now I’m just waiting on parts.