Finally CEL came on..woohoo

Posts specific to the 2.5 litre turbo engine

Re: Finally CEL came on..woohoo

Postby bigBADbenny » Tue Nov 26, 2019 7:32 am

Hey Mick, when pulling the front a/f sensor plug to do this test, is it worthwhile to remove the sensor and replace it with a bung?
Eg so as not to damage the a/f sensor in the exhaust stream but not heated.
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Re: Finally CEL came on..woohoo

Postby Stifull » Tue Nov 26, 2019 10:45 pm

When my youngest daughter had the same lights flashing on her impreza the O2 sensor gave a different code and it actually stipulated it was the rear O2 sensor not the front one whereas both devises all 3 times have had the same P0304 code in the Lib.. and the symptoms are identical to our sti when it had an intermittently failing injector. the failed O2 sensor in daughters car didn't actually produce any symptoms apart from the cel lights coming on..It was only a MUT that found the culprit..
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Re: Finally CEL came on..woohoo

Postby bigBADbenny » Tue Nov 26, 2019 11:11 pm

Yes p0402 is a different CEL. Cat below efficiency.

I think what Mick is referring to is that some CELs can be contraindicative of the actual fault which may be found up or downstream of the affected sensor.

In the case of a/f sensor related CEL’s removing the loom and or sensor makes the car revert to open loop operation, whereby if the car runs well the fault points to the sensor, if not usually vacuum/boost leak, injector or valve train related issues.

Misfire CELs are based on calculations related to the crank and cam sensors, the ecu detecting the crank is slowing down between ignition events.

I guess if one was using logging for diagnosis, and an injector was failing intermittently, you’d see fuelling corrections in the affected rpm ranges.
And this is potentially where you’d force open loop by temporarily removing the a/f sensor plug.
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