AMM & Dirty Air Filter

After installing a new Bosch O2 sensor, the check engine light came on some 1,000 miles later, with 232/221 codes

Engine was running rich, so I verified O2 sensor's heating-element was working, and then verified it was working (rich: 900mV). And verified ECU's bias voltage without 02 hooked-up.

Still rich, so I cleaned AMM with carb cleaner, and blew dust out of air-filter.

I'm getting a new air filter, but either AMM or dirty air-filter was the issue.

I wonder if a dirty air-filter could trip those codes?

I got a similar engine light fault a few hundred miles after fitting a new O2 sensor myself.
After tweaking the way the wire were routed it has been pefect ever since : I guess it just didn't like the way the wires were pulling or something.
Just an idea, and it might save you some work.

Dirty MAF will cause a lean condition. Not too common on these Bosch ones due to the burnoff feature, but a P0171 lean code on Ford can commonly be dirty MAF. I fixed a Frod Escort once that had the PCV dumping pre-MAF and it would foul the MAF. Who designed THAT?

But you said you had a dirty airfilter.

Dirty Rooster said:

After tweaking the way the wire were routed .

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See this thread: Installation: Bosch O2 Sensor with SmartLink?

ZVOLV said:

Dirty MAF will cause a lean condition. Not too common on these Bosch ones due to the burnoff feature....

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Burn-off comes after engine is shut-off (Testing an AMM/MAF). I'll have to test the AMM's burn-off circuit.

Since Check Engine light was on, I have no idea if LH 2.4 was in limp mode, but fuel was being "dumped" via injectors. After clearing codes, check engine light would come on immediately after starting engine-before it was fixed; so LH "knew" something was amiss, and I assume it was evaluating AMM first since O2 would not be upto temperature.

If burn-off circuit is working, then the dirty air filter was restricting air flow, so AMM's mass-air-flow data was below specs for that engine RPM...speculation.

A dirty air filter doesn't help, but I don't see it being the sole cause. Even if you had a new one, would you still get the problem you had, but to a lesser degree.

Graveyard said:

A dirty air filter doesn't help.

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It was the causative agent.