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I have a Pontiac Grandam 2.4 Lire. The ETS light comes on by itself when engine is warm with or without check engine light coming on. The check engine codes vary from P0171, P0134, P0125. Severe black smoke from the exhaust when ETS light on very poor gas milage when on. Starts fine cold or normal. Great performance when lights are off. New Stuff: MAP hose, Cat. Both O2 sensors, Plugs & wires. Can't seem to find any vacuum leaks. 180,000Kms Any hints where to look? Air valve maybe?

ETS light is on because of the way it is running, weather the engine light is on or not.
P0171 is lean. Check fuel pressure.
P0134 o2 sensor insufficient activity. Probalby plugged again from the way it is running, and black smoke.
P0125 coolant temp sensor circuit. This could be the whole problem. If reading open (-40 degrees), then engine would run very rich and may blow black smoke.
Check the fuel pressure regulator (FPR) for leaking gas into the vacuum line.
With the way it is not all the time, i would suspect a bad PCM, or ground problems down buy the starter on the transmission bellhousing. A few small wires grounds that break- quite common.

Hi everyone,
A little bit of success, I cured the ETS light problem.
I changed the idle air valve. Thinking that if it stuck open it would lean out the mixture off idle. That problem is solved, no more black smoke and better gas milage.
I just get the P0134 problem now occasionally only when warmed up under load like going up a hill or hard acceleration onto the highway. I checked the ground wires, replaced the temperature control sensor, and replaced the exhaust cat back with OEM stuff (there was a leak at the "Y" branch). Could it be a faulty NEW O2 front sensor?

Confused about the Idle Air Control valve curing a LEAN condition, and no more black smoke and better gas mileage. Idle valve only controls idle.
Yes, the O2 sensor could be bad or contaminated.

Sorry to confuse you, I believe the old air valve was stuck in the open position, adding more air than usual. I put the new one in and no more ETS light coming on for 2 weeks now and no soot on the back of the white car.