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Discussion Starter #1 (Edited)
1989 900S, LH2.4. Background: I bought the car three months ago and have spent the intervening months dealing with a lot of general neglect. I've had a bunch of stuff apart in the engine bay, including detaching various electrical connectors, but I haven't replaced any fuel injection sensors or components. Battery is recent, primary power and ground connections are clean and tight. Operating voltage is good.

Took the car out for its first test drive after all the work and was greeted with a CEL after about 20 minutes of driving. The light was steady when the throttle was open at all, and blinking with the throttle closed. There was no other indication of any issue, and the engine continued to run normally.

I pulled the fault codes when I got home and found two: 12112 (oxygen sensor self-compensating circuit problem (incorrect air-fuel ratio at idle)) and 12114 (AIC self-compensating fault (system unable to increase idle to an acceptable level)). It is worth noting that the idle seemed to be fine, and only dropped temporarily when the big amperage draw of the radiator fans kicking on occurred.

The oxygen sensor is of unknown age and condition, so that certainly warrants testing. The AIC needs to be checked as well. Beyond these two specific components, are there other likely culprits for these fault codes that I should be looking at? And is this typical behavior for the Check Engine Light on these cars (solid for an existing fault, blinking for an active fault)?


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Quick follow-up. AIC looked clean, but I blasted it with carb cleaner anyway. Hoses were not as snug as I would have liked, so I zip-tied them tight. O2 sensor is switching quickly between ~0.19v and ~0.89v, which seems a little out of range at the low end (spec is 0.4 to 1.0), but that could be due to the engine not being fully warmed.

CEL was not lit in the brief time I had the car idling to check the O2 sensor, but it was also not fully at operating temp. Idle still normal at ~900rpm.
 

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Discussion Starter #4
That won't work.
It has to be hot...Really hot.

But it is registering voltage, meaning it's not completely dead. I'll check it again hot to see if it's in spec.


CEL is out currently. I'll drive it again today and see what happens. Stay tuned.
 

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As I recall, the ECU provides an oscillating default signal during the warm-up phase. It's best to monitor the signal voltage while driving.
If your symptoms diminish after a run on the highway, suspect the O2 sensor.
 

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Discussion Starter #6
As I recall, the ECU provides an oscillating default signal during the warm-up phase. It's best to monitor the signal voltage while driving.
If your symptoms diminish after a run on the highway, suspect the O2 sensor.

That's interesting about the default signal - I've not encountered that before.


That said, I drove the car again this morning - 30 minute drive, mixed roads - and the CEL did not reappear. I'm going to take a wait-and-see approach with it at this point. If the CEL reappears I will hook up the multimeter and check the signal when driving. Given that the O2 sensor is active I'm hoping that the original problem was more the result of a long period of inactivity than anything actually failing. We shall see.
 

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The 2.4 system has a learning/adaptation capacity. If you've killed its memory it may take 50 miles to re-learn parameters. An occasional Check Engine light is normal during re-adaptation.
Speed up the process with a spin on the highway.
 

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Discussion Starter #8
The 2.4 system has a learning/adaptation capacity. If you've killed its memory it may take 50 miles to re-learn parameters. An occasional Check Engine light is normal during re-adaptation.
Speed up the process with a spin on the highway.

What does it take to kill its memory? I had the battery disconnected for weeks while I was working on it - would that do it? Or does it require the specific reset procedure?
 

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What does it take to kill its memory?
Disconnect +12v for about 10 Seconds.....no other backup in the ECU box.


There is also a method to clear the code memory by additional steps with the method used with the code extraction.
Removing the 12v is easier.


That's interesting about the default signal - I've not encountered that before.
If the volts are 'bouncing' as described, the ECU is functioning correctly.


My $0.02 on your idle valve fault code? Might be worth taking a close look at the throttle plate stop and the alignment of the throttle plate. If the idle valve is unable to compensate for increase in revs, I imagine the throttle plate is "too closed".
Idle valve is to compensate base air flow at idle.......not control the entire idle air.
 

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Discussion Starter #10
I did not systematically inspect the throttle position, however I did check that it moves freely off the idle position (no binding). The CEL did not reappear yesterday, which is making me think it was either part of the adaptation process, or that the loose hoses to the AIC (now tightened with zip ties) were throwing it off. Or both.
 
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