Over the last month or so, my check engine light has come on with code 4, air injection inoperative. Anyone else have this problem, and what are the likely culprits? Thanks in advance.

The system uses the O2 sensor to monitor the air injection system to make sure that the pump & diverter valve works.
There is a electro-magnetic vacuum valve & several vacuum lines also that open the diverter valve.
ANY of these as well as the air pump can cause the problem.

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How costly is this repair? Since it only shows up once every few weeks, I have simply been resetting the check engine light and have been ignoring the problem as much as possible. Is it potentially harmful to the engine? How difficult is it for someone to diagnose the correct component that is malfunctioning?

First check is to see if pump comes on when car is warming up.
If not , the problem is more likely electrical.
If you have a vac gauge and are a DIYer, there is a simple test you can do to help diagnose the system:
At the air valve between the pump and the engine, there is a small vac line. Take it off the valve and check it for vac as car warms up [ pump only comes on for up to 110 sec. during warm-up].
If it has vac sometime during warm-up, it means the air valve has received the required sig from the ECU and also lets
you know that there is vac and power to the switch over valve [which let vac get to this line]from the engine. This test also tells you that the pump clutch also has power cuz the SOV and pump clutch are wired to the same feed from the ECU..

If no vac at this line , then you may have a broken vac line from the eng [ top,front] to the switch over valve, or no power to the SOV. Start looking..
[the SOV is located between the hose you have the vac gauge on and the eng vac line]

If you do have vac here and the pump is running, then you may have a bad/stuck air valve

A tech can find these as the system is not a complex as it sounds
I can send you a vac diagram that once looked at,the above info falls into place..
email me if needed

Thanks for the detailed info. I guess my question has more to do with how serious a problem this really is. Do I run the risk of burning up my catalytic converter if I fail to address the issue? You mentioned that the pump only comes on for up to 110 seconds during warm-up. Since I live in Southern California, and the car really does not see extremely cold temperatures, am I doing anything to harm the engine if I simply continue to reset the light when it comes on every 6 weeks or so?

I doubt any problems, but Cal may look at the system at inspection???

The general purpose is to aid the cat to get up to temp.
I don't use the pump [ BB in line], but I do drive the first few miles in 3rd gear range... one, to get cat up, and two, to get into closed loop asap.
because I do this by habit, I also BBed the shift delay as I accomplish the same w/gear selection..[and cuz I don't like their delay set-up]

Arthur

Where did you BB each of these lines? Since the BB inactivates the AIR system (I assume), does your car throw the code and light the CE light?

My car has started this CE light every few days, then I reset code 4 stuff in the last few weeks. The pump had been making noise over the last year or so. Can't say as I've noticed if it's running lately. I'll check tomorrow AM when I start it.

Bob

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If you can hear the air-pump spinning, I would definitely check all the vaccuum lines and their rubber connectors.

I was getting this same symptom and code on our 1993 500E (same engine & emissions) and when I removed the vaccuum line to the air-pump (hard to reach and see) and the rubber elbow connector had a tear in it.

I purchased a bunch of them (cheap) and replaced all the rubber connectors I could find due to the extreme heat and age these pieces are under.

No more CHECK ENGINE.

:-) neil
1988 E36T AMG
1993 500E