There may be a vacuum check valve in between the AIR and EGR switchover valves. Test it for leaks. Also make sure AIR pump is working when cold.
On my car, the LH-SFI module sends a 'turn-on' signal for each function. Not sure how it's done on your car but probably similar since they're the same year.
Had similar problem but only with AIR. Took all components associated with AIR (switchover valve, check valve, vacuum lines, and EGR switchover valve) apart but could find nothing wrong. After re-assemblying, it worked! No codes since.
[Edited by JimF on 04-16-2001 at 06:40 PM]
Regards . . . . JimF
'94 S500 Cpe
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