Car is a 1992 400E with about 140k miles. The check engine light has been coming on maybe once a week for a few weeks now. I check the codes with the LED light and its always the same - 5 - EGR Faulty, and 6 - Idle Speed Control Faulty. I clear the codes and everything is fine for a week or so and then it comes back on - always the same two codes at the same time. Car runs fine whether the CE light on or not - no difference.

It seems unlikely to me that I have two problems always occurring simultaneously. Seems more likely than one problem is causing the other. So which one is the "source" problem? What's my next step - how do I find out which is the source problem? EGRs are a little pricey to just start throwing parts at it. BTW, the engine actuator has been replaced. Thanks in advance for any help.

I would just do a simple hand vac EGR test before anything....
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chances are its a bad connector or a wire short ( soft code ), if engine fails to start then flooding happens and thats when you get these codes they create weak or high fuel mix problems, check all wiring connectors and vacuum pipes also injectors for leaks