94 E420 120k. A couple of weeks ago I left the intake air temp sensor unplugged by mistake. CEL came on after a few trips. I plugged the sensor back in and tried to retrieve the codes using both the built in reader and the home built one. The DM would not respond at all. It smells like it is running rich and I am getting horrible MPGs. I hoped that it was the O2 sensor so I replaced that this weekend and it did not help. Without being able to read the codes I need to go about this differently so here is my question:

What can go bad and cause the CEL to come on, and bad gas mileage, but without causing the car to start or run poorly?

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Will
94 E420
82 300DT
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