Here's the story:

CA 1991 190E 2.3 with 'check engine' light on. Light would reset after disconnecting battery, then come back on after a short drive, turning off then restarting engine. When attempting a diagnostic check using the pushbutton and LED, no light from the LED, despite trying several different methods. Anyway, a separate issue was the drive belt very loose, so adjusted the belt tensioner, and it would quickly lose tension again. I ordered a new tensioner.

Tensioner arrived, and I replaced it, using a new drive belt too. I disconnected the battery to reset the check engine light, and gave the car back to my wife to drive. Many trips since then, over about a week, and the light has stayed off, and the car drives normally. I have looked the fuel and emission system docs over many times, and cannot find any relationship between a loose belt and a reason the check engine light would come on. Am I missing something, or is this just a strange coincidence?

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7