Hi i have a Mercedes C Class 09 plate, deisel, auto. Every now and then wen i stop in traffic or slow down the engine cuts off, its an automatic so i then have to put it into Park then turn the key again for it to start, all the lights on the dash stay on so electrics stay on just the engine cuts out. Any ideas??

  • Have you read the codes? That may help figure out the problem. When was it last serviced?
    – Solar Mike
    Commented Jan 4, 2019 at 6:29
  • Codes? It was serviced in April last year
    – Miss GM
    Commented Jan 4, 2019 at 17:58

1 Answer 1


When the alternator has a bad diode, the output voltage is "spiky." This confuses the ignition module and the spark stops momentarily. The engine will stall at idle or at no-throttle (while slowing down.) It will start up again right away.
In this situation, poor connections at the battery make the problem worse because the battery acts as a load and dampens any spikes.

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