My 1996 406 D8 has recently developed an issue of engine check light (K light) comes on and off once in a while. It's completely random and you never know when it'll lit up again. The moment it lights up engine RPM drops suddenly to below 1 and sometimes engine stops too. Then it's little hard to start. At times car goes fine and this only happens when the vehicle is moving. I can keep the thing in idle speed for hours without any issue. thought it could be
1. intake air flow sensor
2. intake air temp probe
3. TPS sensor
4. Idle up / stepper motor
5. Vehicle speed sensor
6. Engine Control Unit
Ran the car one sensor unplugged at a time. Today I ran with the Vehicle speed sensor unplugged from the socket as the last ditch and still the K light came, and engine stopped few times. Sometimes the idle is rough. I cleaned the stepper motor and it's working fine and even if the stepper motor is unplugged, the issue is there.
Please fellas, I need a big help in determining where the issue is before replacing the ECU coz ECU's are damn expensive here in Sri Lanka. What is left for me to do is clean and re fix all the wiring contacts in the engine bay and clean the throttle body. When the K light comes no matter at what speed the engine reviving up, the rev meter drops to below 1 as if the correct rev signals are not coming to the rev counter. Could it be the TDC(?) sensor ???
That does sound like a dirty contact somewhere and I think you're on the right track checking/cleaning all the contacts. Also check engine running relays 'cos I had two go intermittant on a mates Pussat once so I don't trust them anymore. 
