Coolant Temp Sensor fault code

I have a 1-2-3 code on my 1991 245. The car is completely stock mechanically, no turbo.

The Bentley manual says it is a faulty engine temp sensor. This code comes back every few months. I have replaced the coolant temp sending unit (this car has one) twice with parts store parts. I have replaced the sending unit once with a known good unit from Charlie. I have probed the head to see if there is crud blocking coolant from the sending unit and it is clear. I have opened the wiring harness and re-soldered the group ground at the intake manifold.

The coolant is clean. The car does not run hot or overheat.

I'm out of ideas on how to resolve the code so it stays gone.

The car has no major drivability issues. Occasionally, it will "stall" when I try to accelerate from a dead stop when the ambient air temp. is very high. I think it is a dying spark amplifier, but I have a spare in the tool bag from a junkyard car. That is the only issue the car gives me other than this check engine light code every few months.

Any suggestions?

I'd suggest get out an ohmmeter and see what the coolant temp sensor measures at the three temp test points. You remove the cover over the fuel ecu and ignition and measure from the connector pin on the respective computers to ground. The sender on the engine is a two circuit sender providing the signal for each computer. You measure this with a cold engine, warm engine, and fully hot. I've found senders and wiring that fail under the various temperature test points.

Depending on what location you read the code from is what circuit to check. If you read that in socket 2 then check the fuel side. If a 224 I think it was in socket 6 then the ignition computer. They both can set the check engine light.

Do not front probe terminals.

Go to brickboard .com FAQ. 700 series is the same.