I've got a hard to track down LH 2.4 problem, and its a long story. I'll try to sum it up. The check engine light keeps coming on, and the ECU goes into limp mode. The car still works but it gets horrible fuel economy.
Here are the ECUs I have tried:

Orignial 951: Codes read 123- faulty temp sensor

replacement 942: Codes were for compensating for rich/lean fuel mixture

Second 951: faulty temp sensor and compensating for rich/lean fuel mixture

Pink label 561: No errors so far

I have checked the AMM, the CPS, the temp sensor, the fuel pressure regulator, the timing belt, the O2 sensor, and all other inputs to the ECU. All are within normal tolerance.

The light does not come on right away. Sometimes it takes a few minutes and up to a week for the light to come back on after the ECU it reset. It seems to happen on hot days. The preheat hose is disconnected. The original 951 seems to work fine in my 89 240. The pink 561 is from my 89 and it is the only ECU I know for sure is working normally.

So I am running out of ideas. Do 951s fail by not being able to read the temp signal? The EZ-k does not record any error codes and it is attached to the same temperature sensor.

Any help would be much appreciated.