CEL is on and pulling a 1-2-3 code, which indicates coolant temp sensor.
I have removed connector and reinstalled, cleared CEL and restarted vehicle and no CEL, that is until car heated up and then CEL came on again.

Is it safe to assume that the sensor is bad?

Can the sensor and/or wires be tested-if so how?

If the sensor is bad, can it be replaced without taking off the manifold?

Thanks much!
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1993 240, 1992 240GL, 1985 240DL Wagon, 1983 240DL

While I can not help you with how to tell if your sensor is bad I can help you with replacement procedure as I just did this 2x weeks ago with my '91 740(same engine). By the way I knew my sensor was bad because it effected my idle(was way too high). Without having to remove the intake manifold I used a long extension(about a foot long or more)on my socket wrench. Preferably one with a swivel fitting. You have to use a deep socket as connector gets in way. 19mm or 3/4" . I unplugged any hoses & lines that were in way to get easier acess and visability. Removing and reconnecting the connector is a pain but you have already been there I see so are familiar. Long screwdriver helps to depress connector clip and to pry off & push on connector. This job took all of about 45 " time. Oh yeah forgot to mention very first thing to do is drain out a little coolant as otherwise it will leak out when sensor is removed. Hope any of this helps.
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' 91 740 sedan nearly 157,000 and ' 87 240DL auto nearly 135,000

Yes, this helps, now that I know it is possible to change it out without removing intake manifold.

Thanks
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1993 240, 1992 240GL, 1985 240DL Wagon, 1983 240DL