Thanks.

I queued up with another reputable online MB parts seller. They said the same.

In a post going back to 2002 - former founder(one of them) of mercedesshop.com - benzmac - indicated the M103 motor had 3 coolant temp sensors.

- coolant temp sensor - CIS/EZL - fuel mgt
- coolant temp sensor - Dash temp gauge
- coolant temp sensor - Aux. fans

My 1991 MB 300-SEL M103 has a 4 prong coolant temp sensor at the very back of the cyl. head. Not all M103 motors have this 4 prong sensor. In my case - this 4 prong sensor signals both the CIS/EZL and the Aux. fans - therefore eliminating one of the sensors.

A single pole sensor signals dash temp gauge as mentioned by previous poster.

008 542 3217 - CIS/EZL-fuel mgt - Aux. fans - 2 sensors in one

005 542 2617 - Dash temp gauge
|__replaced by 005 542 1017 - either one is valid if available

Will ohm test 008 542 3217 before replacing. I have a gut feeling that once the O2 sensor is replaced - the CTS/IAT codes will clear up. The O2 sensor is original - 24 yrs. old.