Hi, I changed the thermostat on my '05 CDI because it would not run at the correct temperature most of the time (usually too cold, or slow to warm up, or would run at the right temp for a while then cool off). During the post-swap test, the check engine lamp was illuminated. The only thing I disturbed was to unplug the temp sensor wire from the old thermostat housing during removal; I did start the engine to check for leaks before I plugged the harness back into the new thermostat temp sensor. I added some coolant to top off after the swap was complete, but surprisingly I did not lose as much as I expected I might. Car has 117,500 miles.
The car runs perfectly and the temperature came up quickly and was correct during a road test of about 10 miles, but the check engine lamp stayed on. Any ideas?

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Providing a home for these cars:
1951 Buick Special De Luxe
1977 Rolls-Royce Silver Wraith II
1970 Rolls-Royce Silver Shadow
1986 Mercedes-Benz 420 SEL
2005 Mercedes-Benz E 320 CDI
2012 GMC Sierra 1500 4X4
1927 Pierce-Arrow Series 80
1931 Pierce-Arrow Series 43
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Hopefully it's because you started it with the sensor disconnected; if so it will clear on its own in a few days.

If you go by an autozone they'll read the codes for free.

You can also get a basic reader that will work fine for about $25 on amazon. I recommend one for any DIYer.

Good luck.

"Check Engine" went out after the second start today. Temperature is behaving nicely, also.
Thanks for the advice.

__________________
Providing a home for these cars:
1951 Buick Special De Luxe
1977 Rolls-Royce Silver Wraith II
1970 Rolls-Royce Silver Shadow
1986 Mercedes-Benz 420 SEL
2005 Mercedes-Benz E 320 CDI
2012 GMC Sierra 1500 4X4
1927 Pierce-Arrow Series 80
1931 Pierce-Arrow Series 43
1926 Ford Model T coupe