Replaced my coolant temp. sensor ( Plug broke off, don't know for how long, thought my gauge was haywire) 2001 Blazer with 127000 miles. Gauge now works, but it runs rough when cold. I also had an oil change and had coolant topped off. What might be causing this to happen all of sudden?

Thanks, Jeff

 

Hmmm.. try disconnecting battery for a couple minutes. Report back on whether or not that changes the situation.

 

Double check all of the connections under the hood. You might have knocked a connection loose in the process of replacing the ECT sensor.