Cooling Fans Running - Temperature Gauge not working - my data, experience and rigged fix.

[jaxscuby]

so.i encounter the same...
first replaced ECT with GM..worked for a few months.
the replaced thermostat....worked for a few months.
say over the last year, but within a period of 6 weeks.

Then recently ECT sporadically mis reads and the lovely fans come on.

Clear P0128 General coolant temperature too low..

ECT reads correctly until the next time..

Yes i carry a code reader in the center console now..

 

[ragingtripod]

Another issue that can cause this is the overflow from the radiator to the reservoir. Had this issue on my 2014 gmc yukon. When school started in the fall my yukon would have this issue on cold mornings while taking my kids to school. Started off replacing the temp sensor no change. Then replaced the thermostat again no change. After about a month of pulling my hair out i finally found the answer. The issue happens when the engine temp doesn't get up to operating temp within a certain amount of time. I had replaced my radiator in the spring because the plastic side had broken. The new radiator seemed to work just fine when it was hot out but when it got cold it was working too well. Found that the stock radiator had a flow limiter on the output for the overflow hose which the aftermarket one did not. Instead of replacing the radiator again with a stock one i bought a brass needle valve and installed it in the hose between the radiator and the reservoir. Tightened it down all the way then backed off about 1 turn issue was resolved for about $3.