Honda Civic Temp gauge fluctuating, not heating well

[Kurt Johnson]

2005 Honda Civic 190,000 miles. The car does not heat up fast like it used to. Thermostat has been replaced. On a couple recent occasions, the temp gauge fluctuated between normal, fully hot and cold (up and down and back up and down, etc.) over the course of a few minutes. Car seems to run fine. The repair shop has not been able to replicate the problem and initially seemed stumped, and now says it’s probably the head gasket. They have said the coolant remains pressurized hours after turning engine off. Before I agree to the expensive repair, what should I make sure has been checked? Thanks.

Topic

[Richard Kirshy]

First I would remove and test the thermostat to make sure it is operating correctly.
Next I would inspect and test the temp sending unit, also the connections.
Then vacuum fill the cooling system to ensure there is no air entrapment.
Then test the system.
Measure the coolant temp not by the dashboard gauge.
Might look at something like this:
https://images-na.ssl-images-amazon.com/images/I/41ELTBwl1IL._AC_SY400_.jpg

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