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Replaced thermostat-no heat

I replaced the thermostat in my 08 Mazda and now the heater doesn’t work. Any tips for troubleshooting the problem?

The heater worked before ? or are you attempting to fix a heater problem by replacing the tstat ?

The heater worked before. I had a check engine code which indicated faulty thermostat.

You may need to air-bleed the cooling system. An air bubble in the heater core can cause insufficient coolant flow. With my Supra, the best way is to use ramps and a hill, get the front end as high as possible, and run the engine with rad cap off to push air bubbles out.

Thanks for the tip!

A Code For A Faulty Thermostat ?
Exactly Which Code Is That ?


Newer cars have radiator lower than thermostat so there usually is a bleed screw on thermostat housing. U need to remove trapped air or car could overheat due to poor coolant flow. My dodge thru a code for longer than expected time to warm up. I had done something involving cooling system and got the code. Cars are smart.

I believe your system also has a valve in the intake line to the heater core. You need to be sure that the heater controls are “on” and the key is ON when you bleed the system of air. The valve needs to be open.

@commonsenseanswer it’s usually P0125
I believe all OBD2 vehicles can throw this code.
While the code doesn’t specifically state that the thermostat is faulty, the code sets because the coolant temperature didn’t rise sufficiently with a specified amount of time.

In other words . . . your thermostat is old, and quite often stuck open. The engine will take forever to reach operating temperature. In severe cases, it will never reach operating temperature and will not enter closed loop.

I’ve replaced plenty of thermostats for this very code.

“ECT excess time to closed loop fuel control”

This is from a GM service manual.

Or P0128 Coolant thermostat (Engine temperature stays cold too long). This is the one I usually see.

@Nevada545 yes, I’ve seen that one also.

I don’t recall the exact number but it was the coolant thermostat definition. Thanks for the tips. Hadn’t been this warm in my car in a very long time! Just like babies. …now I know you have to burp them!