2006 Mustang V6 and wierd 'Check Engine' code

Lee Yoder · 12-21-2008

Good Evening All,
I live in South Texas and we have had some chilly (for us) weather lately. Last week, I am driving to work, and have been on the road for about twenty minutes in cold (38 degrees) weather (with the heater running) when the check engine light comes on. I run a diagnostic and all is well and the car is running fine. I go to Autozone to have the code read and it comes up as P0128 (Coolant Thermostat - Coolant temperature below thermostat regulating temperature)

. What is wierd is that the heater was running well, so the coolant temp was up. And the light came on again today in exactly the same driving situation. Am I going to have to replace the Thermostat and has anyone else had this problem?

Thanks,

Spencer88 · 12-22-2008

I would say just replace the Thermostat.. My friends did this and he just replaced it and the light went away.. and yes I'm freezing my butt off its 28* here.

**stlwagon** · 12-22-2008

This diagram should help.

wnewport · 12-22-2008

I run a 180 degree thermostat (stock is around 197.) I got the same code several times when it was cold out. I ended up replacing the thermostat and it hasn't come back yet. In my case I think the setpoint for low coolant temperature is closer to the thermostat setting. When I would downshift and accelerate hard on a cold day after normal driving, the code would come in- like maybe a slug of colder water came through from the radiator.

It could be a faulty sensor, bad thermostat or maybe your fan is coming on when it isn't supposed to. Thermostat is the easiest and cheapest.

**stlwagon** · 12-22-2008

I'd start with the t-stat. If that doesn't do it, it could be the sensor.

Lee Yoder · 12-22-2008

Thanks all, especially Stlwagon, you sir, are a steely eyed missile-man!