CEL code P0128

[jl_1978]

Yesterday, my 2004 Mazda3 CEL came on. I pulled the code at Part Source and was told the code was P0128 (Coolant Temp Below Thermostat Regulating Temp).

I saw there was a service bulletin to update the PCM calibration, but would that likely not be the problem since it's pretty warm outside? The car seems to be driving fine and the temperature guage reaches about half after a few minutes of driving. Mazda wants $115 just to diagnose the problem, so I'm not sure if it's worth it or not.

[TheMAN]

it can happen no matter the temperature
if you can talk them into skipping the diagnostic and just pay them to do the flash, that's what I'd do

that being said though, it's still possible the thermostat is bad... the original one is clamped down together by plastic... it can break... the updated one from mazda is now better constructed and is held together by metal (the housing is still plastic)
so if you're replacing the thermostat, buy only the mazda one... the aftermarket ones are still the plastic junk!

if your engine is slow to warm up, then it's definitely a bad thermostat

[jl_1978]

The temperature gauge reaches almost half way within a couple minutes of driving, which seems pretty normal in this weather. To be honest, I never really paid attention to how quickly it rose before though. I'll see if I can pay them to just do the flash, and if the light is still on, I'll replace the thermometer.

[gabbygenier]

change your thermostat. problem solved.

ive had this issue a while back and if you search, a few other members have had it.

all seems good now but wait till winter and you'll have NO heat inside the car.

got mine for around 70$ at a mazda dealer. reason its a bit more then most cars is cause mazda decided to implement the housing into it and make it one whole unit

[Flagrum_3]

change your thermostat. problem solved.

ive had this issue a while back and if you search, a few other members have had it.

all seems good now but wait till winter and you'll have NO heat inside the car.

got mine for around 70$ at a mazda dealer. reason its a bit more then most cars is cause mazda decided to implement the housing into it and make it one whole unit

+1, and I would suggest having your system flushed and FRESH coolant installed, if it has not already been done.A premature thermostat failure can well be an early sign of bad coolant and of early coolant system corrosion.

_3

[jl_1978]

Thanks for the tips. Does anyone know around how much I should be expecting to pay to flash the PCM calibration?

[rage2021]

quick question when i purchase the thermostat its just the one part im getting right, i dont need a seperate housing/gasket for it? im going to try and get it swappd out soon.