My SES light came on today and I had the code read.. P0128.. From the research I have done it means coolant temp below thermostat requirements... What should I try before replacing the thermostat.. I will mention that I have noticed the past week or so it has taken longer to heat the cab of the vehicle than it usually does. Something else to mention is I added some coolant last night because it was a bit low. My temp gauge never goes over normal and the coolant level for the most part holds..

I just went through the same thing with my daughter's S10.
I reset the DTC & drove it around a bit and it didn't come back but the coolant temp didn't show the proper 190-210F reading on the temperature gauge. The heater didn't work good either until I replaced the thermostat. Now the temperature gauge shows 190F and the heater works very well.

I agree with Chris.
Replace the thermostat. They're not that expensive, and they're easy to change.
I change mine every 4 years when I replace the coolant.

agree with them. thermostat is faulty. replace and double check for leaks since you are losing a bit here and there.

Replaced the thermostat today.. How long before the SES light goes away? Do I have to have it cleared.. It's a 2002 Blazer

Replaced the thermostat today.. How long before the SES light goes away? Do I have to have it cleared.. It's a 2002 Blazer


I think it requires a few driving cycles (on/off) before it'll go off.
If it doesn't go off soon, you could always disconnect the battery for 10 minutes. That should reset it.

I bought an inexpensive ODBII reader at Advance auto parts that reads and erases codes.
Spent less than $50.00 for it. It's paid for itself several times, as my local mechanic charges $90.00 diagnostic time. Had to replace a $90.00 heated oxygen sensor on my daughter's car. The same bill at the mechanic would've been in excess of $300.00.
:iceslolan

Thanks for the advice... I will have to try that if the light doesn't go off soon.. Thanks

The light went out today.. Thank goodness.. I hate that damn SES light.. Thanks to everybody who helped...

When I changed the thermostat I also replaced the Coolant and the stock Radiator Cap.. I purchased a stant cap that I believe was 16 PSI..

Any other suggestions..