2004 Blazer with SES light and read-out code of PO 128

SOURCE: I am getting codes PO 128, PO 300 and PO 506 on my

code P0128 ECT ( electronic coolant temperature ) engine temperature to low ( replace thermostat ).

code P0300 engine miss fire detected ( replace spark plugs and spark plug wires )

code P0506 IAC ( idle air control ) system idle to low ( clean the IAC valve if problem persists replace the IAC valve )

SOURCE: 2006 trailblazer po 411 code check engine light comes on and off over time is it a sensor or the air pump?

Check all your vacume lines one may have a crack in it

SOURCE: The thermostat does not work and the fan stays on

YOU FAULTY COOLANT SENSOR OR PCM.

SOURCE: I HAVE A 2007 CHEVY COBALT THE CHECK ENGINE LIGHT

I had this problem with a 2007 Chevy Cobalt. I changed the coolant sensor and even though the check engine light went out I still had a reading of "---" on the coolant gauge. The problem is most likely one of three things: a low coolant level, a bad coolant sensor, or the thermostat needs to be replaced. The only other possible thing it could be is a bad BCM (Body Control Module) which in that case you would most likely be having other weird problems.

The Thermostat and the sensor are both very easy to get to and replace. Take off the engine cover by removing the oil cap and they are both to the rear of the engine on the driver's side. They are right next to each other.

Hope this helps!

SOURCE: LOW COOLANT LIGHT COMES ON SOMETIMES AND TEMP

There may be a thermostat problem, but your symtoms indicate low coolant level. When coolant is too low, it will not circulate, so you get no heat in car, and the coolant remaining in engine block overheats because no cool fluid coming in.
You need to fill the system up properly then look for leaks.
With cold engine, open radiator cap. Fill with antifreeze directly in radiator. Start engine, top up rad if level drops. Put heat control on Hot. Fan on low, heat on high. Gently rev engine and top up rad as needed. IF as engine heats up, it starts kicking fuild violently out of radiator, then the thermostat is bad. if not, then recap radiator, top up overflow bottle and you're done. Look under car for any sign of leaks. Steady dripping indicates a leak, then address that.