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#1253873 - 10/09/08 04:34 PM Code PO128- Thermostat??
exranger06 Offline

Registered: 01/25/08
Posts: 2987
Loc: Guilford, CT
My sister has a 2001 Saturn L200. The check engine light recently started coming on, so she takes it to Autozone and has them scan for codes. They came up with code PO128, "Engine coolant temp below thermostat temp."

I'm probably gonna replace the thermostat regardless, as I feel it's be a good maintenance thing to do. The only other thing I could possibly think of is maybe the coolant temp sensor. She said the fans were turning on even when it was really cold outside, which I think is probably normal, but it's also possible.

Anyone do a thermostat on one of these before? I know thermostats are generally very easy but I've never done one before, on any car. Any advice/insights/suggestions are very much appreciated.

Also, the thermostat located right where the upper radiator hose connects to the engine right? I know on most cars it is but this Saturn has some strange design elements from what I've seen so far.

Edited by exranger06 (10/09/08 04:43 PM)
2006 Ford Ranger Sport
1992 Honda Accord EX
1994 Ford Bronco XLT (project)
2014 Subaru XV Crosstrek

#1253892 - 10/09/08 05:05 PM Re: Code PO128- Thermostat?? [Re: exranger06]
03cvpi Offline

Registered: 03/04/07
Posts: 145
Loc: Minneapolis
Fairly easy. Mouted on the front of the engine towards the pass side of the car. Don't loose the cardboard tube. The thermostat is pushed in and quarter turn to lock. If you have a mech mirror it's pretty easy to see. Most replacement t-stats come with the tube, if not a needle nose pliers will work, but on install be careful not to crush the t-stat.

#1254329 - 10/10/08 09:46 AM Re: Code PO128- Thermostat?? [Re: 03cvpi]
eljefino Online   content

Registered: 06/15/03
Posts: 27311
Loc: ME
^ That sounds right for a 1.9, is it the same for a 2.2 ecotec or 3 liter V6?

The fans will run when the car is cold if the AC is on, even as part of the defroster. If there's a CTS fault forcing the fans on there'll be a code for that too.

Diagnosing this much farther will probably take more time than the thermostat job. My experience with GM's is they usually fail cold, which is a good failsafe. \:\!

#1254527 - 10/10/08 02:42 PM Re: Code PO128- Thermostat?? [Re: eljefino]
mechtech2 Offline

Registered: 09/05/06
Posts: 19479
Loc: Chicago Area
Make sure the mating surfaces are clean and corrosson free.
I am a fan of using a LIGHT coat of silicone RTV on gaskets and 'O' rings - no comebacks.
You may have a sensor problem. If the thermostat does not fix the problem, look there.