Help diag. trouble code P0128, tried the obvious already

[boostd4]

Hi,

I'm a long time DSMr... had 4 or 5 of them about 10 years ago and moved on to Toyotas. I have the opportunity to pick up a 1999 GSX black on black leather that I know has been taken care of. I can get it on the cheap because it won't pass emissions because of a CEL. Owner doesn't want to deal with it anymore.

Anyway, it's throwing a P0128 code - "Coolant Thermostat below expected temperature".

So far, I've replaced the Thermostat and ECT sensor, and the code still comes back.

I have a strategy, but I wanted to ask if anyone out here has come across this, or if there is a common fix.

It's strange that it's a "too low" temp code. The car has 80k on it and has had the timing belt replaced, but not the water pump. I would assume a bad waterpump would overheat instead of underheat...

I'm also assuming that air pockets in the coolant system would create an overheating instead of underheating situation, so that was ruled out.

The car drives fine, doesn't have any problems, and the temp needle stays where it should on the dash.

What I'm going to try:

Flush/burp coolant
Backprobe the ECT sensor and compare to voltage chart
Backprobe the ECU at the sensor input to make sure the voltage is making it to the ECU

If the voltage is in the operating temp range at the ECU, does that mean it's just a bad ECU?

Any help would be greatly appreciated. If I can fix this issue, I'll basically be able to trade my crappy civic hatch for this car, straight up.

 

[boostd4]

I should add that the previous owner said the light came on after he installed his HKS carbon Ti exhaust, and that he had an exhaust leak (stripped stud at the front of the cat).

I did some searching at other forums and found that someone had a similar code P0125 (low temp or something), and that replacing the O2 fixed it.

I'm curious how an O2 sensor could have anything to do with a Thermostat temp too low code. Perhaps the ECU checks the rear O2 sensor to see when the cat is heated up? I'm a little lost.

 

[ktmrider]

I think your to-do list looks good, especially the check of the sensor.
One note, my 1g has 3 different sensors on the t-housing. One for the ECU, one for the fans, and one for the temp gauge in the cluster. Myself not being familiar with the 2g, are you sure you swapped out the appropriate sensor?
The ECU sensor on my 1g is on the top/above the t-stat with a large 2 pin connector.

 

[boostd4]

I didn't do it, the PO did and I'm taking his word for it. Hopefully backprobing them will sort things out.

I'm between a rock and a hard place right now. If I fix his problem, he'll jack the price or want to keep the car. I was checking the forums to see if it were something common.

I suppose if I test drive it and the tranny doesn't have any grinds, and the compression/leakdown is good, I'll buy it broken. It's not too serious of a code. I think I can get the car for $7500.

1999 Eclipse GSX
Flawless paint, no dings, dents or rust.
80k miles, 2 owners (the most recent I know personally)
Black on black leather interior (rare)
Only mods are K&N FIPK and HKS Carbon Ti exhaust... still on stock clutch so it couldn't have been beat up too badly.

Is this a good deal? I know it needs this code fixed and it needs new shocks all around... what should I offer him?

 

[ktmrider]

Oops, the VFAQ lists the lower 2 pin connection sensor for the ECU and the top one for the fans. The third is a one pin for the temp gauge. I would definitely check it out at Welcome! in the Misc section.
I am not familiar with the late model 2g car values, maybe someone else can chime in?!?