Z06ster

I am posting this primarily as a public service... I recently had a mystery solved and wanted to share it for the benefit of others who happen to need this information...

This post pertains to you if all four items are satisfied:

1) Your Check Engine Light comes on
2) You observe P0128 code in the DIC and no other codes present
3) Your car is running normally, not overheating, and coolant temperature gage is behaving normally.
4) You have installed mod's on the car for cooling such as a DeWitts radiator and/or lower temperature thermostat

Problem Summary:
Check Engine light comes on and P0128 code is thrown

Issue:
The ECU is programmed such that it expects to see 180 or 185 degree temperatures after some time frame of operation (guessing 8-10 minutes but this is not known). If the car does not reach the pre-determined temperature within the pre-determined timeframe, it will throw the P0128 code.

In my particular case, I had a DeWitts radiator installed about 8 months ago. I also have a 170 thermat. I had been driving the car without the appearance of this code and check engine light. Earlier this week, it was a cold Southern California morning and I took the car out. My wife was with me and turned on the heat in the car.... I never turn the heat on in the car. It was shortly after the heater got turned on, the Check Engine Light came on. The use of the heater core obviously prolonged the warm up and hence the Check Engine Light came on and the code got thrown. Reseting the DIC removes the code and check engine light... until this happens again of course, but at least I know what is going on now. If desired, I could use HPTuners to not throw the check engine light.

For the record, I can't take credit for this find... When this first occurred, thinking there could be a potential reliability issue that should be explored, I took the car down to A&A Corvette which is not far from me. Jesse at A&A put on a Tech II scanner and then revealed this all to me. Best of all and as a testament to the superior customer service one gets at A&A, they didn't charge me for this assessment.

m6 c5

Yea I have the same problem as my thermostat never fully blocks off the flow of water. I always leave the ac on auto so the eat turns on as needed and it will still only come on every once in a while. It usually tends to come between 13 and 15 min after I start the car.

Fratbragen

I'd like to say a big thank you for posting this information as I believe I experieced this today. I have a 180 Tstat hooked onto an electric water pump. My temps on normal days are right in the 180-185 range when fully warmed up. However, today I was up on the mountains and it was pretty cool. Had a long down hill run and I believe, when left in gear to cruise down a hill, the fuel is shut off and air is pumped through the engine which really cools down water temps in a hurry. It was after about 10 min. or so the Check Engine light came on. I had some codes that showed up, all were historical, but this one really makes sense. We'll see if the check engine light comes back on. Thanks again.

checkoutmyjunk

Thanks for the write up, I started a thread about this back in march (after you posted this) My question is, what is the fix? I have c5z with a low temp t-stat and now that it's cooling off here in central PA I am seeing this code again. Cool nights in the spring and my CEL came on with code P0128, never got another code all summer then the 1st cool night I dove in the fall and I get the same code. I don't want to change my thermostat ever fall and spring, there has to be a way around this and not effect emissions testing.

m6 c5

You can either get the code tuned out or put in a different T stat

checkoutmyjunk

I was just coming back to post that I found the answer, contacted a local tuner to help me remove the code. Thanks for the response.