P0128 Code

Richg181 · Post by **Richg181** » 02 Jun 2009, 19:39

My 2002 S40 repeatedly gives me a P0128 code: "Coolant temperature below thermostat rating." (This code shows up twice on my scan tool). Does this mean the thermostat or the sensor or both are bad? Thanks for any info you can provide.

chengny · Post by **chengny** » 03 Jun 2009, 02:42

Unless you want to set up a laboratory and are familiar with RTD's (remote temperature detectors) and how they change their internal resistance value with temp variations - there is no way to determine this easily.

You can't even easily identify the thermostat as the culprit anymore either. It used to be that a person could drop the questionable t-stat into a pot of boiling water; if it opened - it was good.

I believe, from what I have read, that the coolant in a Volvo engine must be @ 197 F (+/- 2 degrees) and it must reach and settle out at this value within 20 minutes or the "computer" will throw a P0128. Try duplicating that with a pot of boiling water. (Don't quote me on these parameters) . So jumping the switch is out of the question.

If the sensor is definitely defective, a scan will reveal a P0115.

But, if you think about it - P0128 is a chicken/egg scenario.

Is the t-stat (from which the ECM - or whatever they call it- gets absolutely no input) bad and the temperature sensor is reporting this malfuntion as designed? Or is the t-stat opening and closing as designed (and maintaining the proper coolant temp) while the sensor sends incorrect voltage to the computer?

See what I mean? BTW - I am doing this repair myself as we speak - to clear an 0128. I have both parts on hand, and since the t-stat costs roughly 1/4 the sensor (and I am so chuckin feap) , I will change that component first. If that doesn't clear the code, I will then pull that good t-stat and replace the sender, put the t-stat back in its box and get my $ back for it.

I suppose its possible to have both components fail at the same time but I am not scanning a P0115.

PS - While you have the t-stat out, pick up some super-flush, an in-line kit and flush out your coolant system. Nobody ever touches on this subject but, one can affect a much more thorough system flush with the thermostat is removed.