Here we are at All Tune and Lube Total Car Care with another in our semi-informative series of discussions about CEL codes! Yeah!
Today we’ll be talking about one we see regularly, the P0128 Coolant Temperature Below Regulating Threshold. Doesn’t even sound scary does it? I mean, how can you worry about your engine running too cool? Heck, the problem is when your engine gets too hot isn’t it? Well, yeah, kinda. Overheating an engine is mechanical DEATH! But when an engine runs too cool it presents its’ own set of problems.
What this code is saying is that the computer that controls your engine (the PCM) believes that the coolant temperature isn’t reaching its’ proper level in the amount of time it should, or not getting to that range at all. When an engine is cold it needs a little extra fuel in order to run smoothly so the PCM adjust the fuel trims accordingly. When there is more fuel being supplied to the engine than it optimally needs we call that a rich condition or “running rich.” If your engine is running rich you lose fuel economy, increase exhaust emissions, wear or foul your spark plugs more quickly and accelerate wear of your CATALYTIC CONVERTER! It’s not uncommon for us to see a P0420 cat code within a few months of having repaired a P0128 fault if the cars’ owner let that P0128 go too long.
Usually this code indicates a thermostat that’s suck partially open, but not always. Remember I said that the PCM believes the coolant isn’t warm enough? It gets this information from a device called an Engine Coolant Temperature Sensor, or ECT for those of us who are hip and inside to the lingo. If the ECT is failing it could be sending incorrect information to your ECM like a dam lying liar. Sometimes, not often, it can be a problem with the Intake Air Temperature Sensor (IAT) since the PCM uses a comparison between the ECT and IAT readings. Also, rarely, the cooling fans could be staying on all the time preventing the engine from warming as it should. I’ve heard of instances where incorrect coolant or coolant mix has caused this code but I’ve never seen it.
So there you go. That’s what a P0128 is all about. Simple problem, usually simple fix.
From you Local Mechanic, All Tune and Lube Total Car Care Tempe. Complete Auto Repair and Maintenance.