A P0128 Coolant Temperature Below Thermostat Regulating Temperature code is something we see on a fairly regular basis here at All Tune and Lube Tempe. This and a P0125 is saying that your engines’ computer believes that the coolant isn’t getting up to the temperature required for your engine to run most efficiently within a certain time. As a result the computer will stay in “Open Loop”. This means that your car is running rich (too much fuel). That makes for poor fuel economy and performance and excessive vehicle emissions. Over time this could damage your catalytic converter and will wear your spark plugs out faster.
Usually this code means that you’ll need a new thermostat. That’s the device that regulates the coolant moving through your engine. If the thermostat is stuck slightly open then the engine isn’t able to warm up in the time the computer thinks it should.
I said “usually means” the thermostat but it could be a couple other things too. First check the coolant level. If there isn’t enough coolant in the engine then the engine coolant temperature sensor (ECT) can be in an air pocket and not getting proper readings. Next it could be a bad ECT. To check this item you need to have a scanner that can see the signal the ECT is sending to the computer. Sometimes the engines’ cooling fans are on all the time, keeping the coolant too cold. In some rare cases a P0128 can be caused by a bad gasket at the thermostat housing that’s allowing coolant to bypass the thermostat.
Whatever it is this code is typically simple to diagnose and not frighteningly expensive to repair. So bring it on in to All Tune and Lube Total Car Care Tempe. Complete auto repair and maintenance!