P0128 indicates a problem with your Chevy Malibu's cooling system. Sometimes the problem is as simple as low coolant level. More than often however it's a defective engine coolant temperature (ECT) sensor. We'd recommend checking your Chevy's coolant level first. If the level appears good and your Chevy Malibu does not have an overheating problem we can rule out the cooling system, including the thermostat. You should have an expert verify the coolant system is functioning properly and your Malibu runs cool. You do not want to overheat the engine.
Move on to checking the ECT. At normal operating engine temperature the ECT voltage should read between 0.8 – 1.2 volts. If defective, replace.
It may be possible that P0420 and P0128 are related. If due to a defective ECT your Malibu's engine control system has been unable to regulate proper oxygen sensor voltage switching (unable to maintain the slightly rich then slightly lean air/fuel ratio) the catalytic converter may not operate as expected; thus setting a P0420 (CAT low efficiency code).
Our recommendation would be to repair the P0128 fault, reset the computer, then drive at least 200 miles over the course of a week to verify trouble code P0420.
