what's a rpm p0128 reading

P0128 is a coolant temp. fault code. Computer sees data from a sensor on engine that its monitors the tempuature of engine. In your case the computer thinks the engine is not maintaining the temp. high enough for clean burning of the fuel. Many times its simply a coolant thermostat that is likley stuck open and engine temp. is lower than normal. Can effect the ammount of heat from heater and another symptom. Also could be a problem with that sensor or its wiring circuit. If your temp from heater has had the symtom of taking longer to heat up or if it has a dash guage that indicates temp. is lower that it used to be, good chance that a new thermostat will fix your problem.

you have a thermostat issue or coolant temp sensor issue,have it diag by your mech