In the world of generic OBD-II fault code diagnosis a p0128 is generally engine runs too cold due to a bad thermostat. On this 2006 Honda Civic EX with 1.8 it turns out to be the sensor in the bottom of the radiator. When the sensor glitches it causes the computer to think the engine is running cold causing a rich fuel mixture and running problems. Repair or replace the connector if you see corrosion. Once the sensor has been replaced fill the coolant and check for leaks. Reset the malfunction indicator lamp with an OBD-II compatible scan tool.
If the fault persists then you may need to have the car’s computer software updated by the dealer. See Honda technical service bulletin 09-092.
Click here to read the TSB in PDF format.
Auto parts stores do not carry the correct sensor so you have to buy it at the Honda dealer or get it on Amazon.