CityFix

P0128 on Honda City GM2: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

Short answer: P0128 means “Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)”. The car’s computer has found a fault in the Engine system and turns on the Check engine light (MIL). First check: read the code and freeze-frame data with an OBD-II scanner.

Standard OBD-II code Same meaning on all cars (SAE J2012). Not checked against this car’s factory manual.

Code
P0128
System
Engine
Dashboard light
Check engine light (MIL)
Applies to
Generic OBD-II meaning. Not every code is used by this car.
Confidence
medium (generic OBD-II definition, not checked against the factory manual for this car)
Source
obd-codes.com

Symptoms and when it is set

  1. Check engine light on
  2. You will likely not notice any drivability problems other than the
  3. Illumination. In some cases you may experience symptoms such as
  4. Engine takes longer to warm up
  5. Engine temperature drops while driving at higher speeds
  6. A photo of a typical thermostat

Likely causes

  1. Low engine coolant level
  2. Leaking or stuck open thermostat (likely)
  3. Faulty cooling fan (running too much)
  4. Faulty
  5. Coolant temperature (ECT) sensor
  6. Faulty intake air temperature (IAT) sensor

What to check, in order

  1. Read the code and freeze-frame data with an OBD-II scanner
  2. Check the connector and wiring of the part named in the code
  3. Test the part with scanner live data or a multimeter, using the values in the repair manual
  4. Fix the cause, clear the code and test-drive

Related on this car

Same code on other cars

← P0125 P0130 →

Clearing the code does not fix the fault. If it comes back, the problem is still there. For brakes, airbags and steering, see a trained mechanic.