P0128
Engine coolant stays below thermostat temperature
Severity: MonitorEngine cooling (thermostat) codes
The thermostat keeps the engine cold-running time short by holding coolant in the engine until it warms up, then opening to keep it at a steady temperature. The engine computer watches how fast and how far the coolant warms up. If the engine takes too long to reach its normal temperature, it sets a thermostat code.
How these codes differ
- P0128 is the cooling system's warm-up test: coolant did not reach the expected temperature in the expected time, or dropped below it after warming up. That usually means a thermostat stuck open, but a coolant temperature sensor that reads low can also set it.
- P0128 is about running too cold. Overheating is a separate and more urgent condition; an engine that overheats should be stopped regardless of which code is stored.
Codes that show up together
- P0128 alongside a temperature gauge reading high, steam or an overheating warning does not fit a thermostat stuck open: treat it as overheating and stop, then have the cooling system checked.
Engine cooling (thermostat) codes
P0125
Coolant too cold for closed-loop fuel control
Severity: MonitorP0118
Engine coolant temperature sensor circuit high
Severity: Severity not verifiedP0217
The engine computer has detected an engine over-temperature condition
Severity: StopP1285
Cylinder head temperature above the engine computer’s limit (Ford, Lincoln, Ford-built Mazda)
Severity: StopP1299
Ford and Lincoln: cylinder head over-temperature protection is active (the engine computer's fail-safe cooling mode)
Severity: StopP2681
Engine coolant bypass valve A control circuit open or out of range
Severity: Inspect soonP26A3
Engine coolant bypass valve A range or performance problem
Severity: Inspect soonP26B7
Engine coolant bypass valve C control circuit open
Severity: Inspect soonP0481
Cooling fan 2 control circuit fault
Severity: Inspect soonP26CB
Electric engine coolant pump not performing as commanded, or stuck off
Severity: Inspect soonP0126
Engine not reaching a steady operating temperature
Severity: MonitorP0196
Engine oil temperature reading does not make sense
Severity: Inspect soonP0597
Thermostat heater circuit open
Severity: Inspect soonP2181
Cooling system not holding its expected temperature
Severity: Inspect soonP2600
Coolant pump control circuit open
Severity: Inspect soonP2682
Coolant bypass valve control circuit low
Severity: Inspect soonP01F0
Coolant temperature fell back below the temperature the computer needs for its diagnostics
Severity: MonitorP059F
Active grille air shutter system fault: the movable shutters behind the grille are not moving or reporting as expected
Severity: Inspect soonP2601
Electric coolant pump control circuit fault or poor pump performance
Severity: Inspect soonP0117
Engine coolant temperature sensor circuit reads too low (shorted toward ground)
Severity: MonitorP011A
The two engine coolant temperature sensors disagree with each other
Severity: MonitorP0480
Cooling fan 1 control circuit fault: the computer commanded the fan and did not see the expected response
Severity: Inspect soonP0598
Thermostat heater control circuit low: the computer sees too little voltage in the circuit of an electrically heated thermostat
Severity: Severity not verifiedP05A0
Active grille shutter stuck or out of position: the motorised shutters in front of the radiator are not reaching their commanded angle
Severity: MonitorP111A
On Mazda, the engine coolant temperature has gone above its limit
Severity: Inspect soonP26AB
Engine coolant bypass valve stuck: the valve that steers coolant to the heater core is not moving as commanded
Severity: MonitorP1217
Engine over-temperature condition on Nissan (a cooling system fault, not a transmission fault)
Severity: Severity not verifiedP26A5
Engine coolant bypass valve position sensor range or performance fault
Severity: Severity not verifiedP26A6
Engine coolant bypass valve A: the valve position sensor reads too low
Severity: Severity not verifiedP00B7
Coolant flow through the engine cooling system lower than the computer expects
Severity: Severity not verifiedP0115
The engine computer sees a signal from the coolant temperature sensor circuit that is outside the expected range
Severity: Inspect soonP0116
The engine coolant temperature reading does not make sense for how the engine is running
Severity: MonitorP0119
Engine coolant temperature sensor signal that drops out or jumps erratically
Severity: Severity not verifiedP0195
The engine computer found a fault in the engine oil temperature sensor circuit.
Severity: Severity not verifiedP0482
The control circuit for the third cooling fan has a fault
Severity: Severity not verifiedP0495
The cooling fan is running faster than the computer expects
Severity: Inspect soonP0599
Thermostat heater control circuit voltage too high
Severity: Severity not verifiedP2560
The computer decided the engine coolant level is low
Severity: Severity not verifiedP2602
Coolant pump control circuit low voltage
Severity: Inspect soonP261F
Coolant pump A control circuit stuck on
Severity: Inspect soon
Sources and review
- Final Regulation Order: 13 CCR §1968.2, Malfunction and Diagnostic System Requirements (OBD II), 2004 and subsequent model-year passenger cars, light-duty trucks and medium-duty vehicles (2015 amendments)
- 2023 Integra Owner's Manual: How to Handle Overheating
- Avoiding Hot Water With Cooling System DTCs
- Understanding Engine Thermostats: How They Work and Why They Fail
- Owner's Manual (online): Signs your engine is overheating
- Last reviewed
- Written by
- Saurabh K.
- Editorial review
- Pending
- Technician review
- Technician review pending
This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error