P0128 on a 2021 Honda Cr V

P0128 means the engine warmed up slower than expected or stayed cooler than the computer expected.

Not your vehicle? See general P0128 info.

Most common causes on the 2021 Honda Cr V

Ranked from 311 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Head gasket failure causing coolant loss/intrusion (documented CR-V 1.5T defect, NHTSA PE25016 / Bissell v. Honda litigation)
High severity
Numerous owners of this exact vehicle report a diagnosed failed head gasket letting coolant escape into the cylinders, reducing effective coolant volume and thermal management.
Reported in 7 NHTSA complaints
Parts:head gasket
2
Chronically low engine operating temperature / oil dilution (Honda TSB 21-013 / 22-087)
High severity
One owner's complaint cites official Honda technical service bulletins for chronically low operating temperatures and sludge on this generation of hybrid CR-V, a direct match to this code's failure mode.
Reported in 1 NHTSA complaint
3
Stuck-open thermostat
High severity
The most common general cause of this code; the engine can't reach or hold its regulating temperature.
Common cause
Parts:thermostat
4
Failed engine coolant temperature (ECT) sensor
Medium severity
A sensor reading falsely low reports the engine as never reaching operating temperature even though it has.
Common cause
Parts:engine coolant temperature sensor
5
Low coolant from an external leak
Medium severity
A leaking hose, radiator, or gasket reduces coolant volume, slowing warm-up and holding the reading below threshold.
Common cause
Parts:radiator hose
6
Cooling fan stuck running
Low severity
A fan that won't shut off continuously cools the engine, keeping coolant temperature below the regulating threshold.
Common cause
Parts:cooling fan

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These causes are aggregated from NHTSA owner complaints and general repair knowledge — this is not a diagnosis. P0128 on your car still needs shop confirmation before any repair.

Common symptoms

Check engine light
Weak cabin heat
Poor fuel economy
Temperature gauge low
Long warm-up time

What to check before replacing parts

Check coolant level when the engine is cold

Look for: Low coolant reduces the system's ability to reach and hold operating temperature — top off and recheck before assuming the thermostat or sensor is at fault.

Compare scan-tool coolant temperature to actual warm-up time

Look for: An engine that takes noticeably longer than normal to reach operating temperature (or never gets there) on the scan tool, with normal coolant level, points to the thermostat rather than the sensor.

Inspect thermostat operation

Look for: A thermostat stuck open (or opening at the wrong temperature) lets coolant circulate through the radiator too early, keeping the engine running cooler than it should — this is the most common cause of this code.

Check cooling fan command

Look for: A fan that runs constantly or turns on too early (commanded on when temperature is still low) keeps the engine from warming up normally, mimicking a stuck-open thermostat.

Inspect coolant temperature sensor data

Look for: A sensor reading that doesn't rise smoothly with actual engine warm-up, or that reads implausibly low compared to how the engine feels/behaves, points to a sensor or wiring fault rather than a true cooling problem.

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