P0128 on a 2021 Honda Cr V Hybrid

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 Hybrid

Ranked from 410 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Chronic low engine operating temperature (Honda TSB 21-013 / TSB 22-087)
Medium severity
A factory-documented issue on this hybrid where frequent engine-off/EV-assist operation keeps the engine from reaching normal operating temperature, leading to oil dilution/sludge and a low-coolant-temperature code.
Reported in 1 NHTSA complaint
Parts:thermostat
2
Failed engine coolant temperature (ECT) sensor
Medium severity
One vehicle's diagnosis explicitly included a failed coolant sensor, a direct and common cause of a false low-temperature reading.
Reported in 1 NHTSA complaint
Parts:coolant temperature sensor
3
Stuck-open thermostat
Medium severity
The classic mechanical cause of coolant never reaching regulating temperature.
Common cause
Parts:thermostat
4
Low coolant level from an undetected leak
Medium severity
Reduced coolant volume slows warm-up and can prevent the sensor from reaching full operating temperature.
Common cause
5
Cooling fan stuck running
Low severity
A fan that won't shut off keeps the radiator over-cooled, holding coolant temperature below the thermostat's target.
Common cause
Parts:cooling fan relay
6
Frequent short-trip/stop-start hybrid operation preventing warm-up
Low severity
This hybrid's engine-off EV periods can compound normal short-trip driving, keeping the engine from ever reaching full operating temperature even with healthy hardware.
Common cause

Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.

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.

Rather have a pro handle it? Find a vetted local mechanic.