P0128 on a 2020 Toyota Rav4

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 2020 Toyota Rav4

Ranked from 627 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Coolant bypass valve / integrated electronic thermostat failure
High severity
This batch is dominated by the well-documented 2019-2023 RAV4/RAV4 Hybrid coolant bypass valve defect (Toyota Customer Support Program, TSB 24TE04): the valve sticks or leaks, the engine can't reach/hold operating temperature, and the dash shows 'Engine Maintenance Required' with the bypass-valve circuit code alongside P0128.
Reported in 15 NHTSA complaints
Parts:coolant bypass valve thermostat
2
Water pump failure / coolant leak reducing flow
Medium severity
At least one report in this batch describes a water pump failing outright alongside the bypass valve issue, which will also starve the engine of the flow needed to reach operating temperature.
Reported in 1 NHTSA complaint
Parts:water pump
3
Stuck-open thermostat
Medium severity
A thermostat stuck open (or opening too early) lets coolant circulate through the radiator before the engine reaches operating temperature, keeping ECT readings below the regulating threshold.
Common cause
Parts:thermostat
4
Failed or inaccurate ECT (engine coolant temperature) sensor
Medium severity
A drifting or failed sensor can under-report actual coolant temperature, tricking the PCM into thinking the engine is running cool when it isn't.
Common cause
Parts:engine coolant temperature sensor
5
Low coolant level
Medium severity
Insufficient coolant volume (from an unrelated leak) reduces the system's ability to reach and hold normal operating temperature, especially in cold weather.
Common cause
6
Cooling fan stuck on / running constantly
Low severity
A fan that won't shut off (bad relay, stuck relay, or wiring fault) continuously pulls heat out of the radiator, preventing the engine from reaching temperature.
Common cause
Parts:cooling fan relay
7
Extended cold-weather/short-trip driving
Low severity
Short trips in cold ambient temperatures may not give the engine enough time/load to reach the thermostat's regulating temperature even with healthy components.
Common cause
8
Incorrect-temperature-rated thermostat installed at a prior service
Low severity
A replacement thermostat with a lower opening temperature than OEM spec will open early and hold the engine below the PCM's expected regulating temperature.
Common cause
Parts:thermostat

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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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