P0128 on a 2022 Ram 1500 Classic Regular Cab

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 2022 Ram 1500 Classic Regular Cab

Ranked from 588 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Low coolant level from a heater core leak
Medium severity
Multiple owners had a bad heater core confirmed by pressure test, which reduces system coolant volume and can slow reaching the thermostat's regulating temperature.
Reported in 3 NHTSA complaints
Parts:heater core
2
Stuck-open thermostat tied to an open manufacturer cooling-system recall
High severity
One owner received notice of NHTSA Campaign 25V720000, an 'Engine and Engine Cooling' recall, with the repair part still unavailable — consistent with a known thermostat/cooling defect on this model.
Reported in 1 NHTSA complaint
Parts:thermostat
3
Coolant contaminated with casting sand/debris from manufacturing degrading the thermostat
Medium severity
One owner's mechanic found dirt/casting-sand contamination in the coolant and cited TSBs describing this contaminating engine and cooling components, including the thermostat, on this platform.
Reported in 1 NHTSA complaint
4
Failed engine coolant temperature (ECT) sensor
Medium severity
A drifting ECT sensor can report a temperature below the thermostat's regulating point even when the coolant is properly warmed.
Common cause
Parts:coolant temperature sensor
5
Cooling fan stuck running
Low severity
A fan that won't shut off over-cools the radiator and delays the coolant reaching its target temperature.
Common cause
Parts:cooling fan relay
6
Extended idling or short-trip driving in cold weather
Low severity
Low load and cold ambient temperatures alone can keep coolant below the regulating threshold long enough to set the code.
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.