Diesel Trouble Codes

P0128 — Coolant Thermostat Below Regulating Temperature (6.7 Cummins (SCR / DEF))

Engine coolant does not reach the expected operating temperature in the expected time on 6.7L Cummins SCR/DEF 2013–present.

Updated September 2, 2026

What it means

The PCM monitors engine coolant temperature rise and steady-state temperature. P0128 sets when coolant stays below the calibrated regulating temperature for a calibrated time/distance under normal load and ambient conditions. On this platform the thermostat is a conventional wax-element mechanical unit and the engine uses a mechanical thermostat, gear-driven timing, CP3 (2013–2018, 2021+) / CP4.2 (2019–2020), in-tank lift pump, VGT with electric actuator, MAP-based fueling, grid heater, DOC/DPF/SCR/DEF.12

Most likely causes

  1. Thermostat stuck open / failing to regulate — A mechanical wax thermostat that remains open prevents the engine from reaching normal operating temperature, causing prolonged warm-up and P0128. Coolant degradation/sediment can contribute to thermostat and heater-core failures on this vintage.13
  2. Coolant temperature sensor drift / inaccurate reading — Erroneous low coolant-temperature signal can make the PCM think the engine is not reaching regulating temperature even if the thermostat is functional.1
  3. Low coolant level / air pocket in cooling system — Insufficient coolant or trapped air reduces heat transfer and delays warm-up, leading to below-regulating temperature detection.1
  4. Excessive cooling from high airflow / winter front missing — Unrestricted airflow through the radiator in cold ambient conditions can keep coolant below target during normal driving.1
  5. PCM calibration / false detection after recent cooling-system service — Recent coolant flush or component replacement can introduce air or change warm-up characteristics, causing a temporary P0128 until the system stabilizes.3
  6. Coolant contamination / degraded coolant causing flow restriction — Degraded coolant can form sediment that plugs heater core/EGR cooler and affects thermostat operation on 2013+ Rams where coolant formulation changed.3

How to diagnose

  1. Verify freeze-frame and live data: confirm coolant temperature rise rate and steady-state temp vs. ambient and load.
  2. Inspect coolant level, condition, and for contamination/sediment; check for proper fill and bleed of air pockets.
  3. Command a controlled warm-up drive and log ECT vs. time; compare to expected regulating temperature for the platform.
  4. Check thermostat operation by monitoring ECT behavior at idle and under load; a thermostat stuck open typically shows ECT well below normal and slow rise.
  5. Test coolant temperature sensor accuracy against a known-good reference/ scan tool data.
  6. Inspect cooling system components for external leaks, radiator airflow blockage, and proper winter-front operation if equipped.

Parts and tools you may need

Rough cost and difficulty

Parts and labor vary by model year and shop. Thermostat replacement is a common repair; coolant-related collateral damage (heater core, EGR cooler) can increase cost if sediment/degraded coolant is present.

When to see a pro

If diagnosis points to internal cooling-system contamination, heater-core blockage, or EGR cooler restriction, have a qualified diesel shop perform a full cooling-system evaluation and flush per OEM procedure. Do not attempt high-pressure fuel or emissions-system service.

Sources

  1. Curated DTC seed — 6.7 Cummins (SCR / DEF) (curated) ↩︎ ↩︎ ↩︎ ↩︎ ↩︎

  2. Curated DTC seed — 6.7 Cummins (SCR / DEF) (curated) ↩︎

  3. NHTSA owner report 11511506 ↩︎ ↩︎ ↩︎

Watch a repair

Third-party videos, not ours — for illustration. Confirm anything against the factory service information for your truck.

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