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 October 15, 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

• OBD-II scanner with live data / freeze-frame
• Infrared thermometer
• Coolant test strips / refractometer
• Coolant drain pan
• Basic hand tools for thermostat housing access

Tools you’ll need

• OBD-II scan tool — Reads live ECT and freeze-frame to confirm slow warm-up and steady-state temperature below spec for P0128.
• digital multimeter — Verifies coolant temperature sensor voltage/resistance against spec to rule out sensor drift causing false P0128.
• infrared thermometer — Checks actual radiator/upper-hose temperature vs. scan-tool ECT to detect a stuck-open thermostat or air pocket.
• coolant pressure tester — Confirms proper coolant level and reveals leaks or air pockets that prevent the engine from reaching regulating temperature.

Rough cost and difficulty

Typical US independent-shop figures; labor runs about $120–$180 per hour (dealers charge more). Confirm the diagnosis before buying parts.

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.

Check for recalls

Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.

Sources