Diesel Trouble Codes

P0128 — Coolant Thermostat Below Regulating Temperature (6.7 Power Stroke)

P0128 on a 6.7 Powerstroke means the engine coolant is not reaching the expected operating temperature in the expected time. The code is defined under SAE…

Updated September 1, 2026

What it means

Engine coolant does not reach the expected operating temperature in the expected time.1

Most likely causes

  1. Thermostat stuck open — A stuck-open thermostat prevents the engine from reaching normal operating temperature, which matches the typical failure on this platform.1
  2. Coolant temperature sensor reading low — An inaccurate coolant temperature sensor can report a temperature below the regulating threshold even if the thermostat is functioning.1
  3. Low coolant level or air in cooling system — Insufficient coolant or trapped air reduces heat buildup and can keep reported coolant temperature low.1
  4. Extended idle or very cold ambient operation — Prolonged idle or extreme cold can delay warm-up enough for the PCM to set P0128.1

How to diagnose

  1. Verify freeze-frame data for engine runtime, coolant temperature, and ambient temperature at set.
  2. Monitor live coolant temperature during a normal warm-up drive and compare to expected regulated temperature.
  3. Inspect coolant level, condition, and signs of air ingress in the cooling system.
  4. Check thermostat operation by observing warm-up rate; a slow rise suggests a stuck-open thermostat.
  5. Test coolant temperature sensor voltage/ resistance against spec and compare to infrared temperature at the thermostat housing.
  6. Review for related cooling-system codes or recent cooling-system service that could affect warm-up.

Parts and tools you may need

Rough cost and difficulty

Parts and labor vary by model year and shop rates. Thermostat replacement is generally less costly than sensor or cooling-system repairs. Confirm pricing with a qualified diesel technician.

When to see a pro

If diagnosis is inconclusive, or if cooling-system work is required, have a qualified diesel technician perform testing and repairs. Do not attempt high-pressure cooling-system service without proper training and equipment.

Sources

  1. Curated DTC seed — 6.7 Powerstroke (curated) ↩︎ ↩︎ ↩︎ ↩︎ ↩︎

Watch a repair

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

Really?? A 2020 6.7 P0128 | Drain your coolant & fix NOW! #diy #mechanic — PowerStroke Tech Talk w/ARod

P0128 Code on 2018 Ford F350 Super Duty 6.7 PowerStroke - DIY Thermostat & ECT Sensor Replacement — Humble Home Mechanic

6.7 powerstroke thermostat replacement — FatherDaughterCardRips