P0128 Engine coolant stays below thermostat temperature

Thermostat stuck open / engine running cold

Monitor

An engine that warms up too slowly wastes fuel and gives poor cabin heat but does not overheat, and Hyundai says the condition causes no drivability problems, so it is safe to drive while you book a repair.

It gets more serious if:

  • Stop: The temperature gauge reaches the red, an engine coolant temperature high or overheat warning appears, you see steam from under the hood, or the engine loses power

Source: how we rate severity

P0128 in short

P0128 means the engine took too long to warm up, or never reached its normal temperature. A thermostat stuck open is by far the most common cause. It is safe to drive in the short term, but stop at once if the gauge reaches the red or you see steam.

The thermostat keeps coolant away from the radiator until the engine is warm. The computer watches the coolant temperature sensor, and if coolant does not get close to the thermostat temperature within a set time, it sets P0128.

A thermostat stuck open lets coolant flow through the radiator from a cold start, so the engine warms slowly. Several makers, including Jeep, Hyundai, Subaru and Acura, have bulletins replacing thermostats for this code.

Less often, the coolant temperature sensor reads low, or air trapped in the cooling system after a coolant change confuses the reading.

Most likely causes

  1. Thermostat stuck open or opening too early

    Common general cause
    Why it ranks here
    The monitor checks exactly this, and bulletins from Jeep, Hyundai, Subaru and Acura all replace the thermostat for P0128.
    What would change this
    Rises if the coolant temperature reading climbs slowly and stalls well below normal on a cold start. Falls if it warms up normally.
    How to test the thermostat

  2. Thermostat that opens early after a long cold soak (some Subarus)

    Supported by vehicle data
    Why it ranks here
    Subaru has a bulletin where coolant gets into the thermostat piston after sitting in the cold, so the thermostat opens too early. A countermeasure part fixes it.
    What would change this
    Rises if the code appears after the car has sat in very cold weather and the car is covered by bulletin 09-56-13.
    How to test the thermostat

  3. Air trapped in the cooling system

    Possible — needs testing
    Why it ranks here
    Subaru warns that P0128 can return if air is not bled out when refilling coolant, since an air pocket upsets the sensor reading.
    What would change this
    Rises if the code started soon after coolant work or the coolant level drops after driving.
    How to test the engine coolant temperature (ect) sensor

  4. Coolant temperature sensor reading low

    Possible — needs testing
    Why it ranks here
    The computer relies on this sensor, so a sensor that reads low can fake a cold engine. It is checked separately for plausibility, so it is a less likely sole cause.
    What would change this
    Rises if the sensor reading does not match a known-good temperature reading, or a sensor code is also stored.
    How to test the engine coolant temperature (ect) sensor

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. With the engine fully cold, check the coolant level at the see-through reservoir only.

    Use caution

    Warning:Never open the radiator cap or coolant reservoir cap while the engine is hot. Hot coolant and steam can spray out and burn you. Check only with the engine fully cold.

  2. From a cold start, watch the coolant temperature reading on a scan tool as the engine warms up while you drive.

    Low riskTool: OBD-II scan tool with live data

    • IfTemperature climbs slowly and levels off well below normal operating temperature

      It meansThe thermostat is letting coolant through too early.

      Go to step 3

    • IfThe reading looks wrong on a cold start (for example far from the outside temperature after an overnight sit)

      It meansThe sensor itself is suspect.

      Points to: Coolant temperature sensor reading low

  3. Check whether your vehicle is covered by a thermostat bulletin or service campaign, using the VIN at a dealer or the NHTSA site.

    Low risk

Symptoms

  • Temperature gauge reads lower than usual or takes a long time to rise
  • Weak cabin heat, especially in cold weather
  • Check engine light with no change in how the car drives
  • Temperature gauge in the red, overheat warning or steam from under the hood Gets more serious: stop and have it checked.

How to fix

  1. With the engine fully cold, check the coolant level at the reservoir and top up with the coolant type in your owner's manual.

    Warning:Never open the radiator cap or coolant reservoir cap while the engine is hot. Hot coolant and steam can spray out and burn you. Check only with the engine fully cold.

  2. Ask a dealer whether your VIN has an open thermostat bulletin or service campaign. Hyundai, Jeep and Subaru have issued them.

  3. Watch the temperature gauge. If it ever reaches the red, pull over safely and switch off.

    Warning:Do not open the hood while steam is coming out.

Parts and cost context

  • Thermostat:Some makers use an updated (countermeasure) thermostat, so check for a bulletin before buying the part. Labour varies by engine, so get a quote.
  • Engine coolant temperature (ECT) sensor:Replace only after the reading has been checked against a known-good temperature.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

Jeep

  • 2024–2025 · Compass

    Supported by vehicle data

    Jeep/RAM bulletin 07-006-24 REV. B covers a thermostat stuck open on vehicles built 8 May 2023 to 10 May 2024, setting P0128 with poor cabin heat. The Commander and RAM Rampage are also covered. The bulletin is on NHTSA, but whether it is still current is not verified, so it is labelled vehicle data rather than confirmed.

  • 07-006-24Status unverified

    Thermostat stuck open, P0128

    2024–2025compass, commander · issued · rev. REV. B · NHTSA MC-11008195

    On vehicles built 8 May 2023 to 10 May 2024, the thermostat can stick open, giving P0128, poor cabin heat and a cold-running engine. Replace the thermostat. The bulletin also covers the RAM Rampage.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Hyundai

  • 2017 · Elantra · 2.0L Nu MPI

    Supported by vehicle data

    Hyundai bulletin 16-01-034-2 (service campaign TLL) replaces the thermostat, which can set P0128, even if the code has not set yet. Hyundai says it causes no drivability problems. The bulletin is on NHTSA, but whether it is still current is not verified, so it is labelled vehicle data rather than confirmed.

  • 16-01-034-2Status unverified

    Thermostat replacement, P0128 (service campaign TLL)

    2017elantra · 2.0L Nu MPI · issued

    The thermostat may malfunction and set P0128. Hyundai replaces it even if the code has not set, and says the condition causes no drivability problems.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Subaru

  • 2011–2014

    Supported by vehicle data

    Subaru bulletin 09-56-13 covers P0128 after a long cold soak on the Legacy, Outback, Impreza, XV Crosstrek, Forester and BRZ, fixed with a countermeasure thermostat. The bulletin is on NHTSA, but whether it is still current is not verified, so it is labelled vehicle data rather than confirmed.

  • 2011–2012 · H6

    Supported by vehicle data

    Subaru bulletin 11-111-12R covers P0128 on the H6 Legacy, Outback and Tribeca, caused by small bank-to-bank temperature differences and trapped air. It adds a heater hose pipe with an orifice, and says air must be bled when refilling. The bulletin is on NHTSA, but whether it is still current is not verified, so it is labelled vehicle data rather than confirmed.

  • 09-56-13Status unverified

    P0128 after a long cold soak, countermeasure thermostat

    2011–2014legacy, outback, impreza, xv-crosstrek, forester, brz · FB20, FB25, FA20 turbo · issued

    After the car sits in the cold for a long time, coolant can get into the thermostat piston so it opens too early and sets P0128. Fit the countermeasure thermostat.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • 11-111-12RStatus unverified

    P0128 on H6 engines, countermeasure heater hose pipe

    2011–2012legacy, outback, tribeca · H6 · issued

    Small bank-to-bank coolant temperature differences and trapped air can set P0128. Fit the heater hose pipe with an orifice and bleed all air when refilling, or the code can come back.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Acura

  • 2017 · NSX

    Supported by vehicle data

    Acura bulletin 17-048 says the thermostat inner seal can come loose and hold it open, setting P0128. Replace the thermostat. The bulletin is on NHTSA, but whether it is still current is not verified, so it is labelled vehicle data rather than confirmed.

  • 17-048Status unverified

    P0128 from a detached thermostat inner seal

    2017nsx · issued

    The inner seal can come loose and hold the thermostat open. Replace the thermostat.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • P0125Coolant too cold for closed-loop fuel control

    Shares causes Supported by vehicle data

    P0125 is the other warm-up code. A thermostat stuck open sets either one: Ford names it for P0125, and Jeep, Hyundai, Subaru and Acura for P0128.

  • P0118Engine coolant temperature sensor circuit high

    Shares causes Supported by vehicle data

    Ford lists P0118 and P0128 together on 2.7L EcoBoost engines with ECT sensor or wiring faults, so a bad coolant temperature signal can set both.

  • P0217The engine computer has detected an engine over-temperature condition

    Shares causes Supported by vehicle data

    Ford's 2.7L EcoBoost sensor-wiring bulletin lists P0128 alongside P0217, the engine over-temperature code, when the coolant temperature signal is wrong.

  • P1285Cylinder head temperature above the engine computer’s limit (Ford, Lincoln, Ford-built Mazda)

    Shares causes Supported by vehicle data

    Mazda's Tribute bulletin and Ford's 2019 Edge and Nautilus software bulletin list P0128 alongside P1285 when the temperature reading is wrong.

  • P050AIdle speed off target just after a cold start

    Shares causes Supported by vehicle data

    Kia's 2012–2014 Rio and Soul software bulletins list P0128 with P050A and other codes fixed by the same engine computer update.

All Engine cooling (thermostat) codes

P0128 questions

Is it safe to drive with P0128?

Usually yes in the short term. The engine runs cool rather than hot, and Hyundai notes no drivability problems for its thermostat bulletin. If the gauge ever reaches the red or you see steam, stop safely and switch off.

Is the thermostat always the cause?

It is the most common cause, and several makers fix P0128 by replacing it. Less often the coolant temperature sensor reads low, or air left in the system after coolant work upsets the reading.

Can I check the coolant myself?

Yes, at the see-through reservoir and only with the engine fully cold. Never open the radiator cap or reservoir cap on a hot engine, because hot coolant and steam can spray out.

Do I need a special thermostat?

Sometimes. Subaru and others have countermeasure parts, so check for a bulletin for your VIN before buying a thermostat.

Sources and review

  1. Hyundai 16-01-034-2: thermostat replacement (service campaign TLL), P0128 — Hyundai Motor America / NHTSA ,
  2. Cal. Code Regs. Tit. 13, § 1968.2 — Malfunction and Diagnostic System Requirements (OBD II) — California Air Resources Board (via Cornell LII)
  3. On-Board Diagnostic II (OBD II) Systems Fact Sheet — California Air Resources Board ,
  4. Toyota Camry Owner's Manual (OM06291U, 2.5L A25A-FKS / 3.5L 2GR-FKS) — Toyota
  5. Jeep/RAM 07-006-24 REV. B: thermostat stuck open, P0128 — FCA US / NHTSA ,
  6. Acura 17-048: 2017 NSX P0128, thermostat inner seal detaches — American Honda / NHTSA ,
  7. Subaru 09-56-13: P0128 after long cold soak, countermeasure thermostat — Subaru of America / NHTSA ,
  8. Subaru 11-111-12R: P0128 on H6 engines, countermeasure heater hose pipe — Subaru of America / NHTSA ,
  9. Ford 14407: lack of heat and/or MIL with P0125 from thermostat sticking open — Ford Motor Company / NHTSA ,
  10. Ford SSM 48991: MIL and/or Engine Coolant Over Temperature warning with ECT codes on 2.7L EcoBoost — Ford Motor Company / NHTSA ,
  11. Ford TSB 21-2168: MIL and/or Engine Coolant Over Temperature warning from an intermittent ECT voltage drop, 2018–2020 F-150 2.7L — Ford Motor Company / NHTSA ,
  12. Ford SSM 48991: MIL and/or Engine Coolant Over Temperature warning from the ECT or knock sensor harness, 2.7L EcoBoost — Ford Motor Company / NHTSA ,
  13. Mazda bulletin 01-005-20: MIL with P1285, P1299 and/or P0128, incorrect CHT sensor reading (Tribute) — Mazda North American Operations / NHTSA ,
  14. Ford TSB 19-2046: PCM software update, 2.0L EcoBoost (2019 Edge, Nautilus) — Ford Motor Company / NHTSA ,
  15. Kia TSB ENG118R2 (SA152): Rio, Soul 1.6 GDI MIL on with P0456, P0461, P050A, P2188, P2192, P0128 — Kia America / NHTSA ,
  16. Kia TSB ENG146R1: ECM upgrade, Rio and Soul MIL on with P050A and other codes (2012–2014) — Kia America / NHTSA ,
Last reviewed
Written by
Saurabh K.
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