P01F0: Coolant temperature fell back below the temperature the computer needs for its diagnostics
Coolant Temperature Relapsed Below Diagnostic Monitoring Temperature
Severity: Monitor
Kelley Blue Book describes P01F0 as a fairly small issue that is usually safe to drive with, most often a thermostat or coolant temperature sensor problem. It is an emissions and warm-up issue rather than an overheating one.
It gets more serious if:
- Inspect soon: Inspect soon if the engine loses power or acceleration, the cabin heater blows cool air once warm, or the temperature gauge stays low.
P01F0 in short
P01F0 means the engine coolant temperature dropped back below the level the computer needs for its warm-engine checks. A thermostat stuck open is the most common cause, followed by a coolant temperature sensor fault or a fan that will not shut off. It is usually safe to drive, but get it checked.
Many engine diagnostics only run once the coolant is warm. The computer notes when the coolant has reached that point, and P01F0 sets when the reading later falls back below the diagnostic monitoring temperature, which suggests the engine is not holding its operating temperature.
The usual reason is a thermostat that is stuck open, so coolant keeps flowing through the radiator and cools the engine. A GM patent on low-coolant-temperature diagnostics describes this case, and also says the check is paused when the engine is coasting or under light load, when coolant naturally cools.
A coolant temperature sensor that reads too low, or a radiator fan that runs when it should not, can set the same code. Very cold weather can contribute.
Most likely causes
These are general causes for this code across vehicles. None is specific to one make or model.
Cause 1: Thermostat stuck open
Common general cause- Why it ranks here
- Kelley Blue Book calls the thermostat the most frequent culprit, and the GM patent names a stuck-open thermostat as an example cause of a low coolant temperature fault.
- What would change this
- Rises if the temperature gauge or scan-tool coolant temperature stays low on a long drive and the heater is weak. Falls if the engine warms quickly to its normal temperature.
Cause 2: Coolant temperature sensor reading low
Common general cause- Why it ranks here
- KBB lists a bad coolant temperature sensor as a common cause, and says the code may simply come from a faulty reading.
- What would change this
- Rises if the scan-tool temperature disagrees with the real temperature at the radiator hose. Falls if the readings agree.
Cause 3: Cooling fan that will not shut off
Possible — needs testing- Why it ranks here
- KBB lists a radiator fan that stays on, and debris stuck in the thermostat, as other causes, but with less weight than the thermostat and sensor.
- What would change this
- Rises if the fan runs constantly with a cold engine. Falls if the fan cycles normally.
What to test first
Start at step 1. Each result tells you what it means and where to go next.
Step 1: Warm the engine with a scan tool connected and watch coolant temperature, noting whether it reaches normal operating temperature and then drops back.
IfTemperature climbs normally, then drops while driving or idling
It meansThe engine is losing heat, which points to the thermostat or fan.
IfTemperature never reaches normal
It meansA thermostat stuck open is likely.
IfScan tool temperature is low but the engine feels hot
It meansThe sensor reading is suspect.
Step 2: With the engine running, note whether the cooling fan runs continuously while the engine is cold, and compare the upper radiator hose temperature with the scan tool reading.
Warning:Keep hands, hair and loose clothing away from the fan and belts, and do not open the radiator cap on a hot engine.
IfFan runs constantly on a cold engine
It meansThe fan control is suspect.
IfUpper hose is warm but the reading is low, or far from the scan value
It meansThe sensor or its wiring is suspect.
IfHose stays cool and the fan cycles normally
It meansThe thermostat is likely stuck open.
Symptoms
- Check engine light on
- Temperature gauge slow to rise or reading low
- Cabin heater weak in cold weather
- Reduced power or poorer fuel economy Gets more serious: stop and have it checked.
How to fix
Step 1: Watch the temperature gauge on a longer drive. If it stays low, have the thermostat and coolant temperature sensor checked.
Parts and cost context
- Thermostat:The thermostat is often easy to reach, but a coolant temperature sensor fault looks identical, so check the sensor reading first. Labor varies; get a quote.
Labor varies by vehicle and shop; get a quote before you approve a repair.
Compare used part prices on PartPriceGuideOn your vehicle
We don't have reviewed vehicle-specific evidence for P01F0 yet. The causes and tests above apply across vehicles.
Related codes
No curated relationships for P01F0 yet.
P01F0 questions
Is it safe to drive with P01F0?
Usually yes. The code means the engine runs cooler than the computer wants, not hotter. Get it checked if power drops or the heater stops working.
Sources and review
- P01F0 Code: Coolant Temperature Relapsed Below Diagnostic Monitoring Temperature
- US 9,103,267 B2: Low coolant temperature fault diagnostic systems and methods
- Last reviewed
- Written by
- Saurabh K.
- Editorial review
- Pending
- Technician review
- Technician review pending
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