P01EF
Engine Coolant Heater B Control Circuit Performance.
- Generic
- Powertrain
- Generic code
- Low severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- 1
- AreaFuel and air metering
- EF
- FaultThe specific fault in that area
What it means
P01EF means the control unit has detected that the engine coolant heater 'B' control circuit is not performing as expected. The commanded state of the heater output did not match what the control unit measured or inferred, or the heater did not produce the expected effect. The exact heater location and purpose (cabin heat, warm-up or battery and hybrid system support) depend on the vehicle.
Symptoms
- Warning light on, possibly with a heating system message
- Weak or slow cabin heat in cold weather
- Engine or coolant slow to reach operating temperature
- Higher fuel consumption on short cold journeys
- Heater only works intermittently
- Other coolant heater codes stored alongside
Possible causes
- Wiring fault in the heater B control circuit, such as an open circuit, high resistance, or short to earth or supplylikely
- Corroded, loose or damaged connector at the heater or control unitlikely
- Faulty heater B element or internal driver within the heater unitlikely
- Faulty relay or blown fuse in the heater supplypossible
- Low coolant level or air in the system causing the heater to overheat or cut outpossible
- Faulty coolant temperature sensor giving a misleading reading of heater effectpossible
- Fault inside the control unit driving the heaterrare
How to diagnose
- Read all stored and pending codes and note freeze frame data, including coolant temperature and supply voltage, then check for related heater or temperature sensor codes.
- Check coolant level and condition, and look for leaks or signs of air in the system.
- Visually inspect the heater B wiring, connector and earth points for chafing, corrosion, heat damage or water ingress.
- Check the relevant fuses and relay, and confirm the relay operates and its contacts are sound.
- Use the scan tool's output test, if available for this model, to command the heater B and see whether the control unit sees a response.
- With the circuit commanded on, measure supply, earth and control signal at the heater connector against the manufacturer's wiring diagram.
- Measure the heater element resistance and compare with the manufacturer's specification, and check that it warms up in use.
- Compare coolant temperature sensor readings with a known good temperature reading, and check for manufacturer service information on this code for the specific model.
How to fix
- Repair or replace damaged wiring and correct any short or open circuit found.
- Clean, repair or replace faulty connectors and terminals.
- Replace the coolant heater B unit if its element or driver is faulty.
- Replace the faulty relay or fuse after finding out why it failed.
- Top up or refill and bleed the cooling system to the correct specification.
- Replace a faulty coolant temperature sensor, and only consider the control unit once all other causes are ruled out.
Can I keep driving?
Low severity
The car can normally be driven, but expect poor heater performance and slower warm-up, so book it in soon. If the temperature gauge reads high or other warnings appear, stop and investigate straight away.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.