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Home / DTC / P01EF — Engine Coolant Heater B Control Circuit Performance

P01EF — Engine Coolant Heater B Control Circuit Performance

Detailed page for trouble code P01EF.

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Code

P01EF

Generic P — Powertrain

Engine Coolant Heater B Control Circuit Performance

Brand: Generic
Type: P — Powertrain
Views: UK: 11 EN: 20 RU: 11
AI status
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Page language: EN

Causes

  • Open or shorted wiring in the heater B circuit
  • Corroded, loose or damaged connector(s) at the heater or relay/module
  • Blown fuse or failed relay in the heater supply/control circuit
  • High resistance in heater element (partial failure) or heater element open/short
  • Faulty ground or poor chassis connection
  • Faulty coolant heater element or internal short

Symptoms

  • Malfunction Indicator Lamp (MIL) or engine light illuminated
  • Diagnostic trouble code P01EF stored in ECM
  • Coolant heater B does not heat or only weakly heats
  • Longer engine warm-up and harder cold starts
  • Increased idle roughness or higher emissions during cold start
  • Possible parasitic draw on battery if shorted

What to check

  • Review freeze-frame and scan-tool data; confirm code is current, not historic
  • Verify vehicle-specific service information for circuit design (relay-switched or module-driven)
  • Visually inspect wiring, connectors, and harness for damage, corrosion, pin push-out, or melted insulation
  • Check fuses and relays for the heater circuit; inspect relay contacts and socket
  • Use a DMM to measure resistance of the heater element at the connector with harness disconnected
  • Check for proper battery voltage at the heater supply pin with heater commanded ON

Signal parameters

  • Design varies by vehicle: control may be switched battery (supply) or switched ground (driver in ECM) — expect near battery voltage (~12V) at supply when commanded ON or near 0V at control/driver when the PCM grounds the circuit
  • Heater element DC resistance: typically low (single-digit to tens of ohms); many heaters measure in the range of ~1–30 Ω depending on design (consult vehicle spec)
  • Current draw when energized: commonly several amps (can be 1–20 A depending on heater power) — excessive or no current is a fault
  • Control strategy may use PWM — on command expect a change in duty cycle or voltage; frequency and duty vary by manufacturer
  • If measured voltages or resistance deviate significantly from expected spec, suspect wiring/heater or driver failure

Diagnostic algorithm

  1. Connect a scan tool, read and record freeze-frame and live data. Confirm P01EF is current and note conditions (cold start, battery voltage).
  2. Visual inspection: check connectors, harness, fuses and relays for the heater B circuit. Repair obvious damage.
  3. With harness disconnected at the heater, measure heater element resistance. Compare to vehicle spec. If open or out-of-range, replace heater element.
  4. Check supply voltage at the heater supply pin with heater commanded ON by the scan tool. Expect battery voltage (or PWM). If no supply, check fuse, relay and supply wiring.
  5. Check control/driver pin for expected switching (voltage change or PWM) when commanded ON/OFF. If control signal missing, inspect ECM/PCM grounds and wiring to module/relay.
  6. Perform a ground continuity test from heater ground to chassis. Repair poor ground connections.
  7. If wiring and connectors are good but heater does not operate, bench-test heater by carefully applying battery voltage through a fused jumper to verify heating (observe safety and correct polarity).
  8. Repair or replace failed component(s): wiring, connector, fuse, relay, heater element, or PCM as determined. Use manufacturer-recommended parts and procedures.
  9. Clear codes, re-run the system test and verify heater B operates across required conditions. Confirm no return of P01EF during road/operational cycle.

Likely causes

  • Open or corroded connector at heater B
  • Failed heater element (open or high resistance)
  • Blown fuse or failed relay supplying heater B
  • Broken wiring or short to battery/ground near heater
  • Faulty control output driver in the PCM/ECM

Fault status

⚠️ Status
P01EF — Engine Coolant Heater B Control Circuit Performance: control circuit indicates open/short/high resistance or improper control of coolant heater B.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

Similar codes

P01EA P01EB P01EC P01ED P01EE
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