P2B72

Code

Generic P — Powertrain

Coolant Heater A Control Circuit/Open

Brand: Generic

Type: P — Powertrain

Views: UK: 19 EN: 40 RU: 25

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Page language: EN

Causes

Open or broken wiring in the heater A harness
Corroded or disconnected connector at the heater assembly or control module
Blown fuse or faulty relay supplying the heater circuit
Failed coolant heater element (open winding)
Faulty PCM / heater control module output
Intermittent connection due to damaged insulation or terminal

Symptoms

Malfunction Indicator Lamp (MIL) or CEL illuminated
Coolant heater(s) do not operate (no heat from heater element)
Longer engine warm-up time or reduced cold-start emissions control
Possible stored diagnostic trouble codes related to heater circuit

What to check

Scan for freeze-frame and related codes; note conditions when code set
Visual inspection of wiring and connectors for corrosion, disconnection, or damage
Check relevant fuses and relays for continuity and proper operation
Backprobe connector at the heater and at the control module to measure voltage and continuity
Measure resistance of the heater element with ignition off and battery disconnected
Perform wiggle test on harness while monitoring continuity or voltage for intermittent faults

Signal parameters

When heater is commanded ON: supply-side should show near-battery voltage (approx. 12 V)
When commanded ON (if PCM grounds the circuit): control/output side should switch to close to 0 V (ground) or show a pulsed PWM signal — verify with a scope if uncertain
Heater element resistance: low resistance in the single-digit ohm range for typical direct-heated elements (expect continuity, not open circuit)
Open or very high resistance on continuity check indicates fault

Diagnostic algorithm

Retrieve freeze frame data and any related codes. Verify conditions when the code was stored (temperature, key state, engine off/on).
Visually inspect the coolant heater A connector, harness, and nearby components for damage, corrosion, or disconnection. Repair any obvious issues.
Check fuses and relays for the heater circuit; replace if blown or faulty.
With battery disconnected, measure resistance across the heater A terminals. An open (infinite) reading indicates a failed heater element.
Reconnect battery. Backprobe the heater connector. Command the coolant heater ON with a scan tool and measure voltage at the supply pin and the control pin. Verify the supply has battery voltage and the control changes state when commanded.
If supply voltage is present but no control signal, trace continuity from the heater connector to the control module and check for opens or high resistance. Repair wiring as needed.
If wiring and heater element are good but no proper control signal from the module, test or substitute relay (if present) and inspect ground connections. Consider module output test or replacement only after verifying wiring and loads.
Clear the code, perform a functional test and road/operation cycle to confirm the repair. Re-scan for reappearance of the code.

Likely causes

Disconnected or corroded connector at the heater
Open heater element (internal break)
Blown fuse or failed relay in the heater supply
Wiring harness damage (chafing, rodent damage, broken conductor)

Fault status

⚠️ Status

Control circuit open for Coolant Heater A: no continuity or high resistance detected in heater A circuit; heater may not activate when commanded. MIL illuminated.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours