Code
P20C4
Generic
P — Powertrain
Reductant Heater C Control Circuit High
Views:
UK: 14
EN: 30
RU: 31
AI status
Completed
Completed
100%
Causes
- Open, shorted, or damaged wiring in the heater C control circuit
- Corroded or loose connector at the heater or module
- Defective reductant heater element (internal short or intermittent)
- Faulty PCM/ECM driver or internal relay controlling heater C
- Blown fuse, stuck relay, or loss of proper power/ground to the circuit
- Water/contamination in connector or harness causing high voltage reading
Symptoms
- Malfunction Indicator Lamp (MIL)/check engine light illuminated
- DEF/heater system warning or reduced SCR functionality
- Inability to heat reductant during cold conditions or failed regen events
- Possible limp-home mode or reduced aftertreatment performance
- Stored DTC P20C4 (and possibly related heater or aftertreatment codes)
What to check
- Scan for codes and note freeze-frame and pending codes; record invocation conditions
- Visual inspection of heater C connector, wiring harness, and mounting for damage or corrosion
- Check fuses and relays supplying the reductant heater circuit
- Backprobe the heater C connector to measure control voltage with key ON and while commanding heater on/off via scan tool
- Measure heater element resistance and continuity with connector disconnected
- Perform wiggle test on wiring while monitoring voltage or resistance for intermittent faults
Signal parameters
- Control voltage: should be near battery voltage (~12–14 V) when the heater is commanded OFF and should be pulled low (near 0 V) when the PCM commands the heater ON (specific polarity depends on vehicle design—verify service data)
- Heater element resistance: typically low (single ohms to a few tens of ohms) — check vehicle-specific specification
- Supply fuse/relay circuit: +12 V battery supply present at relay/fuse input
- Command current: heater currents can be several amps; use appropriate meters and safety precautions
Diagnostic algorithm
- Retrieve freeze frame and pending codes with a scan tool; note conditions when P20C4 set.
- Perform a visual inspection of the reductant heater C connector, wiring, and mounting for damage, corrosion, or signs of overheating.
- Check fuses and relays for the reductant heater supply; replace if blown or faulty.
- With the connector disconnected, measure resistance of the heater element to ground and between heater terminals; compare to manufacturer spec. Open or extremely high resistance indicates a heater fault.
- Backprobe the heater control pin at the connector. With key ON and heater not commanded, verify expected idle voltage. Command the heater ON with a scan tool and confirm the control circuit voltage changes as specified (typically pulled low).
- If voltage is higher than expected when commanded ON, check for short to battery or poor ground. Isolate wiring by disconnecting harness sections and re-testing to locate fault.
- Perform continuity and short-to-power/ground checks on the harness between the heater connector and the PCM/relay. Repair any damaged wiring, chafing, or corrosion.
- If wiring and heater element test good, substitute or bench-test the relay (if used) and retest. If circuit driver still shows high voltage only at the PCM pin, suspect PCM/ECM driver failure and refer to manufacturer procedures before replacement.
- After repair, clear codes, perform functional test(s) (command heater on/off), verify proper heating operation and that the code does not return.
Likely causes
- Wiring fault (open, chafe, or short to battery) at heater C harness
- Corroded/poor connector making intermittent/high-resistance connection
- Failed heater element or internal short in the heater assembly
- Defective module driver or control relay/fuse supplying the heater
Fault status
Status
Control circuit for reductant heater C is reporting a voltage higher than the allowed threshold. The PCM has set DTC P20C4, illuminated a malfunction indicator, and may disable or limit heater operation until the fault is corrected.
Repair difficulty: Medium
Diagnostic time: 1-3 hours
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