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Home / DTC / P20C4 — Reductant Heater C Control Circuit High

P20C4 — Reductant Heater C Control Circuit High

Detailed page for trouble code P20C4.

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Code

P20C4

Generic P — Powertrain

Reductant Heater C Control Circuit High

Brand: Generic
Type: P — Powertrain
Views: UK: 12 EN: 21 RU: 22
AI status
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Page language: EN

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

  1. Retrieve freeze frame and pending codes with a scan tool; note conditions when P20C4 set.
  2. Perform a visual inspection of the reductant heater C connector, wiring, and mounting for damage, corrosion, or signs of overheating.
  3. Check fuses and relays for the reductant heater supply; replace if blown or faulty.
  4. 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.
  5. 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).
  6. 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.
  7. 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.
  8. 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.
  9. 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

Similar codes

P20C0 P20C1 P20C2 P20C3 P20C5 P20C6
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