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Home / DTC / P20C3 — Reductant Heater C Control Circuit Low

P20C3 — Reductant Heater C Control Circuit Low

Detailed page for trouble code P20C3.

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Code

P20C3

Generic P — Powertrain

Reductant Heater C Control Circuit Low

Brand: Generic
Type: P — Powertrain
Views: UK: 17 EN: 28 RU: 21
AI status
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Page language: EN

Causes

  • Short to ground in heater C control circuit wiring
  • Open or blown fuse or failed relay supplying the heater heater power
  • Faulty reductant/DEF heater element (open or shorted)
  • Corroded or loose connector or poor pin contact at heater or ECM connector
  • Faulty ECM/engine control module driver for heater C
  • Water intrusion or physical damage to wiring harness near the heater or dosing module

Symptoms

  • Malfunction Indicator Lamp (MIL)/check engine light illuminated
  • Stored P20C3 code (may be accompanied by other DEF/heater codes)
  • Reduced SCR/aftertreatment performance or disabled reductant heating (cold ambient issues)
  • Possible reduced engine power or limp mode in some vehicles if SCR system disabled
  • Possible inability to perform regeneration or reduced emission control effectiveness

What to check

  • Read and record freeze frame and related trouble codes with a scan tool
  • Visually inspect wiring, connectors, and the heater assembly for corrosion, damage, or water ingress
  • Check fuses and relays related to reductant/DEF heater circuits
  • Backprobe heater C connector and measure voltage and continuity with key ON or during commanded heater ON using a scan tool
  • Measure resistance of the heater element at the connector (compare to spec)
  • Perform a wiggle test on wiring while monitoring signal to reproduce fault

Signal parameters

  • Nominal battery/supply voltage at heater power feed: ~12–14 V (key ON/engine running)
  • Control/driver circuit voltage: when not commanded typically open or near battery voltage through pull-up; when commanded the ECM may switch to ground (control voltage near 0 V) — a “low” fault indicates the control circuit is held near ground when it should not be
  • Heater element resistance (manufacturer spec required) — typically a low value (a few ohms up to a few tens of ohms) for positive-temperature coefficient heaters; compare to spec
  • Continuity between heater control pin and ECM driver expected; short to ground or very low resistance indicates a fault

Diagnostic algorithm

  1. Connect a scan tool, record freeze frame, pending and stored codes; note related DEF/heater codes
  2. Perform a visual inspection of the reductant heater C, connectors, and wiring harness for corrosion, breaks, or water intrusion
  3. Verify fuses and relays for the reductant heater circuit; replace any blown fuses and retest
  4. With ignition ON (engine off), backprobe the heater power and control pins at the heater connector: verify battery voltage present on the power feed and check the control/driver pin voltage
  5. Command the heater ON with a factory scan tool or actuator test while monitoring the control pin: verify the ECM is commanding (driver changing state) and observe voltage change; if ECM commands but control remains low, suspect wiring short to ground or internal short in heater
  6. Measure heater element resistance with connector disconnected; compare to manufacturer spec. Very high (open) or very low (short) readings indicate a faulty heater
  7. If resistance and local wiring check good, perform continuity and short-to-ground tests from the ECM driver pin to the heater connector and to chassis ground to identify any short or opens
  8. Inspect and test grounds and ECM connector pins for damage or corrosion; repair damaged connectors or wiring as required
  9. If wiring, connectors and heater check good and ECM driver does not operate correctly, consider ECM/driver module failure and consult manufacturer guidance for replacement/testing
  10. Clear codes and road-test; verify that code does not return and that heater/DEF system functions correctly

Likely causes

  • Short to ground in the heater control wire (most common)
  • Blown fuse or failed relay in heater power feed
  • Corroded connector at heater or module causing low voltage
  • Failed heater element or heater module

Fault status

⚠️ Status
Control circuit low for Reductant Heater C — indicates low voltage on the heater control circuit (possible short to ground, failed heater, blown fuse/relay, connector/wiring fault, or ECM driver failure).
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

Similar codes

P20C0 P20C1 P20C2 P20C4 P20C5 P20C6
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