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Home / DTC / P20C0 — Reductant Heater B Control Circuit High

P20C0 — Reductant Heater B Control Circuit High

Detailed page for trouble code P20C0.

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Code

P20C0

Generic P — Powertrain

Reductant Heater B Control Circuit High

Brand: Generic
Type: P — Powertrain
Views: UK: 17 EN: 17 RU: 23
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Page language: EN

Causes

  • Short to battery voltage on the heater B control circuit
  • Failed heater relay or stuck relay contacts feeding the heater
  • Water ingress, corrosion, or damaged connector at the heater
  • Damaged wiring (chafing, pinched, exposed conductor) causing backfeed
  • Faulty reductant heater element with internal short or abnormal wiring
  • Faulty powertrain/engine control module (ECM/PCM) driver or sensor input

Symptoms

  • Malfunction Indicator Lamp (MIL) / Check Engine Light ON
  • DTC P20C0 stored (may be accompanied by other DEF/heater codes)
  • Reductant heater may be inoperative or on continuously
  • Reduced or disabled SCR/NOx control functionality, possible engine derate
  • DEF system warnings, reduced emissions performance, or cold-weather DEF freeze problems

What to check

  • Retrieve freeze-frame and live data for reductant heater status; note when code set
  • Visual inspection of wiring, connectors, and the heater module for damage, corrosion, or water
  • Check fuses and any dedicated heater relay for proper operation
  • Backprobe the heater B control circuit with scanner commanded on/off and measure voltages
  • Measure resistance of the heater element at ambient temperature with connector disconnected
  • Perform continuity checks for short-to-power and short-to-ground on the control wire

Signal parameters

  • Expected control circuit voltage when heater commanded ON: low (near 0 V) if ECM switches ground, or depending on design a PWM drive low-level
  • Expected control circuit voltage when heater commanded OFF: battery voltage (≈11–14 V) or open/high
  • Typical heater element resistance (varies by vehicle): often in the low single-digit ohms to tens of ohms — consult vehicle spec
  • Heater current draw (varies): can be several amps (commonly 1–20 A) when heating — consult service data
  • Failure condition for this code: control circuit voltage higher than the ECM threshold for a given state or abnormal steady-high voltage when a low is expected

Diagnostic algorithm

  1. Read and record freeze-frame and related codes. Note conditions when the code set (temperature, time after start, commanded heater state).
  2. Visually inspect DEF-heater B connectors, harness routing, and protective sleeving. Repair any corrosion, water intrusion, or damaged insulation.
  3. With ignition ON (engine off) and using a scan tool, command heater B ON/OFF and observe the control circuit voltage. Backprobe the connector to avoid misleading readings.
  4. With heater connector disconnected, measure control circuit voltage while commanding ON. If voltage remains high with connector disconnected, suspect short to battery or relay feeding the circuit or ECM output fault.
  5. Measure heater element resistance at the heater connector. Compare to service spec. Very low resistance may indicate internal short; open circuit indicates broken heater element.
  6. Check for short-to-power on the control wire: disconnect inverter/ECM harness if needed and perform an ohms/voltage test to locate short.
  7. Inspect and test any associated relays, fuses, and power feeds for proper operation and no backfeed from other circuits.
  8. If wiring and heater check good but the control circuit still reads high only when ECM should be driving, consider ECM driver failure. Verify by following OEM diagnostic flow before replacing ECM.
  9. After repairs, clear codes and verify proper operation through several drive/start cycles and with cold-start conditions if relevant.

Likely causes

  • Shorted control wire to battery voltage at or near the heater connector
  • Corroded connector allowing voltage backfeed or intermittent contact
  • Stuck heater relay supplying constant battery power to the heater circuit
  • Open or high-resistance ground path so the ECM sees a high voltage
  • Failed ECM output transistor (internal short to battery)

Fault status

⚠️ Status
DTC P20C0 — Reductant Heater B Control Circuit High. Control circuit voltage above expected threshold detected by control module. MIL illuminated; DEF heater control fault affecting SCR system operation.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

Similar codes

P20C1 P20C2 P20C3 P0493 P20B0
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