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Home / DTC / P20C6 — Reductant Heater D Control Circuit Performance

P20C6 — Reductant Heater D Control Circuit Performance

Detailed page for trouble code P20C6.

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Code

P20C6

Generic P — Powertrain

Reductant Heater D Control Circuit Performance

Brand: Generic
Type: P — Powertrain
Views: UK: 18 EN: 32 RU: 16
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Page language: EN

Causes

  • Open or short in heater D wiring (broken conductor, chafing to ground/battery)
  • High resistance connection or corroded connector at heater D
  • Failed heater element (open or partially shorted)
  • Blown fuse or faulty relay supplying the heater circuit
  • Faulty driver output in the powertrain/aftertreatment control module (ECM/PCM)
  • Low battery/insufficient supply voltage or poor ground

Symptoms

  • Malfunction Indicator Lamp (MIL) or SCR system warning illuminated
  • Aftertreatment/reductant warnings on instrument cluster (DEF heater fault)
  • Reduced or inhibited SCR operation in cold conditions
  • Possible limp mode or reduced engine power in some manufacturer strategies
  • Extended time to reach required DEF temperature or inability to heat DEF
  • Stored DTC(s) related to reductant/heater circuits

What to check

  • Read freeze frame and live data for heater D: commanded state, reported current/voltage/status
  • Inspect fuses and relays related to reductant heater circuits
  • Visually inspect wiring and connector for heater D for damage, corrosion or water ingress
  • Measure heater element resistance with power disconnected
  • Check for battery voltage at the heater connector with heater commanded ON
  • Perform wiggle test on harness/connectors while monitoring live data

Signal parameters

  • Control type: On/Off switched 12V supply controlled by ECM/relay
  • Expected supply voltage (when commanded ON): ~11–14 V (vehicle battery voltage)
  • Typical heater current draw: commonly 0.5–10 A depending on element design (manufacturer-specific)
  • Typical heater resistance: commonly in the low-ohm range (approx. 1–30 Ω depending on element size)
  • Open-circuit: OL/infinite resistance; Short to ground/battery: near 0 Ω or fuse blown
  • Time to warm: heater should show current draw immediately when commanded and raise temperature within manufacturer-specified time

Diagnostic algorithm

  1. Safety: Park vehicle, apply parking brake, wear PPE. DEF (urea) is corrosive—avoid skin/eye contact. If performing powered tests, secure vehicle and use fused test leads.
  2. Connect a scan tool and record DTC P20C6 freeze frame and any related codes. Note engine/battery voltage and commanded heater status.
  3. Inspect harness and connector at the Def Heater D module and the ECM for damage, corrosion, bent pins, or water/crystal build-up. Repair or clean as needed.
  4. Verify fuses and relays for the reductant heater circuit(s). Replace a blown fuse only after determining cause; test relay operation.
  5. With ignition ON and heater D commanded ON (use scan tool to command ON if available), measure voltage at the heater connector supply terminal relative to ground. Expected ~battery voltage. If no voltage, trace upstream (fuse/relay/ECM driver).
  6. With power OFF, disconnect the heater connector and measure resistance of the heater element. Compare to expected range. Infinite/open indicates failed element; very low indicates short.
  7. Check continuity and resistance between heater return and vehicle ground/ECM ground. High resistance suggests poor ground.
  8. If wiring and connectors appear good but problem persists, measure current draw when heater is commanded ON. If current is excessively low or high relative to expected, suspect heater element or control driver.
  9. Perform a bench test: remove heater assembly and apply fused 12 V directly to the heater element to confirm heating and current draw (observe safety and DEF contamination precautions).
  10. If heater element and wiring test OK, suspect ECM/PCM output driver. Verify ECM grounds and software calibrations; review manufacturer service information for known issues or updates.
  11. After repairs or replacements, clear codes, perform heater activation tests with scan tool, and confirm proper operation across cold-start conditions. Road test and monitor live data for reoccurrence.

Likely causes

  • Open circuit at heater element or connector
  • Short to ground or short to battery on heater supply/return
  • High-resistance terminal or corroded connector at heater D
  • Failed heater element drawing no or excessive current
  • Faulty control relay or blown fuse in heater feed
  • ECM/PCM heater driver fault or software/hardware error

Fault status

⚠️ Status
Reductant Heater D control circuit performance fault detected — heater not performing within expected parameters (open/short/high resistance/incorrect current).
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

Similar codes

P20C0 P20C1 P20C2 P20C3 P20C4 P20C5
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