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Home / DTC / P20C7 — Reductant Heater D Control Circuit Low

P20C7 — Reductant Heater D Control Circuit Low

Detailed page for trouble code P20C7.

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Code

P20C7

Generic P — Powertrain

Reductant Heater D Control Circuit Low

Brand: Generic
Type: P — Powertrain
Views: UK: 19 EN: 30 RU: 15
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Page language: EN

Causes

  • Blown fuse or open/failed relay in DEF heater supply circuit
  • Open, corroded or disconnected connector at the heater or PCM
  • Wiring short to ground or high resistance in supply/control wires
  • Failed DEF heater element (open or internally damaged)
  • Poor ground connection
  • Faulty PCM or driver transistor (less common)

Symptoms

  • Malfunction indicator lamp (MIL)/check engine light illuminated
  • Reduced or no operation of DEF heater D (no warming at low ambient temps)
  • Possible DEF system warnings (freeze protection not available)
  • Poor cold-start after low-temp storage of DEF or reduced emission system performance
  • Possible stored diagnostic trouble codes related to DEF heater circuits

What to check

  • Retrieve freeze frame and all related DTCs; note ambient temperature and vehicle status when stored
  • Visual inspection of DEF heater D connector, wiring harness, and ground for corrosion, damage or water ingress
  • Check fuses and relays in DEF heater power circuit
  • Backprobe the DEF heater connector to measure supply voltage and control voltage while commanding heater on/off
  • Measure resistance of the DEF heater element (unplug connector first) and compare to specification
  • Perform continuity and short-to-ground checks on supply and control wires between heater and PCM

Signal parameters

  • Battery/supply voltage at heater power pin: ~11–14.5 V (key on/engine running depending on circuit)
  • Control pin (PCM output) when active: near 0 V (low, ground-switched) or PWM switching between 0 V and battery voltage
  • Control pin when inactive: near battery voltage (open/high) on some designs
  • Heater element resistance (typical range): approximately 2–40 Ω depending on heater design (refer to vehicle spec)
  • Expected heater current when energized: typically several amps (0.5–10 A depending on resistance and voltage)
  • PWM duty cycle: varies with temperature control strategies; frequency and duty vary by manufacturer

Diagnostic algorithm

  1. Read all codes and freeze-frame data; record ambient temperature and vehicle operating conditions.
  2. Perform a detailed visual inspection of the DEF heater D connector, harness, and ground. Repair any corrosion or damage.
  3. Check relevant fuses and relays. Replace any blown fuses and test relays for proper operation.
  4. With ignition off, unplug the heater connector and measure heater element resistance with an ohmmeter. Compare to spec. Replace heater if open or outside range.
  5. Check continuity of the power feed wire from fuse/relay to heater and continuity of the control wire from heater to PCM. Check for shorts to ground on the control wire.
  6. With backprobe and safe procedure, command the heater ON via scan tool and measure voltage on the power pin and control pin. Control pin should be driven (grounded or PWM) by PCM when commanded. If supply is present but control remains low with no command, suspect short to ground or PCM driver fault.
  7. If supply is absent, trace and repair upstream power/relay/fuse wiring before replacing heater or PCM.
  8. If heater is OK and wiring checks fine but control signal is abnormal, consult manufacturer procedures to test/replace PCM or driver module.
  9. Clear codes and perform a function test; verify code does not return under the same conditions.
  10. Safety note: when applying test power or jumpers to heater circuits, use appropriate inline fusing and protective measures to avoid damage or fire.

Likely causes

  • Corroded connector at the DEF heater causing high resistance/low voltage
  • Open or high-resistance heater element
  • Short to ground on the heater control wire
  • Blown fuse or failed relay feeding the heater
  • Faulty PCM output driver after verifying wiring and component

Fault status

⚠️ Status
Control circuit low — low voltage detected on Reductant (DEF) Heater D control circuit.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours
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