Code
P20C5
Generic
P — Powertrain
Reductant Heater D Control Circuit/Open
Views:
UK: 14
EN: 29
RU: 22
AI status
Completed
Completed
100%
Causes
- Open or broken wiring in heater D circuit
- Corroded/loose connector or terminal at heater or module
- Failed reductant (DEF) heater element (open)
- Blown fuse or faulty relay supplying heater power
- Poor ground or high-resistance ground connection
- Short to voltage or short to ground in control wiring
Symptoms
- Malfunction Indicator Lamp (MIL) or DEF system malfunction warning
- One or more DEF heater-related warnings or messages (heater D)
- Possible DEF system derate or inhibited dosing at low temperatures
- Reduced ability to warm DEF in cold conditions; may cause crystallization or poor dosing
- No measurable current draw from heater D when commanded
What to check
- Read DTCs and freeze frame data with a scan tool; record live data for heater control and temperature inputs
- Visually inspect wiring harness, connectors, and heater module for corrosion, damage, or signs of water intrusion
- Check related fuses and relays for continuity and proper operation
- Measure resistance of the heater element with the connector disconnected (compare to expected range)
- With a lab scope or multimeter, check for command voltage or PWM at the heater control circuit while commanding the heater ON via scan tool
- Perform a voltage drop test on the power supply and ground circuits to the heater
Signal parameters
- Typical supply voltage to heater when energized: battery voltage (~11–14.5 V) at connector (may be switched via relay)
- Expected heater element resistance (typical range): approximately 1–30 Ω (manufacturer-specific — confirm spec)
- Expected current draw when energized: roughly 0.5–10 A depending on element resistance and design
- Control signal type: switched power or PWM duty cycle from ECM/PCM; duty 0–100% when commanded; frequency varies by manufacturer
- Activation conditions: heater usually commanded when DEF temperature is near or below freezing (around 0°C / 32°F); exact threshold varies by OEM
Diagnostic algorithm
- Retrieve all stored and pending DTCs and freeze frame; note operating conditions when the code set.
- Perform a visual inspection of DEF heater D assembly, connectors, and harness for damage, corrosion, or signs of water ingress.
- Verify fuses and relays in the DEF/heater power circuit; replace any blown fuse and retest.
- With ignition OFF, disconnect heater D connector and measure resistance across heater terminals; if open (OL) or out of manufacturer spec, replace heater assembly.
- Check continuity between heater power terminal and fuse/relay output, and between heater ground and chassis ground; repair any opens or high resistance.
- With a fused ammeter in-line or clamp meter on supply lead, command the heater ON via scan tool and verify current draw and voltage at the connector; if no voltage present but ECM is commanding, check relay/power feed upstream.
- If voltage is present at the heater but no or low current and heater resistance is correct, inspect for poor ground or high resistance connections.
- Wiggle test harness while commanding heater to detect intermittent faults; repair any affected wiring/clips and retest.
- If wiring, connections, relay and heater element test good but the ECM does not provide a proper control signal, refer to manufacturer procedures for ECM output testing and consider ECM replacement only after confirming harness/grounds are good.
- Clear codes and perform a drive/operational cycle or temperature-based verification to confirm repair; monitor for reoccurrence.
Likely causes
- Open or high-resistance wiring between heater D and power/ground
- Failed heater element (open circuit)
- Corroded/loose connector at heater or module
- Blown fuse or faulty relay feeding the heater
- ECM/PCM output circuit fault (after wiring checks)
Fault status
Status
P20C5 — Reductant Heater D Control Circuit/Open. Control circuit open or interrupted for DEF heater D; ECM unable to energize or detect heater. MIL illuminated.
Repair difficulty: Medium
Diagnostic time: 0.5-2.5 hours
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