Code
P221D
Generic
P — Powertrain
Reductant Heater B Current Too High
Views:
UK: 11
EN: 23
RU: 15
AI status
Completed
Completed
100%
Causes
- Short to battery voltage in Reductant Heater B circuit
- Low-resistance or shorted heater element (internal failure or coolant/DEF intrusion)
- Stuck or welded relay supplying heater (continuous power)
- Corroded/poor connector or damaged wiring causing abnormal current path
- Incorrect replacement heater with wrong resistance
- Faulty heater driver or reductant control module reporting incorrect current
Symptoms
- MIL (check engine light) illuminated
- Reduced/disabled SCR/NOx control or limp-home mode in some vehicles
- Reductant heater not heating correctly or running continuously
- Possible unusual odor from DEF area if element overheats
- Related emission system warnings/messages
What to check
- Read freeze frame and full DTC data; note operating conditions when code set
- Inspect DEF/reductant heater B wiring harness, connectors, and relay for damage or corrosion
- Check fuses and relay(s) for the heater circuit
- Measure heater element resistance at ambient and compare to OEM spec
- Perform a current draw test on the heater circuit using a DC clamp ammeter while commanded ON
- Unplug heater connector and verify current drops to near zero (isolates wiring vs module)
Signal parameters
- Heater control command: PWM or switched output (0–100% duty) from reductant control module
- Measured heater current: typically low when off, expected design range when on (consult OEM for exact amps)
- Heater element cold resistance: typically several ohms to tens of ohms (check OEM spec)
- Supply voltage to heater circuit: nominal battery voltage (approx. 12–14.5 V)
- Driver/relay state: ON or OFF as commanded by control module
Diagnostic algorithm
- Retrieve diagnostic data and freeze frame. Note battery voltage and ambient conditions when code was stored.
- Perform a thorough visual inspection of heater B harness, connector, and relay for damage, corrosion, or signs of DEF ingress.
- With ignition off, disconnect the heater B connector. Measure the heater element resistance to ground and between terminals. Compare to OEM specification.
- Reconnect and backprobe the harness. With the heater commanded ON (use active test if available), measure current draw with a clamp ammeter. If current is above OEM spec, suspect shorted heater or incorrect part.
- If high current is present with heater connected, unplug the connector again. If current falls to zero when disconnected, the fault is in the heater element. If current remains high, suspect a short in wiring or the driver module.
- Test the heater supply relay/driver: inspect operation, swap with a known-good relay where applicable, and check driver transistor outputs from the control module for shorts to battery using a multi-meter per OEM procedure.
Likely causes
- Shorted heater element (most common)
- Damaged wiring harness near heater connector
- Stuck/shorted supply relay or high-side transistor
- Corroded connector causing partial short or alternate current flow
Fault status
Status
Reductant Heater B circuit current higher than allowed. Possible shorted heater element, wiring short, stuck relay, or driver/module fault. Repair wiring or replace faulty component and verify heater current within specification.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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