Code
P221C
Generic
P — Powertrain
Reductant Heater B Current Too Low
Views:
UK: 16
EN: 59
RU: 21
AI status
Completed
Completed
100%
Causes
- Open or high-resistance wiring in Heater B circuit (broken conductor, damaged insulation, corroded connector)
- Blown fuse or faulty relay supplying the heater circuit
- Failed reductant heater element (open or partially open)
- Poor or intermittent ground at heater or module
- Low battery or insufficient supply voltage under load
- Connector contamination, corrosion, or water ingress
Symptoms
- MIL/Check Engine lamp illuminated with P221C stored
- Reductant/DEF system warning message or reduced functionality in cold conditions
- Reduced SCR system performance or increased NOx emissions
- Possible DEF freeze protection failure or warnings during cold start
- No obvious change in engine drivability in many cases
What to check
- Read and record freeze frame and related codes; verify P221C is current
- Inspect fuses and relays for the reductant heater circuit
- Visual inspection of heater B wiring harness, connectors, and ground for damage or corrosion
- Measure battery/resting system voltage and voltage at heater connector when heater is commanded on
- Measure heater element resistance (ohms) with circuit disconnected
- Measure current draw of heater B while commanded on using an inline ammeter or clamp meter
Signal parameters
- Expected supply voltage at heater connector when commanded: ~12 V system voltage (may be PWM modulated)
- Expected heater current draw: vehicle-dependent; commonly in the low single-digit amps up to tens of amps (check OEM spec)
- Heater element resistance when cold: typically low ohms (example ranges vary by vehicle; consult OEM)
- Control signal: either full battery voltage or PWM duty cycle from ECM when heater is active
- Normal behavior: current rises to expected value within seconds of command; low or zero current indicates fault
Diagnostic algorithm
- Retrieve DTC(s), note freeze-frame data and related reductant/temperature codes; clear codes and reattempt test to confirm repeatability.
- With scan tool, command Reductant Heater B ON and observe commanded status and any live data for heater current or voltage.
- Visually inspect fuses, relays and repair/replace if blown or faulty. Confirm relay coil and contacts operate when commanded.
- Inspect harness and connector at heater module and at the ECM for damage, corrosion, water ingress, pin push-out, or bent pins.
- Disconnect heater connector and measure element resistance (ohms). Compare to OEM spec. Open or very high resistance = failed heater element.
- Reconnect and backprobe the power feed and ground at the heater connector. Command heater ON and measure voltage at the connector. Low voltage under command indicates supply or high resistance issue upstream.
- Measure current draw while heater is commanded ON (inline ammeter or clamp around feed). Low or zero current confirms the DTC condition.
- If supply voltage is present but current is low, check ground integrity and resistance; repair ground or wiring as needed.
- If wiring and heater element check OK, test/replace the relay or inspect ECM driver output. If ECM driver suspected, look for related ECM outputs or internal faults before replacing the module.
- Repair or replace the faulty wiring, connector, fuse, relay, or heater assembly as indicated. After repair, clear codes and verify heater operation and that code does not return during functional test and road/temperature conditions.
Likely causes
- Open heater element or heater harness wiring (most common)
- Blown fuse or bad relay preventing full supply to heater
- High resistance connection at connector or ground causing reduced current
- Control module not providing correct voltage/current (less common)
Fault status
Status
PCM detected Reductant Heater B current below expected threshold while heater was commanded ON. Heater may be open, have a high-resistance connection, or be receiving insufficient supply voltage; service wiring, fuses/relays, ground, and heater element.
Repair difficulty: Medium
Diagnostic time: 1-3 hours
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