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DTC codes / P22xx · Fuel and air metering (injector circuits) / P221C

P221CReductant Heater B Current Too Low

P221C means the engine control module has commanded the reductant heater circuit B on, but the current flowing through that circuit is lower than the module expects for the commanded duty cycle. The module monitors the heater circuit and flags the code when measured current stays below the calibrated threshold, indicating an open circuit, high resistance, or a heater element that is not drawing its normal load.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaFuel and air metering (injector circuits)

Symptoms you may notice

  • Illumination of the malfunction indicator lamp, often with no other noticeable symptoms
  • A pending or stored P221C code accompanied by a message to service the emissions system on some vehicles
  • Reduced reductant heater performance in cold weather, which may delay diesel exhaust fluid thawing and limit selective catalyst reduction operation
  • Possible derate or reduced engine power if the fault persists and the module inhibits selective catalyst reduction dosing
  • Longer than normal diesel exhaust fluid thaw time after a cold start in freezing conditions
  • Other reductant heater or selective catalyst reduction codes stored alongside P221C

Common causes, most likely first

  1. Open or high-resistance wiring in the reductant heater B circuit, including a broken wire, corroded terminal, or backed-out pin
  2. Failed reductant heater element B with an internal open circuit or excessive resistance
  3. Blown fuse or tripped relay that supplies power to the reductant heater B circuit
  4. Poor ground connection for the reductant heater circuit, adding resistance and reducing measured current
  5. Faulty heater driver inside the engine control module or reductant control module, unable to deliver commanded current
  6. Contaminated or degraded diesel exhaust fluid affecting heater element heat transfer and current draw
  7. Water intrusion or corrosion in the reductant heater connector causing intermittent high resistance

How to diagnose and fix it

  1. Scan all modules and record freeze-frame data for P221C, then clear the code and verify whether it returns under the same conditions
  2. Inspect the reductant heater B fuse and relay, and check for power and ground at the heater connector with the circuit commanded on
  3. Measure resistance of the reductant heater element B and compare it to the manufacturer specification to confirm an open or high-resistance element
  4. Perform a voltage drop test on the power and ground sides of the heater circuit while loaded to find hidden resistance in wiring or connectors
  5. Check the heater connector and pins for corrosion, water intrusion, or loose terminals, and repair or repin as needed
  6. Command the reductant heater on with a scan tool and monitor current draw and heater temperature to confirm the circuit responds correctly
  7. If wiring and heater element test good, follow the manufacturer diagnostic flow for the control module driver and replace the module only after confirming the driver is faulty

What low heater current really means

  • A low-current code is not the same as a short to ground; P221C points to an open or high-resistance circuit, so replacing a fuse blindly without checking the heater element often misses the real fault
  • The reductant heater element can fail open internally while the connector and wiring look perfect, so a resistance measurement at the heater terminals is the decisive test
  • Corroded terminals at the heater connector are a frequent cause of low current because the added resistance reduces current below the module threshold even though the element is good
  • Some vehicles set P221C only after a cold-start thaw cycle, so the code may not return immediately after clearing if the ambient temperature is warm
  • A weak battery or charging system fault can cause the module to misreport heater current, so verify system voltage is stable before condemning the heater or module

Can I keep driving with P221C?

You can usually continue driving with P221C because the fault mainly affects cold-weather reductant thawing and emissions performance, but avoid long periods of freezing temperatures where the selective catalyst reduction system may be inhibited. If the vehicle enters a derate or shows reduced power, have it diagnosed promptly rather than continuing to drive.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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