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DTC codes / P20xx · Fuel and air metering and auxiliary emission controls / P20BD

P20BDReductant Heater B Control Circuit/Open

P20BD indicates that the powertrain control module has detected an open circuit or excessive resistance in the control circuit for reductant heater B, which is part of the diesel exhaust fluid injection system. The heater is responsible for thawing frozen DEF in cold weather, and the PCM monitors the circuit for expected voltage and current to confirm proper operation.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaFuel and air metering and auxiliary emission controls

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms
  • A warning message on the dash indicating a DEF system fault or reduced engine power
  • Possible reduction in engine power or speed limiting if the fault persists
  • In cold weather, the DEF system may not thaw properly, leading to further faults or nox reduction inefficiency
  • In some cases, the vehicle may enter limp mode or prevent restart after shutdown

Common causes, most likely first

  1. Open or damaged wiring in the reductant heater B control circuit, such as a broken wire or corroded connector
  2. Failed reductant heater B element, causing an open circuit
  3. Faulty reductant heater control module or relay, if equipped
  4. Poor electrical connection at the heater or control module, including loose pins or water intrusion
  5. Blown fuse or fusible link supplying power to the heater circuit
  6. PCM internal fault, though rare, causing incorrect circuit monitoring

How to diagnose and fix it

  1. Verify the code with a scan tool and check for any related codes in the reductant system
  2. Inspect the reductant heater B circuit wiring and connectors for damage, corrosion, or looseness
  3. Check the resistance of the reductant heater B element with a multimeter and compare to specifications
  4. Test the control circuit for voltage and continuity from the PCM to the heater, looking for opens or shorts
  5. Check the fuse and relay supplying power to the heater circuit and replace if faulty
  6. If the heater element is open, replace the reductant heater assembly as needed
  7. Clear the code and perform a road test to verify the repair, ensuring the heater activates under cold conditions if possible

Common Missteps in Reductant Heater Diagnosis

  • A common oversight is assuming the heater element itself is faulty without first checking the control circuit wiring, which often suffers from corrosion or damage due to its location under the vehicle.
  • The code can be set by a poor connection at the heater connector even if the heater tests good, so always perform a voltage drop test under load rather than just a continuity check.
  • On some vehicles, the reductant heater B is part of a combined heater and sensor assembly, and replacing just the heater may not be possible, leading to unnecessary parts replacement if not researched.
  • Environmental conditions such as extreme cold can cause the heater to draw higher current, temporarily setting the code, but if the code returns after clearing, a hard fault is present.
  • The PCM may set P20BD if the heater circuit is open, but a shorted heater circuit might set a different code, so do not assume the heater is always at fault when this code appears.

Can I keep driving with P20BD?

You can generally continue driving with this code, but avoid long trips or cold weather operation until repaired, as the DEF system may not function properly and could lead to reduced engine power or limp mode. If the vehicle enters limp mode, seek repair promptly.

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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