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DTC codes / P20xx · Fuel and air metering and auxiliary emission controls / P20B7

P20B7Reductant Metering Unit Heater Control Circuit Low

P20B7 indicates that the powertrain control module (PCM) has detected an abnormally low voltage or current draw on the control circuit for the reductant metering unit heater, which is part of the diesel exhaust fluid (DEF) injection system. The heater is responsible for keeping the DEF from freezing and ensuring proper flow, so a low circuit condition can prevent the system from functioning correctly and may trigger the malfunction indicator lamp (MIL).

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

Symptoms you may notice

  • Illumination of the malfunction indicator lamp (MIL) on the dashboard.
  • Often no other noticeable symptoms besides the warning light.
  • Possible reduction in engine power or limp mode if the DEF system is disabled.
  • A message on the driver information center indicating DEF system fault or service required.
  • Increased DEF consumption or failure to inject DEF, leading to potential emissions system issues.
  • In cold weather, the DEF may freeze, causing further system faults and poor engine performance.

Common causes, most likely first

  1. Open or high-resistance wiring in the heater control circuit, such as a broken wire or corroded connector.
  2. Failed reductant metering unit heater element (open circuit internally).
  3. Poor electrical connection at the heater or PCM, including loose pins or water intrusion.
  4. Faulty powertrain control module (PCM) driver circuit for the heater control.
  5. Blown fuse or relay supplying power to the heater circuit.
  6. Short to ground in the control circuit causing low voltage detection.

How to diagnose and fix it

  1. Verify the code with a scan tool and check for any related diagnostic trouble codes (DTCs) in the reductant system.
  2. Inspect the wiring harness and connectors at the reductant metering unit and PCM for damage, corrosion, or loose pins; repair as needed.
  3. Check the fuse and relay that supply power to the reductant heater circuit; replace if faulty.
  4. Measure resistance of the reductant metering unit heater element and compare to manufacturer specifications; replace the unit if out of range.
  5. Perform a voltage drop test on the control circuit to identify excessive resistance or an open circuit.
  6. If all wiring and components test good, suspect a faulty PCM and perform further diagnostics before replacement.

Common Misdiagnosis Traps for P20B7

  • A low circuit code does not always mean the heater element is defective; a poor ground connection at the chassis can cause the same symptom and is often overlooked.
  • The reductant metering unit heater is controlled by the PCM via a pulse-width modulated (PWM) signal, so a standard multimeter may not show correct voltage; an oscilloscope is needed for accurate testing.
  • Aftermarket DEF or contaminated fluid can cause heater element failure, so always check fluid quality before replacing parts.
  • This code can be set by a intermittent connection that only fails under certain conditions, such as vibration or temperature changes, making it difficult to reproduce.
  • Related codes like P20B8 (circuit high) or P20B9 (circuit performance) may also be present, indicating a more complex issue with the heater control circuit.

Can I keep driving with P20B7?

You can usually continue driving with this code, but avoid long trips or cold weather conditions until repaired, as the DEF system may be disabled and could lead to reduced engine power or increased emissions. Monitor for other warning messages and address the issue promptly to prevent further damage.

Tools that help

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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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