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DTC codes / P20xx · Fuel and air metering and auxiliary emission controls / P20B0

P20B0Reductant Metering Unit Temperature Sensor Circuit Intermittent/Erratic

P20B0 indicates that the powertrain control module (PCM) has detected an intermittent or erratic signal from the reductant metering unit temperature sensor, which is part of the diesel exhaust fluid (DEF) injection system on selective catalytic reduction (SCR) equipped vehicles. The PCM monitors this sensor to ensure the DEF is at a proper temperature for accurate dosing, and an erratic signal means the voltage readings are jumping or dropping out unpredictably rather than staying within a normal range.

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
  • Reduced engine power or limp mode, especially under load or during regeneration
  • Poor fuel economy or increased DEF consumption due to incorrect dosing
  • Rough running or hesitation if the SCR system cannot maintain proper DEF injection
  • A warning message on the dashboard indicating a DEF system fault or service required
  • In rare cases, the engine may enter a derate condition after a certain number of fault cycles

Common causes, most likely first

  1. Faulty reductant metering unit temperature sensor or its internal circuitry
  2. Damaged, chafed, or corroded wiring harness at the sensor connector or along the harness route
  3. Loose or contaminated electrical connector pins causing intermittent contact
  4. DEF fluid contamination or crystallization affecting sensor readings
  5. Failed reductant metering unit (injector) assembly where the sensor is integrated
  6. PCM software issue or internal driver fault (less common)
  7. Extreme temperature fluctuations or vibration causing sensor signal instability

How to diagnose and fix it

  1. Use a scan tool to retrieve all stored codes and freeze-frame data, then clear codes and test-drive to see if P20B0 returns
  2. Visually inspect the reductant metering unit temperature sensor connector and wiring for damage, corrosion, or loose pins
  3. Measure the sensor resistance or voltage output with a multimeter and compare to manufacturer specifications
  4. Check for technical service bulletins (TSBs) or software updates for the PCM related to this code
  5. Perform a wiggle test on the harness while monitoring sensor data to identify intermittent wiring faults
  6. If the sensor is integrated into the reductant metering unit, replace the unit as an assembly after confirming faulty readings
  7. After repairs, clear codes and verify the sensor signal remains stable during a road test with active DEF dosing

Intermittent sensor faults often misdiagnosed

  • A common oversight is replacing the entire reductant metering unit when the actual problem is a chafed wire hidden behind the engine block or near the exhaust, so always perform a thorough harness inspection first.
  • The code can be set by a temporary low DEF level or air pockets in the DEF line causing erratic sensor readings, so check fluid level and bleed the system before condemning parts.
  • In some vehicles, the reductant metering unit temperature sensor shares a reference voltage with other sensors, and a fault in a different sensor on the same circuit can cause P20B0 to appear.
  • Intermittent faults may not illuminate the check engine light immediately; they can be stored as pending codes and only trigger the light after multiple drive cycles, so a scan tool that reads pending codes is essential.
  • Aftermarket DEF fluid that is contaminated or diluted can cause sensor readings to fluctuate, leading to this code without any hardware failure.

Can I keep driving with P20B0?

You can generally continue driving with P20B0, but avoid heavy loads and long trips until diagnosed, as the fault may escalate to a derate condition that limits vehicle speed. If the check engine light starts flashing or you notice significant power loss, stop driving and seek service immediately.

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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