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DTC codes / P20xx · Fuel and air metering and auxiliary emission controls / P20AF

P20AFReductant Metering Unit Temperature Sensor Circuit High

P20AF means the engine control module (ECM) has detected a voltage signal from the reductant metering unit temperature sensor that is higher than the expected range for the current operating conditions. The reductant metering unit is part of the diesel exhaust fluid (DEF) injection system, and its temperature sensor helps the ECM protect the unit from overheating or freezing; a high signal typically indicates an open circuit, a short to voltage, or a sensor that has failed internally.

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 message on the dashboard indicating a problem with the emissions system or diesel exhaust fluid (DEF)
  • Reduced engine power or limp mode if the fault triggers protective strategies
  • Increased DEF consumption or frequent refill warnings in some cases
  • Difficulty starting the engine or rough running if the ECM cannot verify DEF system readiness
  • Possible failure to complete a regeneration cycle, leading to further emissions-related codes

Common causes, most likely first

  1. Open circuit in the temperature sensor signal wire or ground circuit, such as a broken wire or corroded connector
  2. Short to voltage on the sensor signal circuit, often from chafed wiring contacting a power source
  3. Faulty reductant metering unit temperature sensor (internal failure causing high resistance)
  4. Poor electrical connection at the sensor or ECM connector, including loose pins or water intrusion
  5. Failed ECM driver or internal circuit for the sensor (less common but possible)
  6. Aftermarket or incorrect sensor installed previously that does not meet specifications

How to diagnose and fix it

  1. Use a scan tool to read the live data for the reductant metering unit temperature sensor and compare it to ambient temperature after a cold soak; a reading far above ambient confirms a high signal fault.
  2. Visually inspect the sensor connector and wiring for damage, corrosion, or loose pins, and repair any obvious issues before replacing parts.
  3. Disconnect the sensor and measure resistance across the sensor terminals; compare to the manufacturer's specification for the current temperature (a very high or infinite reading indicates an open sensor).
  4. Check the signal circuit for a short to voltage by measuring voltage at the sensor connector with the key on and the sensor disconnected; any significant voltage indicates a wiring fault.
  5. Check the ground circuit for continuity to chassis ground; repair any open or high-resistance ground.
  6. If wiring and connections are good, replace the reductant metering unit temperature sensor (or the entire metering unit if the sensor is not serviceable separately).
  7. Clear the diagnostic trouble codes (DTCs) and perform a road test or a DEF system verification test with the scan tool to confirm the repair.

What the P20AF code really tells you

  • A high signal does not always mean the sensor itself has failed; wiring harness damage from road debris or heat is a frequent cause, especially near the metering unit.
  • The reductant metering unit temperature sensor is often integrated into the DEF injector or metering unit, so replacing just the sensor may not be possible on every vehicle.
  • This code can be set by a low DEF level or a frozen DEF tank if the vehicle has been sitting in extreme cold, so check fluid level and condition before condemning electrical parts.
  • Some vehicles will not illuminate the check engine light for this code until the fault has been present for two consecutive drive cycles, so a pending code may be the only clue.
  • A failed sensor can cause the ECM to inhibit DEF dosing, which may lead to a derate and eventually a no-start condition if ignored, so address it promptly.

Can I keep driving with P20AF?

You can usually continue driving to a repair shop, but avoid long trips or heavy loads because the ECM may reduce engine power as the fault persists. Do not ignore the warning light, as continued driving with a faulty temperature sensor can lead to more severe emissions system failures and potential derate.

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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