P20AF — Reductant Metering Unit Temperature Sensor Circuit High #

Note: The description for code P20AF may vary depending on the vehicle manufacturer (OEM).

• Reductant Dosing Unit Temperature Sensor Circuit High (General Motors, Ford, FCA/Stellantis)
• SCR Metering Unit Temp Sensor Signal High (BMW, Mercedes-Benz)
• DEF Pump Temperature Sensor Circuit High Input (Toyota, Nissan)

Code P20AF — Overview #

Diagnostic Trouble Code P20AF relates to the Reductant Metering Unit Temperature Sensor Circuit reporting a high electrical signal. The Reductant Metering Unit (also called the DEF dosing pump) is a key part of the Selective Catalytic Reduction (SCR) system, which reduces harmful emissions in diesel vehicles by injecting Diesel Exhaust Fluid (DEF) into the exhaust. A temperature sensor monitors the dosing unit to ensure it operates within safe limits. When the Powertrain Control Module (PCM) detects the sensor reporting unusually high voltage (above the expected range), it sets code P20AF and may illuminate the check engine light. This fault can disrupt the SCR system’s ability to lower NOx emissions and may lead to performance issues or emission test failures.

Common Symptoms for Code P20AF #

Drivers or technicians may notice a range of symptoms if code P20AF is present. In some cases, symptoms are subtle; in others, they become pronounced as the SCR system is affected:

• Illuminated check engine light (MIL on dashboard)
• Reduced engine power or limp mode in some models
• SCR/DEF warning messages or countdown to no restart
• Failed emissions test
• Occasional excessive exhaust smoke or odor (if DEF dosing is impacted)

Probable Causes for Code P20AF #

Code P20AF is most often caused by electrical or sensor faults in the temperature monitoring circuit of the reductant metering unit. The main causes, ranked by likelihood, are:

1. Faulty temperature sensor in the reductant (DEF) metering unit
2. Open or short circuit in the wiring to the sensor (corroded, damaged, or disconnected wires)
3. Poor electrical connections at the sensor or harness
4. Defective reductant (DEF) metering unit (internal electrical fault)
5. Failed PCM/ECM (rare; typically after eliminating other causes)

Step-by-Step Diagnosis for Code P20AF #

Diagnosing P20AF requires a systematic approach to avoid unnecessary part replacements. Here’s a proven process:

• 1. Verify the code: Confirm P20AF is present and check for related SCR, DEF, or sensor codes.
• 2. Visual inspection: Examine wiring and connectors at the reductant dosing unit for corrosion, damage, or loose fittings.
• 3. Check sensor resistance: With the vehicle off and connector unplugged, measure resistance across the temperature sensor. Compare readings with OEM specifications.
• 4. Inspect circuit voltage: Key on, engine off—backprobe the sensor connector for proper reference voltage and ground.
• 5. Test continuity: Confirm there’s no open or short circuit between the PCM and the sensor using a multimeter.
• 6. Substitute component: If all else fails, temporarily swap a known-good sensor or metering unit to see if the code clears.
• 7. Clear codes and retest: After repairs, clear DTCs and perform a road test to ensure the fault does not reoccur.

Possible Repairs for Code P20AF #

Once the root cause is identified, repairs should directly address the diagnosed issue. Common solutions include:

• Replace the temperature sensor in the reductant metering unit (if sensor is faulty)
• Repair or replace damaged wiring/connectors (if corrosion, breaks, or shorts are found)
• Replace the reductant metering unit (if internal failure is confirmed)
• Clean and secure electrical connections (if poor contact is suspected)
• Replace or reprogram PCM/ECM (only after all other causes are ruled out)

Is It Safe to Drive with Code P20AF? #

Driving with code P20AF is generally not recommended for an extended period. While your vehicle may still run, the SCR system will not properly control emissions, potentially leading to increased NOx output and a failed emissions test. Some vehicles will enter a reduced power (limp) mode or initiate a countdown to no restart if the issue is not resolved. Short trips to a repair facility are usually acceptable, but address the fault as soon as possible to avoid further complications.

Vehicles Most Commonly Affected by P20AF #

Code P20AF is most frequently reported on late-model diesel vehicles equipped with SCR/DEF systems. Makes and models where this DTC is more common include:

• General Motors: Chevrolet Silverado/GMC Sierra Duramax, Express/Savana vans
• Ford: F-Series Super Duty PowerStroke
• RAM (FCA/Stellantis): 2500/3500/4500/5500 Cummins
• Mercedes-Benz: Sprinter vans, ML/GL/GLS BlueTEC
• BMW: X5 35d, 328d, 535d
• Volkswagen/Audi: Touareg TDI, Q7 TDI
• Toyota/Nissan: Diesel variants with DEF systems (market dependent)

Common Mistakes & Best Practices for Code P20AF #

Misdiagnosis is frequent with SCR system codes due to their complexity. To avoid wasted time and money, keep in mind:

• Don’t replace the entire metering unit before testing the sensor and wiring.
• Always check for technical service bulletins (TSBs) related to SCR or DEF sensor issues for your specific vehicle.
• Use OEM or high-quality parts for sensor or dosing unit replacements; cheaper components may not last.
• Do not ignore connector corrosion—even small amounts can cause this code.
• Clear codes only after a successful repair and retest to confirm the fix.

Express FAQ for Code P20AF #

For quick answers, here are some of the most common questions about P20AF:

• Q: Can a bad DEF fluid cause code P20AF?
  A: Not directly. P20AF specifically relates to the sensor circuit. However, using contaminated or incorrect DEF can eventually damage dosing components.
• Q: Will clearing the code fix the problem?
  A: No. The code will return if the underlying electrical or sensor issue is not resolved.
• Q: Is this an expensive repair?
  A: Often, it’s a straightforward fix (sensor or wiring). If the entire metering unit or SCR module is faulty, costs can be higher.