What Does P20AC Mean?
The ECM has detected a malfunction in the temperature sensor circuit that monitors the reductant (DEF) metering unit. This prevents the system from accurately monitoring DEF temperature, which is critical for proper dosing and freeze protection.
Common Causes
30%
Damaged or corroded temperature sensor connector causing intermittent or open circuit
30%
Failed reductant temperature sensor with open or short circuit internally
20%
Broken or chafed wiring harness between sensor and ECM
15%
Water intrusion or corrosion in sensor or connector causing electrical fault
5%
Failed ECM sensor input circuit or control module fault
Diagnostic Steps
1
Step 1: Locate and inspect the DEF metering unit temperature sensor connector for corrosion, damage, or loose connection
2
Step 2: Measure sensor resistance with connector disconnected; compare to temperature-resistance chart for thermistor sensors
3
Step 3: Check sensor signal voltage at ECM connector with sensor connected; verify voltage changes with temperature
4
Step 4: Test wiring harness continuity and resistance from sensor connector to ECM connector on both signal and ground wires
5
Step 5: If sensor and wiring test within specifications, monitor live data during temperature change cycle to verify proper response
Estimated Repair Cost
$150 - $500
Parts + labor, varies by vehicle and location
The ECM has detected a malfunction in the temperature sensor circuit that monitors the reductant (DEF) metering unit. This prevents the system from accurately monitoring DEF temperature, which is critical for proper dosing and freeze protection.
The most common cause of P20AC (Reductant Metering Unit Temperature Sensor Circuit) is: Damaged or corroded temperature sensor connector causing intermittent or open circuit
Typical repair costs for P20AC range from $150 to $500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
System cannot protect DEF from freezing or compensate dosing for temperature. Vehicle may enter limp mode in extreme temperatures or prevent starting in cold conditions.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P20AC to identify the root cause.
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Quick Info
Category
Powertrain
System
Diesel Exhaust Fluid (DEF) Injection System / Selective Catalytic Reduction (SCR)
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
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