Review the 'Possible Causes' mentioned above and visually examine the corresponding wiring harness and connectors. Ensure to check for any damaged components and inspect the connector pins for signs of being broken, bent, pushed out, or corroded.
To diagnose the P20B8 code, it typically requires 1.0 hour of labor. The specific diagnosis time and labor rates at auto repair shops can differ based on factors such as the location, make and model of the vehicle, and even the engine type. It is common for most auto repair shops to charge between $75 and $150 per hour.
The Powertrain Control Module (PCM) controls the Diesel Exhaust Fluid (DEF) Heaters via datalink communication with the Diesel Exhaust Fluid Dosing Control Unit (DEF DCU - sometimes referred to as the Selective Catalytic Reduction (SCR) Module). The DEF DCU receives two voltage supplies: One from the SCR Relay 1 (which is activated upon startup). And the other is a fused B+ supply. The DEF Tank electric heater and DEF Line Heater power supply and ground circuits are wired directly to the DCU. The DEF Tank Heater is integrated into the DEF Supply Pump assembly and is not serviced separately.
Activation of the DEF Supply Line and DEF Tank Heaters operate independently of each other. Line heating is based on the Ambient Temperature Sensor input and Tank heating is based on the DEF Tank Temperature Sensor value. Heating of the Line or Tank Heater is initiated when the corresponding temperature value is below -5°C (23°F). Heating stops when the corresponding temperature value goes above -4°C (24.8°F). The DEF Dosing Control Unit monitors these inputs and reports them to the Powertrain Control Module (PCM). The PCM will illuminate the MIL lamp after this diagnostic runs and fails. The PCM will turn off the MIL light when the monitor runs and passes in three consecutive key cycles. The Diagnostic Trouble Code (DTC) will be set when the DEF Dosing Control Unit detects that the SCR Module Heater Supply circuit voltage is above a calibrated value.
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