P20B7 - (Diesel Exhaust Fluid) Reductant Pump Heater Control Circuit Low
Theory of Operation
The Powertrain Control Module (PCM) controls the 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 Relay1 (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 independent 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 is stopped 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.
When Monitored:
With the ignition on, while the SCR Module Heater is commanded off.
Set Condition:
The DEF Dosing Control Unit detects low voltage on the SCR Module Heater Supply circuit.
Possible Causes
SCR MODULE HEATER SUPPLY CIRCUIT SHORTED TO GROUND
SCR MODULE HEATER SUPPLY CIRCUIT OPEN/HIGH RESISTANCE
DEF DOSING CONTROL UNIT
POWER DISTRIBUTION CENTER
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 28 - DTC- Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).
1. ACTIVE DTC
NOTE: If DTC P21CA, P21CB, or P21CC are also active, diagnose and repair that DTC before continuing with this test procedure.
1. Turn the ignition on, engine not running.
2. With the scan tool, actuate the DEF Line Heater or Tank Heater Override Test.
3. With the scan tool, View DTCs.
Is the DTC Active?
Yes
• Go To 2
No
• Perform the INTERMITTENT CONDITION - 6.7L. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).
2. CHECK THE (F203) SCR MODULE HEATER SUPPLY CIRCUIT FOR AN OPEN/HIGH RESISTANCE
1. Turn the ignition off.
2. Disconnect the PDC C5 harness connector.
3. Disconnect the DEF Dosing Control Unit C1 harness connector.
4. Measure the resistance of the (F203) SCR Module Heater Supply circuit between the PDC C5 harness connector and the DEF Dosing Control Unit C1 harness connector.
Is the resistance below 5.0 Ohms?
Yes
• Go To 3
No
• Repair the (F203) SCR Module Heater Supply circuit for an open or high resistance.
• Perform the POWERTRAIN VERIFICATION TEST - 6.7L. (Refer to 28 - DTC-Based
Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).
3. CHECK THE (Z905) SCR HEATER RELAY 1 GROUND CIRCUIT FOR AN OPEN/HIGH RESISTANCE
1. Measure for continuity between (Z905) SCR Heater Relay 1 Ground circuit between the PDC C5 harness connector and ground.
Is there continuity between (Z905) SCR Heater Relay 1 Ground circuit and ground?
Yes
• Go To 4
No
• Repair the (Z905) SCR Heater Relay 1 Ground circuit for an open or high resistance.
• Perform the POWERTRAIN VERIFICATION TEST - 6.7L. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).
4. CHECK THE (F203) SCR MODULE HEATER SUPPLY CIRCUIT FOR A SHORT TO GROUND
1. Measure the resistance between ground and the (F203) SCR Module Heater Supply circuit at the DEF Dosing Control Unit C1 harness connector.
Is the resistance above 10k Ohms?
Yes
• Go To 5
No
• Repair the (F203) SCR Module Heater Supply circuit for a short to ground.
• Perform the POWERTRAIN VERIFICATION TEST - 6.7L. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).
5. CHECK THE PDC OUTPUT
1. Reconnect the PDC C5 harness connector.
2. Turn the ignition on and start the engine.
3. Measure the voltage of the (F203) SCR Module Heater Supply circuit at the DEF Dosing Control Unit C1 harness connector.
Is the voltage approximately equal to battery voltage?
Yes
• Replace the DEF Dosing Control Unit in accordance with the service information.
• Perform the POWERTRAIN VERIFICATION TEST - 6.7L. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).
No
• Check two fuses (F19-30A and F75-10A) in the PDC that both feed the SCR 1 (Heater) Relay for an open. Check the connectors at the DEF Dosing Control Unit and PDC for spread terminals causing an intermittent connection. If the fuses and connectors are good, replace the Power Distribution Center (PDC) in accordance with the Service Information.
• Perform the POWERTRAIN VERIFICATION TEST - 6.7L. (Refer to 28 - DTC-Based Diagnostics/MODULE, Powertrain Control (PCM) - Standard Procedure).