Fix P20B9: Mechanic’s solutions that WORKED

The code P20B9: Reductant Heater 1 Control Circuit/Open indicates a fault in the control circuit of the Reductant Heater 1, usually part of the vehicle’s emissions control system, specifically noting an open circuit which means electrical current is not flowing as it should.

P20B9 quick overview

Meaning	P20B9: Reductant Heater 1 Control Circuit/Open
Is it serious?	Yes, this code is serious as it relates to the vehicle’s emissions system and can affect performance and compliance with emissions standards.
Possible causes	– Faulty DEF heater – Open or short in the heater circuit – Poor electrical connection in the heater circuit – Control module issues
How to diagnose?	Check DEF Fluid Level: Ensure the DEF fluid is not empty or contaminated. Inspect Wiring and Connectors: Examine the wiring and connectors for damage, corrosion, or loose connections. Measure Heater Resistance: Check the resistance of the DEF heater to confirm it’s within specifications. Consult TSBs and Recalls: Look for any Technical Service Bulletins or recalls related to the reductant system. Perform System Resets: After repairs, reset the system using an appropriate scan tool. Seek Professional Diagnosis: If unsure, consult with a dealership or a specialist for diagnostic assistance.

How to fix P20B9 – learn from mechanics

I looked into P20B9 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Chevrolet

Vehicle Information:
2012 Chevrolet Silverado 2500 HD LT 6.6L

Issue + Repair History:

• “Service exhaust fluid system” message displayed on the instrument panel.
• Found code P20B9 indicating a reservoir heater circuit fault.
• Replaced the module with the heater and retested, showing 2.2 ohms resistance.
• The message persisted despite clearing service codes and a clean bill of health on the scan.

Other OBD2 Codes + Test Results:

• P20B9: DEF Reservoir heater control circuit, open or high resistance

Suggestions to Fix Issue:

• Run a DEF fluid quality test with a proper scan tool to confirm the system’s functionality.
• Check for any software updates for the ECM and GPCM that might resolve the issue.
• Perform a Reductant System Malfunction Test and possibly a Reductant System Reset with a TECH2 scanner.
• Consider if outside temperature is affecting the DEF heater’s functionality and wait for it to drop below 17°F for automatic testing.

Final Fix:

• Conducted the Reductant fluid quality self-test, which cleared the “Mode A” code but not the warning message.
• A full drive cycle was completed with all tests passed, yet the message persisted.
• Ultimately, the customer took the vehicle to a dealership where after extensive diagnostics, the issue was resolved by reprogramming the ECM with the latest updates.
• It was concluded that the original repair was correct, but a software fault prevented the message from clearing.

Case #2: GMC

Vehicle Information:
2015 GMC Sierra 3500 HD Denali 6.6L

Issue + Repair History:

• Customer ran out of DEF fluid, resulting in reduced speed and service warning.
• Filled DEF but the warning and reduced speed persisted.
• Found P154A code for intake heater and performed fluid quality test, which passed.

Other OBD2 Codes + Test Results:

• P154A: Intake Heater Circuit
• P20B9: Reductant Heater Circuit

Suggestions to Fix Issue:

• Perform an exhaust regeneration using a scan tool/laptop, resolving the DTC issue first.
• Replace the common faulty heater as it’s necessary for system reset.
• Consider driving the vehicle to complete monitors for the set code to allow the truck to reset.
• Check if the vehicle is still under warranty, especially for DEF systems.
• Perform a warning system reset and ensure the heater code is resolved, which has its own reset procedure.
• Check for extended warranties on reductant heaters.

Final Fix:

• After replacing the intake heater to resolve the P154A code, a P20B9 code for the reductant heater appeared.
• Found the tank heater had high resistance and replaced it.
• Post-repair, the warnings were cleared and the vehicle functioned normally.
• The customer opted for repair despite the possibility of warranty coverage due to the distance to the nearest dealer.

Case #3: Chevrolet

Vehicle Information:
2014 Chevrolet Cruze Diesel 2.0L

Issue + Repair History:

• Customer ran out of DEF fluid, leading to reduced speed and a service warning.
• Replaced the DEF heater due to a failure in the resistance test associated with the P20B9 code.

Other OBD2 Codes + Test Results:

• P20B9: DEF heater 1 circuit high

Suggestions to Fix Issue:

• Conduct an exhaust regeneration using a scan tool/laptop after resolving the DTC issue.
• Ensure the heater is replaced as a faulty one will not clear the issue.
• Perform a Reductant System Malfunction Service Bay Test after replacing the DEF tank.
• Consider driving the vehicle to see if it builds pressure on its own.
• Verify if the vehicle is still under warranty for the DEF system.

Final Fix:

• Found that the procedure listed on Mitchell and Identifix had the process in the wrong order.
• The correct sequence is to purge the system first THEN run the quality test and any updates.
• After following the corrected procedure, the system worked fine.
• The vehicle was also taken to a dealership and confirmed to be under an extended warranty for this particular issue.

Case #4: Volkswagen

Vehicle Information:
2015 Volkswagen Golf TDI S 2.0L

Issue + Repair History:

• Vehicle returned with multiple codes related to the reductant system and a leak at the DEF injector.
• Replaced the module and line as per TSB#26 16 03 and refilled DEF.
• Performed resets with VCDS, including resetting the DEF tank level and a full system test which passed.
• Despite recalibrating and confirming full DEF levels, a warning message persisted indicating “Add AdBlue! Engine will not start in 400 miles.”

Other OBD2 Codes + Test Results:

• 22585 (P203B): Reductant level sensor implausible
• 16936 (P20BD): Control Circuit for reductant heating 2, open circuit
• 16931 (P20B9): Control Circuit for reductant heating 1, open circuit
• 28922 (P20E8): Pressure too low

Suggestions to Fix Issue:

• Check if the vehicle is covered by the extended emissions warranty from VW, which might include sensor issues.
• Consider the possibility that the DEF tank sensor is clogged, preventing the ECM from reading the correct level.
• Verify the effectiveness and completion of the diagnostic tool used (VCDS), and consider using ODIS for a more comprehensive diagnosis.
• Inspect for non-warranty issues like rodent damage, which might affect the wiring or components.

Final Fix:

• The vehicle was sent to the dealer for stage two updates and repairs.
• It was discovered that the vehicle had an extended warranty that would have covered all the work done so far and any additional needs.
• No codes were found during the final check.

Case #5: Ford

Vehicle Information:
2011 Ford F-350 Super Duty King Ranch 6.7L

Issue + Repair History:

• MIL on with driver noticing occasional power loss.
• Stored multiple codes related to the reductant system.

Other OBD2 Codes + Test Results:

• P20BA: Reductant Heater A Control Performance
• P0E8: Reductant Pressure Too Low
• P0B9: Heater A Control Circuit Open

Suggestions to Fix Issue:

• Check heater resistance and circuit for Heater A.
• Inquire about accidental DEF contamination with diesel or other substances.
• Consider a DEF tank heater replacement as it’s a common issue.
• Test powers, grounds, and resistance of heater circuits in or on the DEF tank.
• Follow the flow chart to test the resistance of the heater and check for possible software updates.
• Verify DEF fluid level and look for signs of diesel contamination in the DEF tank.

Final Fix:

• Replaced the DEF heater sender assembly which resolved all codes and improved vehicle performance.
• The assembly from Ford was more cost-effective than aftermarket parts.

Advice	Diagnostic Step Description
Check Heater Resistance	Measure the resistance of the DEF heater to ensure it falls within the specified range.
Replace DEF Heater	If resistance is out of spec or if diagnostics point to a faulty heater, replace the DEF heater unit.
Inspect Wiring and Connections	Check the heater’s wiring and connections for damage, corrosion, or looseness.
Check for Diesel Contamination in DEF Tank	Inspect the DEF tank for signs of diesel contamination, which can affect system performance.
Perform System Resets	After repairs, perform system resets using a proper diagnostic tool to ensure all systems are functioning correctly.
Follow Manufacturer’s Flow Chart for Diagnostics	Use the manufacturer’s recommended flow chart to systematically diagnose the issue.
Test Powers, Grounds, and Heater Circuit	Verify the power supply, grounding, and overall circuit integrity of the heater system.
Consult with Dealer or Specialist for Updates	Check for any software updates or recalls that might pertain to the reductant system.