Fix P204F: Mechanic’s solutions that WORKED

The code P204F: Reductant Quality Performance points out a fault in the Diesel Exhaust Fluid (DEF) system, indicating that the DEF does not meet the required specifications. This is most likely caused by issues with the NOx sensors, DEF quality, or system components.

P204F quick overview

Meaning	P204F: Reductant Quality Performance
Is it serious?	Yes, it can lead to reduced vehicle performance and potential damage to the emission system.
Possible causes	– Faulty or erratic NOx sensors, especially the rear one. – Poor quality or contaminated Diesel Exhaust Fluid (DEF). – Malfunctioning DEF system components such as the pump or injector. – Software issues in the Engine Control Module (ECM). – Damaged or poor ground connections.
How to diagnose?	Check ECM Software: Ensure it’s up-to-date. Monitor NOx Sensors: Use diagnostic tools to check readings, especially from the rear sensor. Inspect DEF Quality: Ensure it meets required specifications and isn’t contaminated. Run Reductant Fluid Quality Test: Multiple times if necessary. Inspect DEF Components: Check the pump, injector, and other related parts for proper function. Conduct a Road Test: Monitor NOx sensor readings and overall system performance. Check Electrical Grounds: Ensure they are secure and not corroded. Consider Factory Resets or Reflashes: Especially after installing new components.
What is Reductant?	Reductant, or Diesel Exhaust Fluid (DEF), is a liquid used in diesel engines to reduce harmful emissions. It’s mainly a mix of urea and water. When added to the exhaust, it turns harmful gases into harmless ones.
What is NOx sensor?	A NOx sensor measures harmful nitrogen oxide levels in a vehicle’s exhaust. It helps the engine control system reduce pollution.

How to fix P204F – learn from mechanics

I looked into P204F 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: Ford

• 2015 Ford Transit-250 3.2L

Issue + Repair History:

• MIL (Malfunction Indicator Lamp) is on.
• Bad smell from the exhaust.
• Code P204f- Reductant system performance bank 1 is present.
• No other codes were found.
• No repair history available.

Suggestions from the Community:

1. Check if the vehicle is consuming an excessive amount of DEF fluid and if the exhaust smells like urea nitrogen. Perform a hard reset and see if MIL returns with KOEO.
2. The code P204f is an information code only. There should be other codes like a p242d. Fix all other codes first, then perform a drive cycle, and the code should clear. Consider taking a DEF sample to ensure its quality.
3. The issue might be with the nox sensor or may require a new DEF tank.
4. This is an information code only and indicates there should be DTCs set in the RDCM. The vehicle may operate in derate mode until DTCs are cleared. Check if the scan tool shows the RDCM.
5. Check the heater control circuit on the 8 pin connector. If the reading is 4.2ohms across pins 1 and 2, a new heater and sender unit is needed. The spec should be between 1 and 2.5ohms.
6. Ensure that someone didn’t mistakenly put diesel in the DEF tank.

Final Fix:

• The tank heater was tested and found to be faulty. The heater was replaced, and the vehicle was shipped. The technician’s scanner did not show the heater code, only the P204f code. Testing in generic mode revealed the heater code.

Case #2: GMC

Vehicle Information:

• 2013 GMC Sierra 3500 HD SLE 6.6L

Issue + Repair History:

• MIL (Malfunction Indicator Lamp) is on.
• Codes P204b and P204f for reductant system performance are present.
• The pump had been previously replaced, and the codes did not return for several weeks.
• Scantool data indicates 65 PSI reductant pressure, which drops off quickly after shutting off the pump.
• The injector was inspected and found not to be bleeding down.
• No signs of reductant leakage were observed.

Suggestions from the Community:

1. On certain vans, the pressure drops quickly once the pump stops. A white buildup usually indicates a leak. The DEF injector should be removed to check for 3 conical spray patterns. A scan tool can be used to operate the DEF injector and check the spray patterns.
2. The DEF quantity test should be run. The pressure should be 500kpa when the pump is commanded. If the on-time percentage is 18% or more, there might be supply side restrictions.
3. The pump typically makes 130psi. There’s a filter in the tank, and it should be checked if the tank was removed and cleaned. The pumps can be fragile.
4. The pumps on these tanks can’t be easily replaced; the entire tank usually needs replacement. If a new pump was forced into an old tank, it might be damaged. The issue might be with the sensors in the tank or the exhaust.
5. If the code p204b is set, there might be an issue with low pressure or the pressure sensor wiring. The 65 psi reading is on the low side. Contamination in the DEF tank might damage the pump.

Final Fix:

• The tank was removed and found to be full of crud. After replacing the tank unit, everything was resolved. Self-tests were performed, and the warning was turned off.

Case #3: Mercedes

Vehicle Information:

• 2013 Mercedes-Benz Sprinter 2500 3.0L

Issue + Repair History:

• Vehicle displayed a message “STARTS REMIN 8” along with several other mechanical issues like tires, brakes, filters, oil change, etc.
• DEF injector was cleaned of crystal build-up, bench tested with a 9-volt battery, and reinstalled. No pending codes were found after a test drive.
• After 508 miles, the customer returned with the codes listed.
• DEF Injector had been cleaned, bench tested, and reinstalled. Since then, the customer drove 508 miles and returned with the mentioned codes.

Suggestions from the Community:

1. Focus on the 2047 code first. The injector might be going open circuit when it’s hot. Conduct an ohm test for wiring from PCM to injector and load test the circuits. Check pin grip at both PCM and injector ends.
2. Using an Autel Maxisys, run a reductant quantity injection test to check the volume and spray pattern on the injector, pump pressure, and other parameters. However, once the “limited starts” message appears, a factory tool might be needed to clear it. Some SCR components might have an extended warranty from the factory.
3. An Autel 908 Pro can reset the start timer. There was a case where a vehicle was locked out with no more starts, and this tool worked.

Final Fix:

• The issue was due to a combination of low-quality/contaminated fuel and low-quality/contaminated DEF fluid. Both the fuel and DEF fluid were drained and replaced. Additionally, both NOX sensors and the DEF fluid injector were replaced.

Case #4: Mercedes

• 2014 Mercedes-Benz GLK250 Bluetec 4Matic 2.1L

Issue + Repair History:

• Vehicle had nine codes, most related to the bluetec system.
• Nox Sensors were tested, replaced, and SCN coding was done using Autel 908p.
• A code remained in the ME indicating the number of starts left.
• The DEF injector was cleaned, tested, and reinstalled.
• After the “learn in” for the adblue metering, the warning disappeared, and no codes were detected during a scan.
• However, the Adblue countdown persisted, showing 6 starts left.

Suggestions from the Community:

1. Check the faults and events in the CDI control unit and the SCR control unit. Allow the CAN network to sleep for several hours.
2. Xentry might be needed to reset the Adblue countdown.
3. After all repairs and clearing adaptations, drive the vehicle on the highway for up to 50 miles for the system to reset.
4. Despite the Autel’s claims, it might not be able to reset the Adblue countdown. Consider sending the vehicle to Mercedes for a reset.
5. Some technicians have been able to reset the Adblue countdown using the Autel 908pro.

Final Fix:

• The vehicle was sent to Mercedes. They confirmed that all repairs were done correctly. The only action needed was to reset the Adblue countdown and perform adaptations and learn-in processes using the factory tool. After the reset, a long drive was necessary for the system to recalibrate. The Autel tool was found to be lacking in this capability, despite its claims.

Case #5: Chevrolet

• 2011 Chevrolet Silverado 3500 HD WT 6.6L

Issue + Repair History:

• The vehicle had issues with the DEF/SCR system.
• Initially, the vehicle displayed a DEF light with a message “Service DEF System NOW, Speed Limited to 55Mph.”
• Four codes were found, with P207F not being present initially.
• During the Reductant System Leak Test, no pressure was built.
• Fresh DEF was added to the tank.
• The DEF tank was removed and replaced with a new tank assembly from GM, which included the DEF pump assembly/module and level sensor/heater module.
• After the new parts were installed, pressure was built in the system.
• The DEF quality test was run multiple times, with varying results.
• The P207F code, indicating “Incorrect Reductant Composition,” was set after the second DEF quality test.
• The vehicle had been previously flashed in May 2014.

Suggestions from the Community:

1. Ensure the ECM software is up-to-date.
2. Check for a bad ground at g400.
3. Reflash the system since a new unit was installed.
4. Monitor NOx sensor readings during a road test. The rear NOx sensor is often the cause of P207F. During a coast-down after a full-throttle acceleration, the rear NOx sensor reading should go very low (under 20ppm). If not, the rear sensor might be faulty.
5. Perform a “scan tool Reductant System Reset procedure” to reset the tank level sensor calibration when the tank is replaced or serviced.
6. The vehicle might still be under the diesel 5/100mi warranty, which covers DEF components and NOx sensors.
7. The Reductant Fluid Quality Test must be run three consecutive times to ensure the Reductant System history resets and completes estimating the ammonia load.

Final Fix:

• The vehicle’s issue was resolved with a completely new DEF tank assembly, which included the level sensors/heater module and the pump module.
• The DEF quality test was run multiple times, and after the third test, the warning light went out, and the SCR catalyst monitor became Ready.
• The rear NOx sensor did not need replacement.
• The vehicle had been driven about 750 miles since the repair without any issues.

How to diagnose P204F

I made you an overview of solving this DTC, based on all the advice that was suggested by certified mechanics to address this issue in previously mentioned service cases.

• Update ECM Software: Ensure it’s current.
• Monitor NOx Sensors: Especially the rear one; replace if faulty.
• Run Reductant Fluid Quality Test: Multiple times for system reset.
• Inspect DEF Components: Use fresh DEF and ensure proper system pressure.
• Reflash/Reset System: After new component installation.
• Conduct Road Test: Monitor system performance.
• Check Grounds: Especially locations like g400.
• Consider Warranty: Some components might be covered.