Easy one-page printout for the garage or repair shop
This summary uses the existing diagnostic information shown in full below.
This code means that the car's system that cleans up harmful gases before they leave the car isn't working well enough.
The P20EF code indicates a problem with the Selective Catalytic Reduction (SCR) system, specifically that it is not reducing nitrogen oxides (NOx) as efficiently as it should before the exhaust reaches the catalyst in Bank 1.
The SCR system is designed to reduce NOx emissions from diesel engines by injecting a urea-based solution (DEF) into the exhaust stream. This code is triggered when the system's pre-catalyst NOx reduction efficiency falls below a predetermined threshold, indicating that the system is not performing as expected.
• Check Engine Light is on
• Increased NOx emissions
• Possible reduction in engine performance
• Faulty SCR catalyst
• Defective NOx sensor
• Improper DEF quality or level
SCR: Selective Catalytic Reduction
NOx: Nitrogen Oxides
DEF: Diesel Exhaust Fluid
• Check for other related codes
• Inspect and test the NOx sensors
• Verify the quality and level of DEF
• Replace the SCR catalyst if faulty
• Replace or clean the NOx sensors
• Refill or replace the DEF with the correct quality
• OBD-II scanner
• NOx sensor tester
• DEF refractometer
$500 - $1500
Moderate
• Ford F-250
• Chevrolet Silverado 2500
• Ram 2500
• Regularly check DEF levels
• Use high-quality DEF
• Perform regular maintenance on the SCR system
Emissions Control
• Have the vehicle inspected by a professional if the Check Engine Light is on
• Do not ignore the warning as it can lead to more severe engine problems
SCR components may be covered under emissions warranty. Check with your dealer.
This code has become more common with the increased use of SCR systems in diesel engines to meet emissions regulations.
Moderate – Addressing this issue is important to maintain emissions compliance and vehicle performance.
Operating a vehicle with malfunctioning emissions controls may be illegal and could result in fines.
• Vehicle at operating temperature
• Steady driving conditions
The SCR system is located in the exhaust system, typically under the vehicle.
Q: Can I drive with a P20EF code?
A: While the vehicle may still be drivable, it is not recommended as it can lead to increased emissions and potential damage to the SCR system.
Q: What is DEF?
A: DEF stands for Diesel Exhaust Fluid, a solution used in SCR systems to reduce NOx emissions.
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A wired color-screen reader with readiness hotkeys, live data, code definitions, and data logging.
Good for emissions-related codes because it exposes readiness monitors and live powertrain data.
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Specialty · Smoke machine
Helpful for finding EVAP and vacuum leaks that are hard to see with visual inspection alone.
Helpful for tracking down EVAP and vacuum leaks that can trigger emissions faults.
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Specialty · O2 sensor socket
Helpful when the code points toward exhaust or oxygen-sensor work and space is tight.
Useful when the diagnosis points toward oxygen-sensor removal or replacement.
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Fitment Required · Vehicle-fitment part
Relevant when catalyst-efficiency or emissions faults persist after proper diagnosis.
Catalyst-efficiency faults sometimes point toward converter problems after proper testing.
Catalytic converters are highly vehicle-specific and should not be purchased by code alone.
View options on AmazonThese parts may be related to this fault category, but a code by itself does not confirm exact fitment or guarantee that replacing a part will solve the problem.