TL;DR

P20EE means the SCR isn’t reducing NOx enough. Check DEF quality, NOx sensors, injector, leaks, and ECM updates. Use iCarsoft CR MAX to verify data and clear codes.

The P20EE trouble code is an OBD-II diagnostic code commonly associated with diesel vehicles equipped with Selective Catalytic Reduction (SCR) systems, indicating a problem with the NOx (nitrogen oxides) reduction process. This article provides a comprehensive examination of P20EE, covering its definition, causes, symptoms, diagnostic steps, and solutions. Supported by real-world case studies—such as issues in the Ram 2500 and other models—this guide offers practical insights for vehicle owners and technicians to effectively resolve this fault.

1. What is P20EE?

P20EE is an OBD-II diagnostic trouble code (DTC) defined as "SCR NOx Catalyst Efficiency Below Threshold (Bank 1)." It indicates that the Engine Control Module (ECM) has detected that the SCR system on Bank 1 (typically the only bank in most diesel engines) is not reducing NOx emissions as effectively as expected. The SCR system uses Diesel Exhaust Fluid (DEF, also known as AdBlue) to convert NOx into harmless nitrogen and water, and a failure to meet efficiency thresholds triggers P20EE.

Role of the SCR System

The SCR system is part of the exhaust aftertreatment process in diesel vehicles, designed to reduce NOx emissions. DEF is injected into the exhaust stream, where it reacts with NOx in the SCR catalyst to produce nitrogen and water vapor. NOx sensors before and after the SCR catalyst monitor its efficiency. If the reduction in NOx levels is below the expected threshold, the ECM triggers P20EE.

Common Trigger Conditions

• SCR catalyst efficiency falls below the minimum threshold.
• Issues with DEF quality, injection, or NOx sensors.
• Problems with the SCR catalyst or related exhaust components.

2. Potential Causes and Specific Issues of P20EE

P20EE can stem from various factors. Below is a detailed list of common causes, supported by real-world examples:

• (1) Contaminated or Poor-Quality DEF
  Issue: Low-quality or contaminated DEF reduces the SCR system’s ability to convert NOx.
  Example: In a 2018 Ram 2500, P20EE was triggered due to contaminated DEF purchased from a non-reputable source, reducing SCR efficiency.
  Cause: Improper DEF storage or use of incorrect fluid.
• (2) Faulty NOx Sensors
  Issue: A malfunctioning upstream or downstream NOx sensor provides incorrect data, leading to a perceived efficiency issue.
  Example: A 2017 Ford F-250 exhibited P20EE because of a failing downstream NOx sensor, reporting inaccurate NOx levels.
  Cause: Sensor wear or exposure to high exhaust temperatures.
• (3) DEF Injector Issues
  Issue: A clogged or malfunctioning DEF injector fails to deliver the correct amount of DEF, impairing NOx reduction.
  Example: A 2019 Chevrolet Silverado 2500HD triggered P20EE due to a clogged DEF injector, caused by crystallized DEF residue.
  Cause: Poor DEF quality or lack of system maintenance.
• (4) Damaged or Degraded SCR Catalyst
  Issue: The SCR catalyst is damaged, contaminated, or degraded, reducing its efficiency.
  Example: A 2016 Volkswagen Touareg TDI had P20EE due to a contaminated SCR catalyst, caused by engine oil entering the exhaust from a failing turbo.
  Cause: Oil or coolant contamination, or prolonged exposure to high temperatures.
• (5) ECM Software or Exhaust System Issues
  Issue: ECM software glitches or exhaust leaks affect SCR performance.
  Example: A 2020 Mercedes-Benz Sprinter showed P20EE due to an ECM software issue, resolved with a firmware update.
  Cause: Outdated software or exhaust leaks introducing excess oxygen.

3. Symptoms of P20EE

Symptoms of P20EE are often related to emissions and performance limits. Here are common signs and their associated causes:

• Check Engine Light or DEF Warning On
  Symptom: MIL or DEF warning light illuminates.
  Cause: ECM detects low SCR efficiency.
• Reduced Engine Power (Limp Mode)
  Symptom: Vehicle enters a reduced power mode.
  Cause: ECM limits performance to protect the engine and comply with emissions regulations.
• Increased DEF Consumption
  Symptom: DEF tank depletes faster than normal.
  Cause: System overcompensates by injecting more DEF.
• Failed Emissions Test
  Symptom: Vehicle fails emissions inspection.
  Cause: Elevated NOx emissions due to poor SCR efficiency.
• Exhaust Odor
  Symptom: Noticeable ammonia smell from the exhaust.
  Cause: Excess DEF injection or incomplete NOx reduction.

4. Vehicles Prone to P20EE and Case Studies

P20EE is common in diesel vehicles with SCR systems. Below are notable examples:

• Ram 2500 (2016-2020)
  Issue: Contaminated DEF.
  Symptom: MIL on and increased DEF use.
• Ford F-250 (2017-2021)
  Issue: Faulty downstream NOx sensor.
  Symptom: Limp mode and failed emissions test.
• Chevrolet Silverado 2500HD (2017-2020)
  Issue: Clogged DEF injector.
  Symptom: Reduced power and DEF warning.
• Volkswagen Touareg TDI (2014-2017)
  Issue: Contaminated SCR catalyst.
  Symptom: Exhaust odor and MIL on.
• Mercedes-Benz Sprinter (2018-2022)
  Issue: ECM software glitch.
  Symptom: MIL on with no noticeable performance issues.

User Instances (Quick Wins)

• 2018 Ram 2500: Drained cloudy DEF, refilled with ISO 22241 fluid. Using iCarsoft CR MAX, post-SCR NOx dropped by ~75% at 2,000 rpm cruise; code cleared.
• 2017 F-250: Downstream NOx sensor stuck at high ppm. Replaced sensor, performed drive cycle; CR MAX BT confirmed 85–90% SCR efficiency.
• 2019 Silverado 2500HD: Crystal buildup at injector. Cleaned line/injector, ran commanded dosing test with CR Eagle; DEF usage normalized.
• 2020 Sprinter: No faults in dosing, efficiency low after regen. Dealer ECM update + leak fix at V-band clamp; efficiency restored.

5. Steps to Diagnose P20EE

Diagnosing P20EE requires a systematic approach. Follow these steps:

Step 1: Read Trouble Codes and Data Stream
Tool: OBD-II scanner (e.g., iCarsoft CR MAX / CR MAX BT).
Action: Connect to the OBD-II port, retrieve P20EE and related codes, and monitor live data.
Key Data:

1. Upstream vs. downstream NOx levels: Should show significant reduction post-SCR.
2. DEF injection rate: Should match expected values (e.g., 1–3 mL/s under commanded dosing).
3. SCR efficiency: Typically target >80% during steady-state cruise once the system is warm.

Example: The Ram showed only a ~50% NOx reduction, indicating low efficiency.

Step 2: Visual Inspection
Checkpoints:

1. DEF tank and fluid: Check for contamination or crystallization.
2. DEF injector: Look for leaks or buildup.
3. Exhaust system: Inspect for leaks or damage before the SCR catalyst.

Example: The Silverado had crystallized DEF around the injector.

Step 3: Test the NOx Sensors
Tool: Scan tool; backprobe only as specified.
Action:

1. Verify sensor response with changing load and temperature.
2. Compare upstream and downstream readings: expect a marked drop post-SCR at operating temp.

Example: The F-250’s downstream NOx sensor showed erratic readings.

Step 4: Test the DEF System
Action:

1. Test DEF quality with a refractometer (~32.5% urea).
2. Run a commanded dosing test; look for fine, even spray.

Example: The Ram’s DEF tested at ~20% urea, indicating contamination.

Step 5: Inspect the SCR Catalyst
Action:

1. Check for contamination: oil, coolant, soot.
2. Perform regeneration cycles if applicable and verify temp sensors.

Example: The Touareg’s SCR catalyst was contaminated with oil.

Step 6: Verify the ECM
Action:

1. Update ECM software with dealer tools.
2. Substitute known-good components to isolate faults.

Example: The Sprinter’s issue resolved after a software update.

6. Solutions and Implementation

Apply these repairs based on diagnostic findings:

• Replace Contaminated DEF and Flush System
  Part: Fresh DEF (~$20/gal).
  Steps: Drain/flush/refill, clear codes, run drive cycle.
  Result: Ram’s SCR efficiency improved after DEF replacement.
• Replace the NOx Sensor
  Part: OEM NOx sensor (~$200–$400).
  Result: F-250 limp mode resolved post-replacement.
• Clean or Replace the DEF Injector
  Part: Injector (~$150) or cleaner (~$10).
  Result: Silverado’s dosing restored; warnings off.
• Replace the SCR Catalyst
  Part: OEM SCR catalyst (~$800–$1500).
  Result: Touareg required new catalyst due to contamination.
• Update ECM Software
  Tool: Dealer equipment; apply TSBs where applicable.
  Result: Sprinter’s P20EE cleared after update.

7. Repair Costs and Precautions

• DEF Replacement and Flush: $50–$150
• NOx Sensor: $200–$400
• DEF Injector Cleaning/Replacement: $10–$200
• SCR Catalyst Replacement: $800–$1500
• ECM Update: $0–$150

Precautions

• Use only ISO 22241-compliant DEF from reputable sources.
• Avoid overfilling the DEF tank to prevent crystallization.
• Check for exhaust leaks ahead of SCR; even small leaks can ruin efficiency calculations.

8. Preventive Measures

• Use high-quality DEF and store it properly (cool, dry, sealed).
• Inspect the DEF system during annual maintenance (lines, injector, tank cap/seal).
• Perform regular DPF/SCR regeneration cycles as recommended.
• Keep ECM software updated per manufacturer releases.

Recommended Tools

For fast, accurate diagnosis and live data on NOx sensors, SCR efficiency, and dosing tests, try the iCarsoft CR MAX or CR MAX BT. These scanners help confirm fixes and clear P20EE safely.

Learn More about CR MAX

FAQ: P20EE & SCR NOx Catalyst

What does P20EE mean?

It indicates the SCR system isn’t reducing NOx enough on Bank 1. Common causes include bad DEF, a clogged injector, faulty NOx sensors, leaks, or a weak catalyst.

Can I drive with P20EE?

Short trips are possible, but continued driving may trigger limp mode and increased emissions. Diagnose and fix promptly to avoid catalyst damage.

Will a DPF regen clear P20EE?

A regen can help if efficiency loss relates to soot/temperature. If DEF, sensors, or leaks are the issue, regen alone won’t solve it.

How do I test DEF quality?

Use a DEF refractometer; proper concentration is ~32.5% urea. Replace fluid if out of spec or contaminated.

Do I need to replace the SCR catalyst?

Only after verifying DEF quality, injector function, NOx sensors, exhaust leaks, and software. Replace the catalyst if it’s contaminated/damaged and efficiency stays low.

Which scanner do I need?

iCarsoft CR MAX / CR MAX BT read NOx sensor data, run dosing tests, and show SCR efficiency so you can confirm repairs before replacing parts.

What are common P20EE related codes?

Often seen with P2BAA, P20E8, P207F, and sensor heater/performance codes for NOx/temperature sensors.

How do I prevent P20EE from returning?

Use sealed, fresh DEF; fix exhaust leaks; keep software up to date; and periodically inspect dosing hardware.