SCR NOx Catalyst Efficiency Below Threshold Bank 1
The P20EE code indicates that the Selective Catalytic Reduction (SCR) system's NOx catalyst is operating below the required efficiency threshold for Bank 1. This typically means the catalyst is not converting nitrogen oxides (NOx) into harmless nitrogen and water as effectively as needed, often due to contamination, aging, or issues with the Diesel Exhaust Fluid (DEF) system. The Powertrain Control Module (PCM) monitors NOx levels before and after the catalyst; when the conversion rate drops below a calibrated limit, this code is set.
1. Most Common Causes
- Faulty or degraded SCR catalyst
- Contaminated or low-quality DEF fluid
- DEF injector clogged or malfunctioning
- NOx sensor failure or inaccurate readings
- Exhaust leaks upstream of the SCR catalyst
2. Symptoms You Will Notice
- Check Engine Light illuminated
- Reduced engine performance and fuel economy
- Increased exhaust emissions
- Possible limp mode activation
3. Step-by-Step Diagnosis
Step 1: Scan the vehicle for all stored codes and freeze frame data.
Step 2: Inspect the DEF fluid level and quality; check for contamination or incorrect fluid.
Step 3: Visually inspect the DEF injector, lines, and SCR catalyst for physical damage or leaks.
Step 4: Use a scan tool to monitor NOx sensor readings before and after the catalyst during a test drive.
Step 5: Perform a DEF system test to verify injector operation and fluid flow.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
Driving short distances is generally safe, but prolonged driving may cause further damage to the SCR system and lead to reduced performance or limp mode. It's best to diagnose and repair promptly.
How much does it cost to fix?▼
Diagnosis typically costs $50-$150. Repairs vary: DEF fluid top-up or replacement is cheap ($20-$50), while replacing a NOx sensor ($200-$400) or SCR catalyst ($500-$2000+) is more expensive.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.