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 its expected efficiency threshold for Bank 1. This typically means the catalyst is not converting nitrogen oxides (NOx) into harmless nitrogen and water as effectively as required, often due to contamination, aging, or issues with the diesel exhaust fluid (DEF) system.
01. Most Common Causes
- Faulty or degraded SCR catalyst
- Contaminated or low-quality DEF fluid
- DEF injector clogged or malfunctioning
- Exhaust leaks upstream of the SCR catalyst
- Faulty NOx sensors providing incorrect readings
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Reduced engine performance or power
- Increased fuel consumption
- Failed emissions test
- Possible DEF warning light or message
03. Step-by-Step Diagnosis
Step 1: Scan for additional codes and freeze frame data to identify related issues.
Step 2: Inspect DEF fluid level and quality; check for contamination or incorrect fluid.
Step 3: Visually inspect the DEF injector, lines, and SCR catalyst for leaks, damage, or blockages.
Step 4: Test NOx sensor readings using a scan tool to compare pre- and post-SCR values.
Step 5: Perform a forced regeneration or SCR efficiency test per manufacturer procedures.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
It is generally safe to drive short distances, but continued operation may lead to reduced performance, increased emissions, and potential damage to the SCR system. It's best to diagnose and repair promptly.
How much does it cost to fix?▼
The cost varies widely depending on the root cause. Simple fixes like replacing DEF fluid or a sensor can cost $50-$150, while replacing the SCR catalyst can exceed $1,000.