SCR NOx Pre-Catalyst Over Temperature Bank 1
The P20ED code indicates that the Selective Catalytic Reduction (SCR) system's pre-catalyst NOx sensor on Bank 1 has detected an exhaust temperature exceeding the maximum allowable threshold. This typically occurs when the diesel exhaust fluid (DEF) injection is insufficient or the catalyst is degraded, leading to excessive heat in the exhaust stream. Prolonged over-temperature can damage the SCR catalyst and downstream components.
01. Most Common Causes
- Faulty DEF injector or dosing system
- Defective NOx sensor
- Exhaust gas recirculation (EGR) system malfunction
- Engine over-fueling or incorrect timing
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Reduced engine power or limp mode
- Increased exhaust temperatures
- Poor fuel economy
03. Step-by-Step Diagnosis
Step 1: Scan for additional codes and check freeze frame data.
Step 2: Inspect DEF level, quality, and injection system for leaks or blockages.
Step 3: Test NOx sensor readings and compare with known good values.
Step 4: Check EGR system operation and coolant temperature sensor.
Step 5: Verify engine timing and fuel trim data for over-fueling conditions.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive long distances as high exhaust temperatures can damage the SCR catalyst and other components. Limp mode may reduce power, making driving unsafe.
How much does it cost to fix?▼
Repair costs range from $150 to $600 depending on the root cause. A DEF injector replacement may cost $200-$400, while a NOx sensor can be $150-$300 plus labor.