SCR NOx Pre-Catalyst Efficiency Below Threshold Bank 2
The P20F3 code indicates that the Selective Catalytic Reduction (SCR) system's NOx pre-catalyst efficiency is below the expected threshold for Bank 2. This typically means the upstream NOx sensor (Bank 2) is detecting higher NOx levels than the downstream sensor, suggesting the SCR catalyst is not effectively reducing NOx emissions. Common causes include a faulty NOx sensor, degraded SCR catalyst, DEF quality issues, or exhaust leaks.
01. Most Common Causes
- Faulty NOx sensor (Bank 2 upstream)
- Degraded or contaminated SCR catalyst
- Poor quality or incorrect Diesel Exhaust Fluid (DEF)
- Exhaust leaks before or after the SCR catalyst
- SCR system wiring or connector issues
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Reduced fuel efficiency
- Increased NOx emissions
- Possible reduced engine power or limp mode
03. Step-by-Step Diagnosis
Step 1: Scan for additional codes and freeze frame data.
Step 2: Inspect NOx sensor wiring and connectors for damage or corrosion.
Step 3: Test NOx sensor readings using a scan tool; compare upstream and downstream values.
Step 4: Check DEF quality and level; ensure proper DEF injection.
Step 5: Inspect exhaust system for leaks between the NOx sensors.
Step 6: Perform SCR catalyst efficiency test per manufacturer procedure.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
Driving short distances is generally safe, but prolonged driving may cause reduced performance and increased emissions. It's best to diagnose and repair promptly to avoid further damage.
How much does it cost to fix?▼
Diagnosis typically costs $50-$150. Replacing a NOx sensor can range from $200-$600, while SCR catalyst replacement can be $1,000-$3,000 or more.