P20EE
System
Powertrain (Diesel Emissions)
Scanner required
Full-system scanner (engine, ABS, SRS, transmission, service resets)

SCR NOx Catalyst Efficiency Below Threshold

Understand P20EE, common causes, and the right TOPDON scanner to diagnose it.

SCR system not reducing NOx sufficiently — DEF quality, injector, or catalyst issue on diesel.

Common symptoms

  • Check engine light
  • DEF warning
  • Derate on commercial diesel

Typical causes

  • Bad or diluted DEF
  • Failed SCR catalyst
  • DEF injector clogged
  • NOx sensor fault

What scanner do you need?

Full-system scanner (engine, ABS, SRS, transmission, service resets)

Use live data and freeze frame to confirm the fault before replacing parts. ABS, SRS, and network U-codes usually require a full-system scanner.

Diagnostic tips

  • Record freeze frame data when the code first sets
  • Clear the code only after fixing the root cause
  • Check for related pending codes that point to a common issue
  • Verify battery voltage is above 12.4V before chasing sensor codes

Recommended TOPDON scanners

Frequently asked questions

Can I diagnose P20EE with a basic code reader?
P20EE may set in the engine module, but related diagnostics often benefit from an enhanced or full-system scanner with live data and freeze frame.
Is it safe to drive with P20EE?
It depends on the code and symptoms. Flashing check engine lights and misfire codes should be addressed immediately. Consult a professional if unsure.