P20EE

P20EE means the SCR NOx catalyst on Bank 1 is converting less NOx than the ECM's efficiency model requires - it is the catalyst-level efficiency code, distinct from quality (P207F) or dosing (P204F) faults. Cheapest-first ladder: this is the single most over-condemned code on modern diesels, so work the ladder honestly. First refractometer the DEF, then verify upstream NOx sensor reads 200 to 600 ppm under cruise and downstream reads under 50 ppm; if downstream is reading 150+ ppm with good DEF and verified dosing, the SCR catalyst may genuinely be aged. Before condemning the cat, swap or test both NOx sensors (they fail more often than the catalyst by roughly 4 to 1). Caveat: a $3,500 catalyst replacement on a $400 sensor fault is the most expensive mistake on this code.

Vehicle-specific patterns: 2011 to 2016 Duramax LML set P20EE constantly from failed downstream NOx sensor 2, GM bulletin PI1149 explicitly says to replace the sensor before the SCR; 2011 to 2014 Ford 6.7L Powerstroke throw P20EE from sulfur poisoning after extended idle, Ford TSB 13-7-15; 2010 to 2016 Mercedes Sprinter OM642/OM651 BlueTEC set P20EE from cracked SCR substrate after repeated DEF crystallization events; 2009 to 2014 VW/Audi 3.0 TDI set the code from genuinely aged SCR catalysts at 120,000+ miles. Estimated repair: $380 to $3,800.