SCR NOx Catalyst Efficiency Below Threshold (Bank 1)

Modern diesels inject DEF (diesel exhaust fluid — basically urea + water) into the exhaust to chemically convert NOx into nitrogen and water inside the SCR catalyst. The PCM watches NOx sensors before and after the SCR — there should be a big drop in NOx going across the catalyst. When the drop isn't big enough, P20EE sets. Could be bad DEF, weak dosing, contaminated SCR, or — very commonly — a failed downstream NOx sensor reading wrong.

Downstream NOx sensor reading vs upstream NOx sensor reading shows insufficient NOx reduction across the SCR catalyst over the monitor window.

Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.

1. 1Pull NOx readings upstream vs downstream during a hot highway cruise. Quantify the reduction.
2. 2Check DEF quality sensor data — concentration below 32% = bad fluid, replace tank.
3. 3Inspect DEF injector for crystal buildup. Clean or replace.
4. 4Verify DEF dosing rate is happening when commanded (watch dosing PID under load).
5. 5If upstream NOx looks reasonable and dosing is happening but downstream stays high — suspect downstream NOx sensor or SCR catalyst itself.

• Contaminated / wrong DEF (water, washer fluid, or expired)
• Failed downstream NOx sensor (very common — they're consumable)
• DEF dosing injector clogged with crystallized urea
• DEF pump or supply line failure
• Contaminated SCR catalyst (sulfur or oil)
• Exhaust leak before / between NOx sensors

$50 (DEF top-off) to $4,000+ (SCR replacement). NOx sensors typically $400–$900 each.