Powertrain, generic (extended)

P20EE SCR NOx Catalyst Efficiency Below Threshold (Bank 1)

Check engine lightOn

EmissionsRelevant

Drive cycleRequired

Limp modePossible

P20EE is the main monitor CARB requires for the NOx-reducing catalyst: the share of NOx removed by the SCR catalyst on bank 1, judged by the NOx sensors before and after it, has fallen below what the system needs. Ford's 2023 6.7 L diesel fails it when cumulative efficiency drops under 55 % with the SCR inlet at 200–320 °C. GM's 2018 diesels compare measured with expected efficiency, partly via an EWMA filter, as a Type A code. Ram and Jeep 3.0 L and 6.7 L diesels had bulletins replacing the SCR catalyst (18-064-15, 18-038-16), Isuzu N-Series and the 2015 Cruze diesel got new software. Because poor DEF also cuts efficiency, GM pairs this code with a DEF quality warning, and a speed limit may follow. Check DEF quality, dosing, leaks and NOx sensors before condemning the catalyst.

Likely causes

Diluted or contaminated DEF, or a clogged or leaking dosing valve

Upstream or downstream NOx sensor reading wrong

SCR catalyst aged, overheated or poisoned

Exhaust leak between the two NOx sensors

Calibration needing an update (several manufacturer bulletins)

Symptoms

Malfunction indicator lamp on
DEF or exhaust fluid quality warning in the instrument cluster
Speed limitation countdown on some vehicles if the fault persists

Repair estimate

DifficultyHardWorkshop job

Parts and labour€100–4,000

Time1–5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

scr catalystnox sensor upstreamnox sensor downstream

References: fordservicecontent.com, gsi.ext.gm.com, static.nhtsa.gov, static.nhtsa.gov.