Diesel Trouble Codes

P20EE — Diesel Particulate Filter Efficiency Below Threshold (Bank 1) (6.7 Cummins (SCR / DEF))

Diesel Particulate Filter Efficiency Below Threshold (Bank 1)

Updated September 2, 2026

What it means

On 6.7L Cummins SCR/DEF trucks P20EE is an SCR NOx efficiency code, not a DPF efficiency code. DPF efficiency on this platform is monitored with P2463/P2458/P246C. P20EE typically presents as SCR/NOx system performance, not DPF restriction.1

Most likely causes

  1. SCR NOx conversion efficiency below threshold due to degraded/poisoned SCR catalyst or excessive NOx load — NOx sensors report insufficient NOx reduction across the SCR catalyst. Typical on this platform as SCR/NOx system performance rather than DPF restriction.1
  2. DEF dosing quantity/quality issue leading to low NOx conversion — Insufficient or incorrect reductant delivery reduces SCR efficiency. Platform SCR codes include P204F, P203F, P20E8, P20EE, P2BAD, P2201.12
  3. DEF supply pressure low or intermittent — Low reductant pressure faults are common in the SCR system. P20E8 is a DEF reductant pressure low code on this platform, commonly from failing DEF supply module/pump, clogged DEF filter, frozen/contaminated DEF, leaking lines/connectors, or faulty DEF pressure sensor.3
  4. NOx sensor performance/range issue — NOx sensor faults can cause calculated SCR efficiency errors. Platform NOx sensor code is P2201 and is grouped with P20EE/P2BAD/P204F family.12
  5. Exhaust temperature/DPF regeneration behavior affecting SCR inlet conditions — Abnormal exhaust temps or incomplete DPF regeneration can alter SCR inlet conditions and NOx conversion. DPF efficiency monitoring on this platform uses P2463/P2458/P246C, not P20EE.1

How to diagnose

  1. Verify code context and freeze frame: confirm P20EE is present with SCR/NOx related companions such as P204F, P2BAD, P2201, P20E8. P20EE on this platform is SCR NOx efficiency, not DPF efficiency.
  2. Check DEF system basics: level, quality, and recent freeze/contamination history. Low DEF level can trigger limp/derate behavior on this platform.
  3. Inspect DEF supply module pressure and delivery: review live reductant pressure data and look for P20E8 or pressure-related faults. Typical causes include failing DEF pump/supply module, clogged filter, frozen/contaminated DEF, leaks, or faulty pressure sensor.
  4. Evaluate NOx sensors: compare pre- and post-SCR NOx sensor readings at steady cruise and during a controlled drive. Sensor rationality faults are indicated by P2201 and can accompany P20EE/P2BAD.
  5. Review SCR catalyst health indicators: monitor exhaust temperatures, DEF dosing quantity vs. commanded, and NOx conversion efficiency during steady-state operation. Degraded/poisoned catalyst or excessive NOx load can set P20EE.
  6. Check for related aftertreatment faults: confirm DPF status is not the primary driver. DPF efficiency on this platform is P2463/P2458/P246C; P20EE is not a DPF efficiency code here.
  7. Inspect for exhaust leaks upstream of SCR and verify EGR operation: leaks or excessive EGR flow can raise NOx load into SCR. Platform has EGR valve+cooler, DOC/DPF/SCR+DEF.

Parts and tools you may need

Rough cost and difficulty

Parts and labor vary by model year and root cause; SCR catalyst, DEF supply module, NOx sensors, and labor can be high. Diagnosis should confirm the specific failed component before replacement.

When to see a pro

If derate/limp mode is active, or if NOx sensor or SCR catalyst replacement is indicated, have the system evaluated by a diesel aftertreatment specialist. Do not attempt high-pressure fuel or safety-critical repairs without proper training and equipment.

Sources

  1. Curated DTC seed — 6.7 Cummins (SCR / DEF) (curated) ↩︎ ↩︎ ↩︎ ↩︎ ↩︎

  2. Curated DTC seed — 6.7 Cummins (SCR / DEF) (curated) ↩︎ ↩︎

  3. Curated DTC seed — 6.7 Cummins (SCR / DEF) (curated) ↩︎