P20EE — SCR NOx Catalyst Efficiency Below Threshold (Bank 1) — 6.7 Cummins (SCR / DEF)
SCR NOx catalyst efficiency below threshold on the 6.7L Cummins SCR/DEF: what P20EE means, likely causes and how to check them.
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
- 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
- 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
- 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
- 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
- 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
- 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.
- Check DEF system basics: level, quality, and recent freeze/contamination history. Low DEF level can trigger limp/derate behavior on this platform.
- 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.
- 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.
- 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.
- 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.
- 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.
Specs and test values
| Check | Normal reading | When to measure | Source |
|---|---|---|---|
| DEF urea concentration | 32.5% (check with a DEF refractometer) | Sample from the tank | Ram service bulletin 25-005-14 (Diesel Exhaust Fluid) |
| DEF freeze point | About 12°F (−11°C); freezing alone is normal and the system is built to thaw it | Cold weather | Ram service bulletin 25-005-14 (Diesel Exhaust Fluid) |
Common mistake to avoid
Replacing sensors, the doser or the SCR catalyst before testing the DEF. Old, heat-stored or diluted DEF loses strength and sets the same efficiency codes. Check it with a refractometer first. Source: Ram service bulletin 25-005-14 (Diesel Exhaust Fluid).
Parts and tools you may need
- Diagnostic scan tool with Cummins/SCR live data
- NOx sensor test harness
- DEF refractometer
- DEF pressure test adapter
- Exhaust backpressure/temperature probes
Tools you’ll need
- OBD-II scan tool — Reads P20EE freeze frame and SCR/NOx companion codes, plus live reductant pressure, DEF dosing quantity, and pre/post-SCR NOx sensor data for efficiency evaluation.
- digital multimeter — Checks NOx sensor heater and signal circuits and DEF pressure sensor voltage for rationality faults that can accompany P20EE.
- fuel pressure gauge — Adapted as a DEF pressure test gauge to verify DEF supply module pressure and delivery against commanded values when P20E8 or dosing issues are suspected.
- refractometer — Verifies DEF concentration/quality to rule out contaminated or diluted reductant causing low SCR NOx conversion.
Rough cost and difficulty
| Likely repair | Parts | Shop labor | Who usually does it |
|---|---|---|---|
| Scan-tool forced (parked) regeneration | — | 0.5–1.5 hrs | DIY with tools and a manual |
| NOx sensor | $250–$800 | 0.5–2 hrs | DIY with tools and a manual |
| DEF pump / supply module | $400–$1,200 | 1–3 hrs | DIY with tools and a manual |
| SCR catalyst | $2,000–$4,500 | 1–4 hrs | Shop |
Typical US independent-shop figures; labor runs about $120–$180 per hour (dealers charge more). Confirm the diagnosis before buying parts.
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.
Check for recalls
Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.
Sources
Repair videos: P20EE on the 6.7 Cummins '13+
Third-party videos, not ours — picked for this engine. Confirm anything against the factory service information for your truck.
Spotted an error, or something missing? Tell us
Run a 6.7 Cummins (SCR / DEF)? This goes straight to the editor — it is not published.