P20EE – SCR NOx Efficiency Below Threshold (Full 2025 Fix Guide)

P20EE – SCR NOx Efficiency Below Threshold (Full 2025 Fix Guide)

If your truck throws P20EE, you’re facing one of the most misunderstood and misdiagnosed diesel problems.

P20EE affects:

  • Cummins

  • Detroit

  • Paccar MX-13

  • Volvo/Mack

  • Freightliner

  • International

  • Duramax

  • Powerstroke

  • RAM 6.7

It means your truck is not properly removing NOx from the exhaust — and it WILL eventually lead to:

  • Loss of power

  • Failed regens

  • “DEF Quality Poor”

  • SPN 5246 (derate)

  • 5 MPH limit if ignored

The good news: P20EE is almost always fixable at home with the right diagnostic tools.

🚨 What Does P20EE Actually Mean?

P20EE = SCR NOx Efficiency Below Threshold

Translation:

👉 The SCR catalyst is NOT reducing NOx enough
👉 OR the sensors think it isn’t
👉 OR the DEF system isn’t dosing properly

This code does NOT automatically mean the SCR is bad.

⚠️ Symptoms of P20EE

  • “SCR Fault – Engine Will Derate”

  • Frequent regens

  • Failed regen

  • DEF light

  • Check engine + MIL

  • Loss of power under load

  • Exhaust temps not rising

  • “DEF Quality Poor” (P207F)

  • SPN 4364 / 5246

🔍 What Causes P20EE? (Real Reasons)

Based on thousands of dealer-level diagnostics, these are the TRUE causes:

1. Bad NOx Sensors (MOST COMMON)

If inlet/outlet NOx readings are wrong, the ECM thinks the SCR isn’t working.

Signs:

  • Outlet NOx equals inlet NOx

  • Sensor slow to respond

  • Voltage flat-lined

  • High ppm values after regen

NOx sensors fail CONSTANTLY — the #1 cause of P20EE.

2. DEF Not Dosing Properly

A failing DEF pump or clogged doser causes low DEF injection.

Causes:

  • Weak DEF pump

  • Clogged injector/doser

  • Bad DEF

  • DEF pressure low

  • Crystal buildup

If DEF isn’t sprayed right → NOx won’t drop → P20EE triggers.

3. Exhaust Leaks Before the SCR

Even a tiny leak causes oxygen contamination = NOx efficiency failure.

4. Failed or Contaminated SCR Catalyst

This is rare, but if the catalyst is coated with oil, coolant, or excessive soot, it cannot reduce NOx.

5. DPF Not Regenerating Properly

Low exhaust temps → poor SCR function.

6. Bad Temperature Sensors (EGTs)

If the ECM cannot validate exhaust temps → NOx efficiency cannot be calculated.

7. Incorrect EGR Flow

If EGR is not working, NOx values skyrocket → P20EE appears.

8. Wrong or Contaminated DEF Fluid

Bad DEF causes low NOx conversion and eventually triggers derate.

❌ What NOT to Do When You See P20EE

❌ Don’t replace the SCR immediately

Most shops misdiagnose this — SCR is rarely the cause.

❌ Don’t trust a cheap OBD2 scanner

They cannot read NOx values or DEF dosing performance.

❌ Don’t clear the code and keep driving

It’ll return and eventually trigger a 5 MPH derate.

🛠️ How to Fix P20EE (Dealer-Level Procedure You Can Do at Home)

You need:

  • JPRO

  • Diesel Tech

  • Cummins Insite

  • Detroit DDDL

  • Paccar Davie

  • Volvo Tech Tool

These show:

  • Inlet & outlet NOx ppm

  • DEF dosing performance

  • Pump pressure

  • Exhaust temps

  • SCR efficiency %

  • Soot level

  • DPF status

⭐ STEP 1 — Check Inlet & Outlet NOx Readings

Normal:
Outlet NOx should be 70–95% lower than inlet.

If outlet = inlet → outlet NOx sensor failed.

⭐ STEP 2 — Run DEF Doser Test

Confirms:

  • Spray pattern

  • Pump pressure

  • Flow rate

  • Injector response

If doser fails → P20EE is valid.

⭐ STEP 3 — Inspect DEF Fluid Quality

Look for:

  • Crystals

  • Odor

  • Cloudiness

  • Water contamination

If in doubt → drain & refill.

⭐ STEP 4 — Check EGT Sensors

Bad temp sensors = ECM cannot verify SCR temps.

Common signs:

  • EGT1/EGT2 don’t match

  • Slow readings

  • Value stuck

⭐ STEP 5 — Check for Exhaust Leaks

Look near:

  • Flex pipe

  • Joints near SCR

  • DPF exit

  • Doser flange

Leaks = oxygen contamination = P20EE.

⭐ STEP 6 — Repair Root Cause

Most common repairs:

  • Replace NOx sensor

  • Clean/replace DEF doser

  • Replace EGT sensor

  • Fix exhaust leak

  • Correct EGR issue

  • Clean/replace DEF

⭐ STEP 7 — Perform Required SCR/DEF Resets

You MUST reset:

  • NOx sensor

  • SCR efficiency

  • DEF dosing system

  • Aftertreatment learned data

Skipping resets → P20EE returns instantly.

⭐ STEP 8 — Perform a Forced Regen

A forced regen:

  • Raises exhaust temps

  • Validates SCR temps

  • Stabilizes NOx conversion

  • Clears derate logic

💻 Tools Required to Fix P20EE

You MUST have:

✔ Diesel diagnostic laptop

✔ Nexiq USB-Link 3

✔ Full aftertreatment reset capability

✔ Ability to test NOx, DEF pump, doser, EGT, VGT

DieselScanners kits include everything:

  • SCR resets

  • DEF pump/doser tests

  • NOx sensor programming

  • Forced regens

  • Aftertreatment diagnostics

  • Lifetime support

📞 Need help choosing a kit? Call: (800) 987-4850

💲 Why Shops Misdiagnose P20EE & Overcharge

Most shops instantly blame the SCR catalyst.

Actual costs:

Bad Diagnosis Cost
SCR Catalyst $2,000–$6,000
DEF pump $900–$1800
Doser $250–$650
NOx Sensors $300–$800 each
Forced Regen $150–$300
Aftertreatment diagnostic $250–$500

Your diesel laptop:

👉 Shows EXACT cause
👉 Prevents unnecessary part swaps
👉 Clears P20EE & derate
👉 Pays for itself instantly

🚛 Fix P20EE Before It Triggers Derate

If untreated, P20EE leads to:

  • SCR failure

  • Repeated regens

  • DEF system faults

  • SPN 5246 5 MPH derate

  • No power

  • Emissions shutdown

Fix it now with DieselScanners.

📞 Call: (800) 987-4850
👉 Shop DieselScanners Diagnostic Kits

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