OBD-II Diagnostic Trouble Code
P20EE — NOx Catalyst Efficiency Below Threshold (Bank 1)
P20EE means the SCR catalyst on bank 1 is not reducing NOx as well as it should. The ECM compares the NOx sensor upstream of the SCR with the one downstream, and the conversion is below the required threshold. This is a result code: the cause is often poor DEF dosing, bad DEF or a faulty NOx sensor rather than the catalyst itself.
Severity 4/5
Truck-relevant
Stop within 50 mi
Severity
Stop within 50 mi
NOx Catalyst Efficiency Below Threshold (Bank 1)
Code
P20EE
Category
Powertrain
Subsystem
Aftertreatment
Truck-relevant
Yes
What this means for truckers
P20EE (NOx Catalyst Efficiency Below Threshold) — the SCR/NOx catalyst isn't reducing NOx enough. On 2010+ trucks this is a top-3 emissions code. Causes: aging SCR catalyst (life is typically 600K-800K miles), failed NOx sensor, DEF quality. Fix path mirrors P204F.
Before you replace parts — try this first
P20EE on the Aftertreatment system is most often poor quality or diluted def — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Check-engine and SCR warning lights, emissions derate countdown and eventually reduced power and speed. DEF use may look normal, low or high depending on the cause.
Common causes
- CommonPoor quality or diluted DEF ($150–$600)
- CommonRestricted or crystallized DEF doser or low dosing ($150–$1200)
- CommonFaulty upstream or downstream NOx sensor ($400–$900)
- OccasionalExhaust leak between the NOx sensors ($150–$600)
- OccasionalContaminated or degraded SCR catalyst ($3000–$8000)
How to fix
Test DEF with a refractometer
When to use: First step
Run the OEM DEF dosing test
When to use: To confirm dosing quantity
Compare NOx sensor readings at key-on and while running
When to use: To find a faulty sensor
Run the OEM SCR performance test and replace the catalyst only if all other checks pass
When to use: Last step
Affects most
- Ford F-3501 complaint mention this code
- GMC Sierra 35001 complaint mention this code
- RAM 55001 complaint mention this code
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P2002 — Diesel Particulate Filter Efficiency Below Threshold (Bank 1)
- → P2003 — Diesel Particulate Filter Efficiency Below Threshold (Bank 2)
- → P2031 — Exhaust Gas Temperature Sensor Bank 1 Sensor 2
- → P2032 — Exhaust Gas Temperature Sensor Bank 1 Sensor 2 Low
- → P2033 — Exhaust Gas Temperature Sensor Bank 1 Sensor 2 High
- → Complaints on Ford F-350
- → Complaints on GMC Sierra 3500
- → Complaints on RAM 5500
Frequently Asked Questions
Can I drive with code P20EE?
Drive with caution. P20EE is severity 4 — stop within ~50 miles. Most commercial diesel codes at this severity (turbo, fuel rail, transmission solenoid) can damage adjacent expensive components if left unrepaired. Get to a service location before highway-speed loads.
How much does it cost to fix P20EE on a commercial truck?
Total repair cost ranges $150–$8000 depending on the root cause. The cheapest fix is usually the first one listed below — Poor quality or diluted DEF — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P20EE fail a DOT inspection?
Yes — P20EE is an emissions-related code, and DOT inspectors check emissions compliance. An active emissions code with a check engine light can result in an out-of-service violation under FMCSA Part 393. The truck typically can't be moved until the code is cleared. Repeated emissions violations escalate to CSA scores.
What's the difference between P20EE and related codes?
P20EE is closely related to: P2002 (Diesel Particulate Filter Efficiency Below Threshold (Bank 1)), P2003 (Diesel Particulate Filter Efficiency Below Threshold (Bank 2)), P2031 (Exhaust Gas Temperature Sensor Bank 1 Sensor 2), P2032 (Exhaust Gas Temperature Sensor Bank 1 Sensor 2 Low). These codes share the same subsystem (aftertreatment) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P20EE is set without the companions, the issue is more isolated.
How do I prevent P20EE from coming back after a repair?
For P20EE (NOx Catalyst Efficiency Below Threshold (Bank 1)), prevention centers on the aftertreatment system. Run quality DEF (verify supplier — BlueDEF, PEAK, Yara Air1). Don't skip parked regens — when the dash asks for one, do it. Avoid extended idle time which fouls the DPF. Replace NOx sensors at the OEM interval (typically 400K miles).