OBD-II Diagnostic Trouble Code
P20EF — Selective Catalytic Reduction (SCR) NOx Catalyst Efficiency Below Threshold Bank 2
P20EF means the SCR catalyst on bank 2 is not reducing NOx as well as it should, based on upstream and downstream NOx sensors. Bank 2 only applies to systems with two SCR paths, such as some V-engines; most single-bank heavy trucks never set it.
Severity 4/5
Truck-relevant
Stop within 50 mi
Severity
Stop within 50 mi
Selective Catalytic Reduction (SCR) NOx Catalyst Efficiency Below Threshold Bank 2
Code
P20EF
Category
Powertrain
Subsystem
Aftertreatment
Truck-relevant
Yes
What this means for truckers
Stop within ~50 miles on a Cummins X15, Detroit DD15, PACCAR MX-13. Driving longer risks damaging adjacent components (turbo housing, head gasket, aftertreatment) and turning a $300 fix into a $3,000 fix.
Before you replace parts — try this first
P20EF on the Aftertreatment system is most often poor quality 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.
Common causes
- CommonPoor quality DEF ($150–$600)
- CommonRestricted doser or low dosing on bank 2 ($150–$1200)
- CommonFaulty bank 2 NOx sensor ($400–$900)
- OccasionalExhaust leak ($150–$600)
- OccasionalDegraded SCR catalyst ($3000–$8000)
How to fix
Test DEF quality
When to use: First step
Run the dosing test for bank 2
When to use: To confirm dosing
Compare NOx sensor readings
When to use: To find a faulty sensor
Run the SCR performance test and replace the catalyst if needed
When to use: Last step
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
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
Frequently Asked Questions
Can I drive with code P20EF?
Drive with caution. P20EF 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 P20EF 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 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 P20EF fail a DOT inspection?
Yes — P20EF 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 P20EF and related codes?
P20EF 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 P20EF is set without the companions, the issue is more isolated.
How do I prevent P20EF from coming back after a repair?
For P20EF (Selective Catalytic Reduction (SCR) NOx Catalyst Efficiency Below Threshold Bank 2), 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).