What are the most likely causes of P20EF?

Low DEF level or poor DEF quality; DEF dosing / injector problem; Faulty NOx sensor reading

How do you diagnose code P20EF?

1) Check DEF level and quality 2) Check DEF dosing and the DEF injector 3) Check the NOx sensors

P20EF

CAUTION

SCR NOx pre-catalyst efficiency below threshold (Bank 1)

The truck's control module (ECU) reports a fault on: SCR NOx pre-catalyst efficiency below threshold (Bank 1).

P20EF means the emissions system detected that NOx reduction upstream, in the pre-catalyst or first SCR stage (Bank 1), is below what it expects. It is part of the SCR / DEF system on heavy-duty diesel trucks. DERATE rates it CAUTION: drive to a shop today, because emissions faults like this can cut power if left unrepaired.

SCR / DEF

Can I keep driving?

Drive to a shop today. The engine may cut power (derate) if the fault stays active.

Most likely causes

Low DEF level or poor DEF quality
DEF dosing / injector problem
Faulty NOx sensor reading
Exhaust leak
Worn or damaged SCR catalyst

Recommended tests

1. Check DEF level and quality
2. Check DEF dosing and the DEF injector
3. Check the NOx sensors
4. Inspect the exhaust for leaks
5. Check the SCR catalyst condition

Data: DERATE verified base, updated October 2026

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