P20EE: SCR NOx Catalyst Efficiency Below Threshold (Bank 1)

How P20EE sets

The engine control module (ECM) compares NOx sensor readings before and after the SCR catalyst while diesel exhaust fluid (DEF) is being injected. If the downstream reading does not drop enough, conversion efficiency is judged low and P20EE sets. Efficiency limits and monitoring conditions are manufacturer-specific.

Common causes of P20EE

Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.

1. Poor-quality, diluted, or contaminated DEF
2. Clogged or failing DEF injector
3. Faulty upstream or downstream NOx sensor
4. Weak DEF pump or low dosing pressure
5. Degraded or contaminated SCR catalyst
6. Exhaust leaks around the NOx sensors
7. Outdated engine software

P20EE symptoms

• Check engine light on
• DEF or emissions system warning message
• Countdown to reduced speed or power
• Often no change in drivability at first

How P20EE is diagnosed

1. Scan for all codes and freeze frame; address NOx sensor and DEF system codes first.
2. Test DEF concentration with a refractometer and check for contamination.
3. Inspect the DEF injector for crystal buildup and verify spray pattern and dosing quantity.
4. Compare upstream and downstream NOx sensor readings during a steady drive.
5. Check for exhaust leaks near the SCR and NOx sensors.
6. Run any factory SCR efficiency test and check for software updates.

Typical fixes for P20EE

• Drain and refill with fresh, correct-specification DEF
• Clean or replace the DEF injector
• Replace a faulty NOx sensor
• Repair DEF pump or dosing pressure problems
• Update ECM software
• Replace the SCR catalyst if confirmed degraded

Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.

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P20EE FAQ

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