P20F2: SCR NOx Catalyst Efficiency Below Threshold (Bank 2)

What it means

SCR NOx Catalyst Efficiency Below Threshold (Bank 2): the control module has detected this fault condition.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test
• The engine or gearbox may go into limp mode

How it works

The SCR catalyst in bank 2 uses reductant to convert nitrogen oxides in the exhaust. The module estimates its efficiency using NOx sensor readings before and after the catalyst. This code sets when the bank 2 catalyst does not achieve the monitored NOx reduction threshold.

Common causes

Most likely first.

1. SCR catalyst thermally aged
2. Catalyst poisoning from sulphur or oil ash

Symptoms

• Check engine light on
• Failed emissions inspection
• Possibly slightly increased fuel consumption
• Sulfur (rotten egg) smell from exhaust under load

How to diagnose it

1. Read freeze-frame data and live upstream and downstream NOx readings for bank 2 to confirm the conditions associated with low efficiency.
2. Check for related NOx-sensor, reductant-quality, dosing, and exhaust-temperature codes before assessing the catalyst.
3. Inspect the bank 2 exhaust between the NOx sensors for leaks that could affect the efficiency calculation.
4. Check bank 2 NOx sensor wiring and connectors, and compare live readings for plausibility under stable operating conditions.
5. Verify reductant quality and dosing against the manufacturer's specification, then evaluate catalyst performance before replacing the SCR unit.

Common fixes

• Repair an exhaust leak between the bank 2 NOx sensors.
• Repair damaged wiring or connections to a bank 2 NOx sensor.
• Replace a NOx sensor confirmed inaccurate by testing.
• Correct a reductant quality or dosing fault found during diagnosis.
• Replace the bank 2 SCR catalyst if it fails an efficiency check after related faults are ruled out.

Can I keep driving?

The vehicle may remain drivable if it runs normally, but its emissions are higher and it may fail an inspection. Avoid heavy loads if power is reduced, and have the fault diagnosed before an emissions warning or starting restriction appears.