P21B5: NOx Adsorber Over Temperature (Bank 2)

What it means

The NOx adsorber temperature for bank 2 exceeded the calibrated limit. The code does not establish the cause or predict component damage.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The NOx adsorber on bank 2 stores nitrogen oxides in the exhaust and periodically heats up to release or reduce them. The control module tracks its temperature using exhaust temperature sensor data and sets this code when the reported or calculated bank 2 adsorber temperature exceeds its calibrated limit.

Common causes

Most likely first.

1. Excessive NOx adsorber desulfation or regeneration heating
2. Incorrect exhaust temperature sensor reading
3. Fueling or combustion fault causing excessive exhaust heat
4. Exhaust restriction or flow problem

Symptoms

• Check engine light or NOx warning on
• AdBlue/DEF level or quality warning
• Possible no-start countdown after ignore period
• Failed emissions inspection (NOx)

How to diagnose it

1. Record freeze-frame data and check live bank 2 adsorber and exhaust temperatures to see when the over-temperature condition occurs.
2. Compare the bank 2 temperature readings with other exhaust temperature sensors from a cold start; an implausible reading can point to a sensor or circuit fault.
3. Inspect the bank 2 temperature sensor, connector, and wiring for heat damage, corrosion, loose connections, or contact with the exhaust.
4. Check for other stored or pending codes and review regeneration or desulfation history for repeated or unusually prolonged heating events.
5. Assess fueling and combustion data for faults that could raise exhaust heat, and check the exhaust for restriction or abnormal flow using the manufacturer's procedures.

Common fixes

• Repair damaged or loose wiring and connectors at the bank 2 exhaust temperature sensor.
• Replace a bank 2 temperature sensor only after confirming its reading is inaccurate.
• Correct any fueling or combustion fault that is driving exhaust temperatures too high.
• Repair an exhaust restriction or flow problem identified during testing.
• Address a regeneration or desulfation control fault after confirming the cause.

Can I keep driving?

Avoid heavy loads and continued driving if a NOx or temperature warning appears, or if the engine runs abnormally; excessive heat can damage exhaust after-treatment components. Resolve the fault promptly, since emissions performance may worsen and an AdBlue/DEF-related no-start countdown may begin.