P21B4
- Powertrain
- Manufacturer-specific
- Emissions
Get it looked at now
Driving may cause further damage or leave you stranded. Arrange service promptly rather than waiting.
NOx Adsorber Over Temperature (Bank 1)
This code is in our catalogue but has not yet been through editorial review, so the guidance below is limited to what our structured data supports. We publish deeper diagnostic content only after a code has been reviewed — here is why.
A temperature sensor at the NOx adsorber on bank 1 reports a value above the safe limit. Sustained over-temperature destroys the storage washcoat.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Driving may cause further damage or leave you stranded. Arrange service promptly rather than waiting.
- Urgency
- Get it looked at now
- Will it pass emissions
- No
- Check engine light
- Solid
- Check engine light or NOx warning on
- AdBlue/DEF level or quality warning
- Possible no-start countdown after ignore period
- Failed emissions inspection (NOx)
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Runaway desulphation regeneration | Very common | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
- NOX Adsorber Catalyst
- Exhaust Temperature Sensor
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Awaiting review
Found something wrong? Tell us — we publish corrections.