Blow-off and diverter valves: UK vs US vs Germany
A recirculating valve returns metered air to the intake; an atmospheric one vents it. That single difference is what every market actually regulates.
All three markets reach the same verdict, conditional. What differs is the rule each one uses to get there, which is the third column.
| Market | Verdict | The rule that decides it |
|---|---|---|
United Kingdom DVSA / GOV.UK |
Conditional | No specific ban; atmospheric must not be excessively noisy A recirculating valve changes nothing that is tested. Venting to atmosphere can upset fuelling on some cars, and the engine must keep running cleanly. |
United States EPA, NHTSA and state codes |
Conditional | Recirculating is the safe choice A vent-to-atmosphere valve releases metered air, which can affect emissions and running. In CARB states an emissions-relevant valve should be CARB-compliant. |
Germany StVZO / KBA |
Conditional | Closed recirculating valve is generally unproblematic An open venting valve changes noise and mixture formation and needs approval, which usually does not exist for open operation, so the type approval lapses. |
All three treat recirculating as a non-event and atmospheric as the problem, for the same two reasons: noise, and metered air leaving the system.
Why the answers differ
Is an atmospheric blow-off valve legal?
Every market here regulates exactly one distinction, and it is the same one: recirculating versus atmospheric. A recirculating diverter valve returns metered air to the intake, changes nothing measurable and is treated as a non-event in all three. An atmospheric valve releases air the sensor has already counted, which can upset fuelling, and makes a noise that was not in the type approval. So the UK objects on noise, the US on emissions and running, and Germany on both while noting that approval for open operation essentially does not exist. The practical takeaway is consistent across all three: the valve that makes the noise is the one that creates the legal problem, and the one that does not is unregulated.
Verdict at a glance
The same three markets, grouped by where they land.
Conditional
ConditionalUnited Kingdom, United States and Germany. Permitted, but only once the condition in the table above is met.
Sources
Primary legislation and regulator guidance behind each market's verdict.
- UKRoad Vehicles (Construction and Use) Regulations 1986, reg. 97 (avoidance of excessive noise)
- UKMOT inspection manual: cars and passenger vehicles (GOV.UK)
- USAftermarket, performance and add-on parts (California Air Resources Board)
- USVehicle modifications and equipment (NHTSA)
- Germany§ 49 StVZO - Geräuschentwicklung und Schalldämpferanlage (gesetze-im-internet.de)
- Germany§ 47 StVZO - Abgase (gesetze-im-internet.de)
- Germany§ 19 StVZO - Erteilung und Wirksamkeit der Betriebserlaubnis (gesetze-im-internet.de)
Other cross-market comparisons
This is a comparison of national rules, not legal advice for your vehicle. Rules change, and the US row in particular summarises federal law where one exists and the common pattern across states where it does not. Confirm the position in your own state or region before you buy or fit anything. Sources last reviewed July 2026.