Cat-back exhaust: UK vs US vs Germany vs Norway
A cat-back sits downstream of the catalytic converter, so it is a noise question rather than an emissions one. Every market allows it and every market caps how loud the result may be.
All four 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 | Cat stays fitted, noise must not exceed approval It must still pass the noise, nuisance and emissions checks, and must not have been altered to increase noise beyond the type-approved level. |
United States EPA, NHTSA and state codes |
Conditional | Effective muffler required, state dB limits The catalytic converter stays in place, the vehicle keeps an effective muffler, and some states enforce a specific decibel cap. |
Germany StVZO / KBA |
Conditional | EC type approval or an ABE for your vehicle Without approval the type approval lapses and only an individual inspection helps. Cat and particulate filter must stay complete, and export or motorsport only systems are not road-legal. |
Norway Statens vegvesen |
Conditional | E-marked system intended for the model Noise must stay within the level the car was approved at. Systems that can change noise level while driving, such as an open valve, are not permitted as a way around the limit. |
The practical difference is what proves compliance: a subjective noise check in the UK and US, and a marked approval on the part itself in Germany and Norway.
Why the answers differ
Is a cat-back exhaust legal?
Because a cat-back sits downstream of the catalytic converter, it is a noise question rather than an emissions one, and all four markets allow it. What differs is what proves compliance. The UK and US assess the result, an effective silencer and a noise level that has not been deliberately raised above the type-approved figure, which in practice means a subjective judgement at test or roadside. Germany and Norway assess the part, requiring an approval marking on the system itself for the vehicle it is fitted to. The Norwegian rule adds the sharpest clause in the set: a system whose noise level can be changed while driving, an open valve, may not be used to get around the limit, which is a direct answer to valved exhausts.
Verdict at a glance
The same four markets, grouped by where they land.
Conditional
ConditionalUnited Kingdom, United States, Germany and Norway. 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. 54
- UKMOT inspection manual: 8. Nuisance (GOV.UK)
- UKCatalytic converters and diesel particulate filters (GOV.UK)
- USVehicle modifications and equipment (NHTSA)
- USTampering and aftermarket defeat devices (US EPA)
- Germany§ 19 StVZO - Erteilung und Wirksamkeit der Betriebserlaubnis (gesetze-im-internet.de)
- Germany§ 49 StVZO - Geräuschentwicklung und Schalldämpferanlage (gesetze-im-internet.de)
- Germany§ 21 StVZO - Betriebserlaubnis für Einzelfahrzeuge (gesetze-im-internet.de)
- NorwayKjøretøyforskriften kap. 24 - Støy (lovdata.no)
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.