Is Downpipe Legal in the UK on a Audi S4/S5?
A downpipe is road-legal on a Audi S4/S5 only if it keeps a working, type-approved catalytic converter. A de-cat downpipe is not road-legal.
Why?
A downpipe sits at the very front of the exhaust, around the catalytic converter. On the Audi S4/S5, whether one is road-legal comes down to a single question: does it keep a working, type-approved catalytic converter?
Removing or gutting the cat (a "de-cat" downpipe) means the car can no longer meet the emissions standard it was designed to, which is an offence under the Construction and Use Regulations. And because the Audi S4/S5 was first used in the modern emissions era, a missing or obviously modified cat is also an automatic MOT failure.
A downpipe that retains a high-flow, type-approved sports cat can be road-legal; a de-cat or cat-delete downpipe is for off-road or track use only.
What decides if it's legal
- Legal only if a type-approved catalytic converter stays fitted and working.
- De-cat / cat-delete downpipes: not road-legal (emissions offence).
- A missing cat fails the MOT on emissions-era cars.
- Declare the change to your insurer.
Legal alternatives
- A cat-back exhaust, which leaves the catalytic converter in place.
- A high-flow sports-cat downpipe that is type-approved and road-legal.
Downpipes that fit the Audi S4/S5
On the Audi S4/S5 these are road-legal only with a catalytic converter fitted (a catted or sports-cat downpipe). The catless / de-cat versions are for off-road or track use only.
Related UK legality guides
Sources
This page is general guidance, not legal advice, and reflects our reading of UK rules for a typical Audi S4/S5. Confirm with the manufacturer, your insurer and the latest DVSA/GOV.UK guidance before modifying.