Downpipe and vehicle inspection: what you need to know

Updated: 30 September 2026.

At vehicle inspection in France, a downpipe is not judged on its brand but on four points: the visible presence of the catalytic converter, the absence of leaks, the gas measurement (CO and lambda on petrol, opacity on diesel) and the OBD reading. A downpipe with a 200-cell catalytic converter keeps emissions control and can stay within the limits; a decat downpipe removes an emissions control device, which is a major defect at inspection and an offence on public roads.

This article follows the official French inspection method, point by point. For the wider question of removing the catalytic converter, see decatting your car: allowed or not?.

What the inspector checks, in order.

In France, car inspection follows the decree of 18 June 1991 and its technical instructions. The one covering the exhaust (IT VL F8, “Nuisances”) sets out the method. For a petrol car, this gives:

For a petrol car first registered after 2 July 2002, the limits are 0.3% CO at idle and 0.2% at high idle. A useful detail: the instruction states that a vehicle fitted with a mounting point for a lambda sensor or catalytic converter is tested as an emissions-controlled vehicle “even if the catalytic converter and/or the lambda sensor have been removed”. Removing the catalytic converter therefore does not change the measurement method.

Downpipe with a 200-cell catalytic converter vs decat.

With a 200-cell downpipe, the catalytic converter is present and visible. With the engine warm and properly tuned, CO and lambda can stay within tolerances. The sensitive point is OBD: the lambda sensor placed after the catalytic converter may judge its efficiency insufficient and turn on the warning light (code P0420). With the light off and compliant readings, the car is presented in good conditions, although nobody can guarantee the result on the day.

With a decat downpipe, three points fail at once: the emissions control equipment is missing (8.2.11.a.2), the OBD warning light comes on in the vast majority of cases, and the gas readings often exceed the limits. It is designed for the track.

With a 100-cell downpipe, emissions control is reduced: depending on the engine, it may or may not pass the measurements. Whether it is accepted depends on the vehicle and the country's rules.

And on diesel: opacity and particulate filter.

On a diesel, the downpipe usually carries the oxidation catalytic converter and often the particulate filter. The inspection measures smoke opacity (item 8.2.22): without a manufacturer value, the limit is 0.70 m⁻¹ for a Euro 6 vehicle and 1.50 m⁻¹ for a vehicle first registered since 1 July 2008. Visible absence of the particulate filter falls under item 8.2.21.a.2, a major defect. Rog does not make diesel parts.

What the law says, plainly.

In France, two texts apply:

• Article L318-3 of the Highway Code (Code de la route): a fine of €7,500 applies to “carrying out or having carried out on a vehicle modifications that remove an emissions control device, degrade its performance or mask any malfunction”. The fixed penalty is €200.
• Article R318-1: a vehicle must not emit pollutant gases in conditions that inconvenience the public; class 4 offence (contravention de 4e classe), with possible immobilisation of the vehicle.

A downpipe is therefore a part like any other; it is the removal or degradation of emissions control that causes the problem. A downpipe with a sports catalytic converter keeps the converter, a decat one removes it. Outside France, the rules and the inspection change from one country to the next: check with the testing body in your country (see also noise checks in Europe).

Engine by engine, with no promises.

How a car behaves at inspection depends on the engine and on how its ECU monitors it. Three common families:

• BMW B58 (M140i, M240i, M340i, Supra 3.0): one turbo, a downpipe carrying the catalytic converter, and a petrol particulate filter on recent versions. The 200-cell unit is the road configuration; do not touch the particulate filter.
• BMW S55 (M3 F80, M4 F82, M2 Competition): two turbos, two downpipes with catalytic converters. Same logic: 200 cells for the road.
• Volkswagen and Audi EA888 (Golf GTI and R, S3): one downpipe with a catalytic converter, plus a particulate filter on the most recent versions.

In every case, have any fault code diagnosed before the test: a code alone with a compliant result is a minor defect, a warning light that stays on is a major one.

The checklist before inspection.

1. Engine warning light off with the engine running, and coming on properly with the ignition on.
2. No leaks at the downpipe flanges (soot marks, hissing noise).
3. Engine at temperature, oil at the right level: the inspector checks it.
4. Valves closed: noise is judged by ear (item 8.1.1), and a valved exhaust system with the valves closed stays close to its original sound.
5. Invoice for the part to hand, stating its type of catalytic converter.

For the rest of the exhaust, see will my sports exhaust pass vehicle inspection?

Rog downpipes.

On our valved exhaust systems, the downpipe is chosen as an option (listed as “Manifold” on the product pages): 200-cell catalytic converter for the road, 100-cell accepted or not depending on the vehicle and the country, decat for the track. Examples of extra costs for the 200-cell version in stainless steel, checked on 30/09/2026: + €650 on the BMW M140i exhaust, + €780 on the Volkswagen Golf 7.5 GTI exhaust, + €820 on the BMW M3 F80 / M4 F82 exhaust. Made to order from your VIN, 20 working days, free shipping, 3-year warranty. To understand the part itself: downpipe: what it is and what it does

Frequently asked questions.

Does a downpipe pass vehicle inspection?
A downpipe with a 200-cell catalytic converter keeps the converter: it can pass if the readings are within limits and the OBD warning light is off. A decat downpipe results in a major defect.

Is a downpipe allowed?
The part itself is not banned. What is punished in France is removing or degrading emissions control, or masking its malfunction (Article L318-3). A decat downpipe is therefore for track use only.

Can the inspector spot a decat downpipe?
The inspector visually checks that the emissions control equipment is present (item 8.2.11), measures the gases and reads the OBD. A missing catalytic converter usually shows up in at least one of these three checks.

Should you refit the original downpipe for the inspection?
With a 200-cell downpipe in good condition, with no leaks and the warning light off, it is generally not necessary. With a decat version, the car is not intended for the road in any case.

Is downpipe noise measured at inspection?
No, not for a car: noise is assessed by ear. An “abnormally high” noise is a major defect, which is why it makes sense to present a valved exhaust system with the valves closed.

Sources.

All consulted on 30/09/2026.

1. Technical instruction IT VL F8 index G “Nuisances”, periodic inspection of light vehicles in France (23/12/2020), published by the FNA: https://fna.fr/wp-content/uploads/2023/05/IT-VL-F8G-NUISANCES-20201223.pdf
2. Légifrance, Article L318-3 of the French Highway Code (version in force since 26/01/2023): https://www.legifrance.gouv.fr/codes/article_lc/LEGIARTI000047053619
3. Légifrance, Article R318-1 of the French Highway Code: https://www.legifrance.gouv.fr/codes/article_lc/LEGIARTI000023821569
4. Edmunds, code P0420: https://www.edmunds.com/obd-dtc/p0420.html