Chapter 8 — Section 7: Exhaust, fuel and emissions

Section 7 ties routine maintenance to air quality standards and the environmental requirements of the Road Traffic Act 1988. The tester checks both the physical condition of the exhaust and fuel systems and the actual chemical output from the tailpipe. Failures here often point to something deeper than the exhaust itself, anything from a vacuum leak to a tired engine. The two aims are keeping exhaust gases away from the cabin, and confirming combustion stays within the limits set when the car was first registered.

01Physical condition and system security

Before the gas analyser or smoke meter comes out, the tester checks the hardware by eye, the full length of the exhaust from manifold to tailpipe. They're looking for corrosion that could cause a failure, or leaks that let fumes into the cabin or bypass the test equipment. Aftermarket changes that significantly increase noise are also grounds for failure. Fuel system integrity matters just as much: any leak, however small, is an immediate fail because of the fire risk. The fuel cap's seal and tether are checked, and where there's a locking cap the tester may ask for the key to confirm it works properly.

02Petrol emissions: the gas analyser

For petrol engines the tester uses a gas analyser to measure carbon monoxide and hydrocarbons, along with the lambda value. Cars first used after 1 August 1995 typically go through a fast idle test at around 2,500 to 3,000rpm followed by a normal idle check. A modern Euro 6 petrol engine is typically expected to stay around 0.2% CO at fast idle and 200ppm HC. A high HC reading usually points to unburnt fuel, often from a misfire, while high CO tends to suggest a rich mixture or a failing catalytic converter.

03Diesel emissions and the smoke meter

Diesels are tested for smoke opacity rather than gas composition, measured as a k-value in m⁻¹, which reflects the density of particulate in the exhaust. The tester runs a series of free-acceleration tests, revving the engine from idle to its governed maximum. For vehicles first used after 1 January 2014 the limit is 0.7m⁻¹, with older cars allowed higher figures depending on age and whether the engine is turbocharged. Revving the engine to its governor doesn't harm it; if a car can't withstand that, it's arguably not roadworthy anyway. A fail is recorded if the average of three readings exceeds the limit, or if the smoke is dense enough to obscure the equipment's sensors.

04Hybrid vehicles and cold starts

Hybrids and plug-in hybrids can be awkward to test because the petrol or diesel engine may not run continuously at idle. Testers often use a maintenance or service mode through the car's own software to keep the engine running while the probe is in place. The engine needs to be at normal operating temperature first; a hybrid that's been sitting in a cold test bay for a couple of hours can fail simply because the catalytic converter hasn't reached its working temperature of around 200 to 300°C, giving misleading readings.

05DPF and EGR: tamper checks

Exhaust gas recirculation and the diesel particulate filter (DPF) are both checked for signs of interference:

• The tester confirms a DPF is present on any vehicle where one was fitted as standard.
• A DPF that's been cut open and re-welded, or replaced with a straight-through pipe, is an automatic Major defect.
• Visible removal or bypassing of EGR coolers or piping also fails the car.
• A lit engine management light specifically relating to emissions is a fail, not an advisory.
• Excessive smoke of any colour from a DPF-equipped car is usually taken as a sign the filter has been removed or has failed.

06Advisories and minor defects

Emissions results are often pass or fail, but testers do use advisories for borderline cases: a minor exhaust gas leak, or corrosion that hasn't yet weakened the system. On diesels, a smoke reading close to the limit might be flagged as an advisory, suggesting the injectors or intake could do with attention before the next test. Fuel pipes with light corrosion but no leak are often noted the same way, as an early warning rather than a failure.

Passing Section 7 needs the hardware to be sound, the emissions control components to be working, and the engine itself in reasonable tune. None of that is exotic, but it does mean a neglected engine will eventually show up here even if nothing is visibly wrong.