What are Euro Emission Categories, and what does S/S mean?

At Dowleys, we always aim to give our customers as much information as possible, to help make an informed choice when buying a vehicle.

You may therefore have seen some details in the names of our vehicles which refer to "Euro 6" and "S/S". For example, "SEAT Ibiza 1.0 TSI XCELLENCE Lux Euro 6 (s/s) 5dr" or "Suzuki Vitara 1.4 Boosterjet MHEV SZ-T Auto Euro 6 (s/s) 5dr".

At first glance these details may seem like mere jargon but in fact they refer to important features of the vehicle, covering emissions and economy.

Read on for more information on why understanding these terms can help motorists make a smart choice of vehicle.

Euro emission categories (Euro 1–6/7)

The Euro emission categories are a series of European Union standards that set limits on the amount of harmful pollutants vehicles can emit. Although Britain has left the EU, these standards are still recognised in the UK, and have important functions which we will explain below. They mainly control:

• Nitrogen oxides (NOx)

• Carbon monoxide (CO)

• Particulate matter (PM)

• Hydrocarbons (HC)

Since first establishment in 1992, each new "Euro" stage has set progressively stricter limits, so a Euro 6 car produces lower emissions than a Euro 4 car, for example.

• Euro 1 (1992) – the first step, introduced catalytic converters.

• Euro 2 (1996) – tighter CO and HC limits.

• Euro 3 (2000) – separate NOx limits for the first time.

• Euro 4 (2005) – further reductions in NOx and PM.

• Euro 5 (2009/2011) – diesel particulate filters became common.

• Euro 6 (2014 onward) – big NOx reductions for diesels.

• Euro 7 (expected 2025/26) – will apply stricter rules, including brakes and tyres emissions.

Why do Euro emission categories matter?

These standards are important because they are factors in costs associated with motoring..

• Vehicle tax
  While one might have assumed that compliance with Euro emission categories should postitively affect a vehicle's liability for tax (e.g. the higher a vehicle's compliance, the lower the cost of of vehicle tax might be), UK vehicle tax rates were actually based on C0₂ emissions, which are not covered by Euro emission standards. From 2025, new changes to vehicle tax futher complicate the picture, depending on when the vehicle was first registered.

  
  Rates for cars first registered on or after 1 April 2017 are determined by one of three vehicle tax bands - zero, standard or premium. A sliding scale of tax increasing with C0₂ emissions is applied on new cars, and in subsequent years tax will apply at a set standard rate (even for EVs) or at an additional Premium rate for vehicles worth over £40,000, over the first 5 years the standard rate is applied.
  
  For cars registered between 1 March 2001 to 31 March 2017, the rates are split into different CO₂ bands – the lower the tailpipe emissions, the lower the vehicle tax. Again, higher Euro category vehicles may well have an advantage on reducing tax rates, assuming this means generally lower emissions, but it is not a factor on tax calculations as the other pollutants are not considered, only C0₂ emissions.

• ULEZ/LEZ compliance
  This is a topic where Euro standards really do lead to direct reduction in costs. Drivers whose vehicles enter Ultra-Low Emission Zones (ULEZ) in London must ensure their vehicles meet specific Euro standards for emissions (e.g. Euro 4 for petrol cars, Euro 6 for diesel cars), to avoid paying a daily charge (£12.50 at the time of writing, August 2025). A LEZ charge applies to commercial vehicles. Other UK cities including Birmingham, Bristol, Bath, Bradford, and Sheffield have Clean Air Zones (CAZs) with similar, but different, regulations. In Scotland, Low Emission Zones (LEZs) are in force in Aberdeen, Dundee, Edinburgh, and Glasgow.
   
• Resale values

What does S/S mean?

When you see something like “Euro 6 S/S”, the S/S stands for Stop/Start system.

• This is a fuel-saving and emissions-reducing feature.

• When the vehicle is stationary (e.g., at traffic lights), the engine automatically shuts off.

• It restarts instantly when the driver presses the clutch or accelerator.

It doesn’t change the emission category, but it’s often listed in specs because it helps cars meet fuel economy and CO₂ targets.

In short:

• Euro category = emissions standard.

• S/S = Stop/Start system fitted.

Table to follow

Notes

• ULEZ (London & expanding cities):

  • Petrol: Euro 4 and above compliant

  • Diesel: Euro 6 only compliant

• VED (road tax): Based largely on CO₂ emissions, but Euro standard also matters for older bands.

• S/S (Stop/Start): Often added from Euro 5 onwards to help cars meet CO₂ targets.