Volvo 850 Manual page 146

As part of the intensive efforts being invested in the improvement of the environment, it is important that you as a car
owner understand which components may affect the contents of the exhaust gas and which measures need to be adopted
to limit the emission of harmful exhaust gases.
Low exhaust emission therefore requires ...
... as regards service:
That the car receives regular service in accordance with the Volvo
Service Programme. The implication of this concept is dealt with in
greater detail on 7:2 and in the Service booklet. We recommend that
you schedule your car for service just prior to the annual roadworthi-
ness (MOT) test.
as regards engine components:
...
That the engine receives proper lubrication. 7:6 describes the replace-
ment of engine oil and the oil filter.
That the exhaust system is totally free from leakage and is in sound
condition.
as regards the fuel system:
...
That the pipes and connections are sound and free from leakage.
That the fuel and air filters are not blocked.
That the engine controls function smoothly.
That cars with catalytic converters are always driven on unleaded
petrol.
as regards the ignition system:
...
That the spark plugs are undamaged and have the correct electrode
gap.