Can the fuel I put in affect my car's emissions?

I accidentally put E05 petrol in the car immediately prior to its NCT test last week. About 11 litres was put in with the remaining seven litres of E10 left in the tank. The CO emissions were off the charts (1.41 instead of 0.11 in March). I was wondering if, given that E05 is more like 97 octane, would it be the reason for the high numbers? If I had known that E05 was like 97 octane I would not have got it as it was not an emergency.

Regards

Joe Ball

May 2025 Filed under: NCT

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Expert answer

Hi Joe,

There's no easy answer to this as every car has a different set of sensors and calibration. In short, the level of ethanol in the fuel can certainly affect a car's emissions, but we're surprised by the jump in the level recorded and don't think that alone would account for it unless something very odd is going on with the engine control unit.

Shane O' Donoghue - Complete Car Advisor