Hi Thanks for the response.
According to the Mrs who drives the car its when its warm. I did not do any further checks to ensure that its firing on all four cyclinders as I replaced two
earlier this year and my assumptiom was that it might be one of the last two
despite no warning lights. I had them as spares as I had bought a full set so thought I had nothing to lose by fitting them. Am I right in thinking that if it was mis firing then the warning light would come on?