do coils simply work or not, ive lost around 5-7mpg and performance is down, assuming its the cheap lemark coils as they have done 18 months and 15000 miles, 1 of them has defenately gone as it was missing heavily under load and went away when i swapped in the spare ngk i had, but whats bothering me is the last time i had a coil go it went very quickly got flashing cel then mins later off a cylinder with constant cel,
this time no flashing light or cel even though the coil was clearly at fault, could the remaining 3 coils still be misfiring and causing issues even without a cel, thinking maybe they are on there way out and just about to fail completely