There is no interval to replace the coils. If they aren't broken, no need to reaplce them. Have the fault codes pulled on the car first to start with the diagnosis of the problem.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record