Check engine light is on
Engine misfires
Rough idle
Hard starting
Engine stalls
Poor acceleration
Reduced fuel economy
Backfiring
Strong fuel smell from exhaust
Visible cracks or damage on the coil
Burn marks or corrosion on the coil
Engine runs worse when the coil is hot
Trouble code points to a specific cylinder misfire
Swap the coil with another cylinder and see if the misfire moves
Test coil resistance with a multimeter
Test spark output with a spark tester