If a coil (in a coil-pack arrangement) is bad then the engine is probably misfiring on two cylinders.
You can purchase plastic pliers that are designed specifically for pulling sparkplug wires while the engine is running.
Start the engine, pull one plug wire from the coil pack using the plastic pliers. (don't use metal, you'll get a shock that will change your attitude) Go through the plug wires until you find one that DOESN'T make the engine run worse when you pull the wire. To verify that THAT is the bad coil, pull the OTHER wire and see if it doesn't affect the way the engine is running.
Engine misfire. Possibly a bad coil or spark plug.
If the check engine light is flashing the engine is misfiring. Most likely a bad ignition coil.
If the reason you want to check the coils is because of a check engine misfire code swap the coil with a coil from another cylinder, clear the codes, drive if the check engine light comes back on see if the code follows the coil you moved. If so replace the coil.
it could be the coil
When i stops firing. The check engine light will come on when it goes bad.
Check engine light comes on, mileage drops, may end up toasting a cylinder coil which is a cylinder misfire or flashing check engine light.
Could be a weak ignition coil Check for fuel and spark
Hi the flashing coil light doubles as an engine management light the computer has probably got a stored fault code and put the engine into safe mode. Diagnostic check required.
check, plugs, coil, distributor
The computer stores the codes until it is reset manually. The check engine light does not have to be on to check the codes.
could the coil pack be bad There are hundreds of things that could turn the light on. You need to have it checked with a scan tool.
The heater core will not have an effect on the check engine light. See related question below for more info on misfires and engine light.