2005 Citroen C3 CITROEN C3 ENGINE LIGHT

Hi i hope you can help, i reciently broke down and was recovered by the rac. they said the problem was prob. the coil pack as it was running on three cylinders.

I replaced this and all was well untill two hours later after driving the car i now have the engine management light on. the car seems to be running ok any ideas what else could be wrong,

thanks in advance