The root of a check engine light

I'm sure everyone and their mom has had a check engine light come on, I was wondering if anyone has been down the specific road I'm on.

The light came on about 3 weeks ago, I took it into my mechanic and he said it was some coolant temp issue or something that wouldn't really be a problem.

I may have felt some shudders in the engine in the time that passed, but 3 weeks and about 800 miles later the light started blinking and the engine was shaking a lot, especially when idling.

I thought it was a transmission issue and took it back to the shop. They found that all 3 ignition coils were shot and replaced them for about $500.

On the way home from the shop, the light came back on. I will have the codes checked again tomorrow.

Has anyone else had a problem like this? I'm thinking that the coils were not the original problem, just a symptom.