Hello and welcome to JustAnswer!
Which side of the engine was the coil replaced on? Was it the left (driver's side) or right?
Do you have a test light to do some electrical testing?
What were the compression numbers on the other cylinders?
The other problems aren't related.
I didn't know a mechanic was working on it, I had figured that you were. Do you have the car now or is it still at a mechanic? If you have the car and a test light I'd like to do some testing.
Do you have a test light and want to do some electrical testing, or would you just like some possible causes?
This code sets when the control circuit for the number 4 ignition coil isn't a complete circuit from the ASD relay through the coil back to the engine controller (PCM). It may or may not be related to the (possible) low compression that was found. We need to look into the electrical circuit first.
Start by disconnecting the number 4 coil and find the dark green/orange wire in the connector. Attach the alligator clip end of the test light to ground, use the probe end of the test light on the dark green/orange wire. Start the engine and see if the test light lights up.
If it doesn't that means there is a broken wire that we'll have to find.
If it does light up then attach the alligator clip of the test light to battery positive and probe the other wire in the coil connector. Start the engine and watch the test light, it should rapidly flash while the engine is running. If not, then there is either a broken wire or the coil driver is burnt out of the PCM. The next thing we would need to do from there is go right to the PCM and check the coil driver there.
how do we check the coil driver in the PCM? test light also? i might take the car back with me. the mechanic insists on the low compression issue. i know they're out there to make money, but I've spent enough for penalties and other fees from the DMV. So the last thing I want to hear is there is a problem with the motor.
Sorry to hear about the bad luck. Thanks for the update, and good luck with your car!