Check engine light on. Code P0300. Replaced spark plugs, replaced air filter, replaced fuel filter. The coils were checked by disconnecting the electrical connection of each one and # 4 and #3 coil had no change in idle. Now code P1515 comes on. What does it mean if no change in idle and why the P1515 code? Runs rough and accelerates slow and gets very poor gas mileage. Please help
Sounds like you have a defective coil. If the secondary is open, you won't get spark, but by unplugging it you created an open circuit in the primary circuit. Only the primary circuit current is monitored by the Engine Computer. That's why it had to be unplugged to set the fault code.
Can you switch two coils to see if the problem goes to two different cylinders? That would prove the module that drives the coils is working.