P0351 Toyota -

Question

I have Toyota Avalon 2007 model. My engine light was on three weeks ago and I took my car to mechanic to check and rectify the problem. After connceting computer to my car error code related to coil pack secondary/primary ignition of all 6 cylenders came up along with randum/multiple cycliner missfirig error. We changed spark plugs and checked coil pack with new one but same error appeared. The problem still has not solved

I would like to know what is the source of this problem and can be due to car's computer failure or not?