Am I being taken?

My 2005 PT Cruiser was sputtering and hesitating, so I took it to my mechanic who did a diagnostic on it. He said one of my cylinders wasn't firing. He also suggested I change the timing belt (at 164000 km) as preventative maintenance. When everything was put back together, it wouldn't start - just kept turning over but not starting. Mechanic said a wire might have been pinched when putting it back together. Then. He told me that he had misdiagnosed the original problem and that my main computer was the problem and had to be replaced. How likely is this?