My neighbor has a 2005 Grand Prix with a 3.8 liter V-6 that has 120,000 miles on it. Just as he got home, car began to run rough and engine check light came on. OBD2 readout was P0300 which is engine misfiring randomly. No other codes. Further investigation showed that a mouse had built a nest under the intake shroud over the top. No damage to injector wires etc was found. I discovered that #5 cylinder was not firing. Changing plug didn't not help. Further checking revealed that #2 cylinder was not firing either. Both of these cylinders fire off the same coil. I replaced the coil and I still had the same results. #2 and #5 cylinders had no spark. At this point, I am at a point where I don't know where to go next. Any suggestions?

This link may offer a few ideas:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-11.html#post2028083