With timing off, cylinder and exhaust port & CATs, loads up with unspent fuel. This will sometimes set off an O2 or CAT DTC. After driving around in normal lower RPM (city driving), to dry CATs. Running on HWY in lower gear at high RPM ~4,500 to 4,800 RPM for 5 to 7 minutes, than back off to normal cruise. Will burn off gunk. It's like turning and oven to clean.