Reply To: 2003 Nissan Code P 0420

[hondaguy453]

You can confirm you have a clogged or restricted cat by hooking up a vacuum gauge. Note vacuum at idle, then increase engine RPM to about 3000 RPM. Vacuum should drop, then steadily climb back up to the reading you had at idle. If it doesn’t climb back up, or it does climb, but drops back down, this would indicate that your exhaust is restricted. A more invasive approach would be to remove the cat, if it’s mounted to the exhaust of course, and to check if it’s clogged.