2000 Mercury Marquis Repair Question
Asked on November 1, 2007
PO420 on 2000 Grand Marquis
I recently had the PO 420 code show up on my Grand Marquis. I've replaced the downstream O2 Sensor on bank one. It didn't fix the problem and the PO420 code showed back up. I've replaced the Cats on the left side and it still didn't fix the problem. PO420 showed back up again. I've checked the pins & connections on the CPM and all seems to be fine and I can't find any vacuum leaks anywhere . Any ideas as to what else the problem could be??
Replied on August 18, 2007
If you hear the pump buzzed for that few secs-=it means it came On. If you remove the filter and see it exit-it don't mean a thing-it has to produce the correct pressure.
You need to hook-up a fuel pressure gauge to the rail and verify the pressure with and w/o the fuel pressure regulator.