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I was just gifted a 2004 DeVille by my father and just ran in to some trouble with misfires after a long road trip. The misfires happen while at an idle, low and high speed.

these are the codes I pulled:

P0455: EVAP System leak detected (large leak)
P0420: Catalyst System Efficiency Below Threshold Bank 1
P0410: AIR System A
P0306: Misfire Detected (Cylinder 6)

I also have a warning on my dash that the gas cap is open(it isn't)

What sort of self diagnosis can I do to try and isolate the problem

Thanks
 

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2001 Deville base
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P0455: EVAP System leak detected (large leak)
Look at the gas cap. Even if the seal has small cracks in it, it will throw a code. Other than that, check for damage to the fuel filler neck.

P0420: Catalyst System Efficiency Below Threshold Bank 1
When was the last time the O2 sensors were replaced? These are cheap.

P0410: AIR System A
This should not create any misfires. If your state does emissions testing, you'll need to replace it. About $200. Mine is broken and I don't have plans to replace it.
Bad design.

P0306: Misfire Detected (Cylinder 6)
I would put a long screwdriver to my ear and find out if the fuel injector on #6 is working. If it is (clack-clack-clack), I would definitely check the plug.
Then check to see if any connectors are loose, any wires frayed, etc.

These are just the baby, visual steps to rule out the obvious. Most of my problems were "fixed" by replacing gas caps, cleaning connectors, etc.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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Check the ignition coil assembly and the #6 fuel injector for function. The left (front) bank is cylinders 2-4-6-8.

AIR is a blower system that sends fresh air to the exhaust manifold(s) during cold startup to quickly light off the catalytic converter.

In your area of cold weather and road chemicals it's possible that the EVAP large leak is due to a rusted out fuel fill pipe. Maybe a cracked vacuum or fresh air line.

Also check the EVAP purge line test port - should have a green valve cap over next to the power brake booster. If that cap is loose or missing it's a big vacuum leak when EVAP is operating.

EVAP purges only during road load speeds over 30 mph with cruise load gas pedal applied. Then, the EVAP solenoid (6) shifts to fresh air intake, the purge valve (1) opens and intake manifold vacuum purges the stored vapor in the charcoal canister. EVAP operates only during conditions when the extra fresh air will not affect driveability, and EVAP does not operate constantly - only for a set, short period.

Here's your EVAP line drawing.

2002 EVAP.gif
 
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