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The P0455 isn't pointing to an "exhaust" leak, it's pointing to a leak in the EVAP (emissions control/recovery) System. This system needs to be able to hold a complete vacuum for self-diagnostic tests. When it cannot, the P0455 is set and states there is a "Large Leak" in the system somewhere. First and foremost- Check the Fuel Cap and make sure it is tight and sealing the system. Check the underside of the fuel cap for any cracks or signs of defects that may keep the cap from sealing correctly. If there are any doubts about the inspection of the cap, replace it. Keep in mind, you will need a cap that meets OE Specs. Those "universal" or "locking" fuel caps sold at after market parts stores do not meet OE specs, and will not seal the system correctly. If you are sure the cap is good, then the next course of action is to have the EVAP System "smoked". This is where a machine forces smoke through the system and any leaks will reveal themselves with a stream of smoke.
In the meantime, if you'd like to clear the Engine Light anyway- you can remove the "NEG" cable from the battery for a minimum of 5 minutes. Keep in mind, if the P0455 was not addressed or was not repaired correctly, the light WILL return within 50-75 miles of combined city and highway driving.
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Hi! I noticed this thread was still open and I wanted to check in and see how you made out. Did you have any other questions on this issue?