# 2002 Mercury Grand Marquis - Misfires

**URL:** <https://community.cartalk.com/t/2002-mercury-grand-marquis-misfires/164612>\
**Category:** Maintenance/Repairs\
**Tags:** mercury\
**Created:** [May 20, 2020, 7:48pm UTC](https://community.cartalk.com/t/2002-mercury-grand-marquis-misfires/164612 "2020-05-20T19:48:53Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![StevieMK](https://avatars.discourse-cdn.com/v4/letter/s/3ec8ea/32.png) [@StevieMK](https://community.cartalk.com/u/StevieMK)\
**Post date:** [May 20, 2020, 7:48pm UTC](https://community.cartalk.com/t/2002-mercury-grand-marquis-misfires/164612/1 "2020-05-20T19:48:53Z")

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My 4.6 just started misfiring on cyl 5 (code 305).  
I hooked up a code reader and it also gave me  
(code 453) Evaporative Emission System Pressure Sensor / Switch High

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<div class="post-metadata">

**Author:** ![Mustangman](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/mustangman/32/44130_2.png) [@Mustangman](https://community.cartalk.com/u/Mustangman)\
**Post date:** [May 21, 2020, 2:11pm UTC](https://community.cartalk.com/t/2002-mercury-grand-marquis-misfires/164612/2 "2020-05-21T14:11:23Z")

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> [@StevieMK](#):
>
> My 4.6 just started misfiring on cyl 5 (code 305).

Start by removing spark plug #5. If it not oily but a bit black with carbon, replace it. Does that solve the P0305? Yes? Done. If it doesn’t, replace the coil to #5

If the plug is oily, remove the other 7 and run a compression test. #5 may be on its way out.

The P0453 is a bad pressure sensor for the evap system. Not sure where this sensor is on a Merc. It may be on top of the tank by the fuel pump inlet.