# Ford Focus check engine light

**URL:** <https://community.cartalk.com/t/ford-focus-check-engine-light/65074>\
**Category:** Maintenance/Repairs\
**Created:** [May 30, 2011, 4:08pm UTC](https://community.cartalk.com/t/ford-focus-check-engine-light/65074 "2011-05-30T16:08:26Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![justcruzin](https://avatars.discourse-cdn.com/v4/letter/j/838e76/32.png) [@justcruzin](https://community.cartalk.com/u/justcruzin)\
**Post date:** [May 30, 2011, 4:08pm UTC](https://community.cartalk.com/t/ford-focus-check-engine-light/65074/1 "2011-05-30T16:08:26Z")

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My 2004 Focus passed its emissions test in Maryland in the late fall last year. Shortly thereafter, the check engine light came on. My mechanic put it on the computer and said it needed a new catalytic converter. It only had 86,000 miles on it. I watched and waited for a few months and had the fuel filter changed during a routine oil change. A couple of days after the fuel filter was replaced the check engine light went off and stayed off for 4 months, but it came back on a couple of days ago. I have been watching the mileage carefully and it has been steady at about 26 MPG for years. Can anyone make a suggestion about this?

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**Author:** ![keith](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/keith/32/5518_2.png) [@keith](https://community.cartalk.com/u/keith)\
**Post date:** [May 30, 2011, 4:22pm UTC](https://community.cartalk.com/t/ford-focus-check-engine-light/65074/2 "2011-05-30T16:22:55Z")

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You need to get the code read and let us know what the code it. We need the number preceded by the letter P. Many car parts stores like AutoZone will read the code for you and give you a printout of what it means. But before you do anything, post the number here, sometimes the printout can be misleading.

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**Author:** ![justcruzin](https://avatars.discourse-cdn.com/v4/letter/j/838e76/32.png) [@justcruzin](https://community.cartalk.com/u/justcruzin)\
**Post date:** [May 30, 2011, 8:01pm UTC](https://community.cartalk.com/t/ford-focus-check-engine-light/65074/3 "2011-05-30T20:01:43Z")

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Thanks. I’ll do that ASAP and get back to you!

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**Author:** ![justcruzin](https://avatars.discourse-cdn.com/v4/letter/j/838e76/32.png) [@justcruzin](https://community.cartalk.com/u/justcruzin)\
**Post date:** [May 30, 2011, 10:12pm UTC](https://community.cartalk.com/t/ford-focus-check-engine-light/65074/4 "2011-05-30T22:12:47Z")

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OK. The parts store computer gave us this message: P0420 Catalyst system efficiency below threshold bank 1.

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**Author:** ![keith](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/keith/32/5518_2.png) [@keith](https://community.cartalk.com/u/keith)\
**Post date:** [May 30, 2011, 11:00pm UTC](https://community.cartalk.com/t/ford-focus-check-engine-light/65074/5 "2011-05-30T23:00:26Z")

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This can be a leak in the exhaust system, most likely a failed gasket between the exhaust manifold and the precat. It could also be the rear oxygen sensor. Could also be the cat. Sometimes water gets into the rear oxygen sensor connector and that can do it too.

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**Author:** ![justcruzin](https://avatars.discourse-cdn.com/v4/letter/j/838e76/32.png) [@justcruzin](https://community.cartalk.com/u/justcruzin)\
**Post date:** [June 1, 2011, 12:36am UTC](https://community.cartalk.com/t/ford-focus-check-engine-light/65074/6 "2011-06-01T00:36:05Z")

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Thank you. We’ll look into it.