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Check Engine light won't turn off - even though repair most likely accomplished

2004 Ford Focus, 2 Liter Zetec engine.
0. Situation: Car initially running well but want to improve mileage, so I installed premium plugs.

  1. Ran roughly on test run, so headed home. Check Engine light came on before fully warmed up.
  2. Found and corrected loose plug wire.
  3. Engine now purrs like a kitten, but Check Engine light stays on.

What should I do now?

if you don;t have a scan tool go by auto zone or advance auto they will cut it off free of charge.

You should note the codes just in case they come back. The exact codes, and not what they think may be wrong.

Most Fords will reset if you disconnect the battery for a minute or so…

It takes three drive cycles with out a DTC to turn off the check engine light. If you haven’t done the three complete cycles, have patience. If it doesn’t go out after three drive cycles, then get the code(s) read and report them back here.

It is remotely possible that you knocked a vacuum line or a connector loose or something.

I agree with keith here.

How do “premium plugs” improve gas mileage?

A lot of people have issues with plugs that aren’t OEM. I’d go back to the OEM plugs myself.

Thank you everybody. Caddyman’s suggestion worked.