Mechanics

DIAGNOSTIC SHOWS P0420,HOW DO I REPAIR THAT.

1997 Mercury Marquis • 90,000 miles

Catalyst system Efficiency Below Threshold Bank 1 is what shows up. Took it in auto zone did a diagnostic and showed cadalack coverter. But today showed the P0420
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Debbie taufi
June 8, 2012.



Best to have a pro look at it. It can be a converter bad, a coolant leak cooling off the converter, a bad O2 sensor, many things.

Hmac300
Jun 8, 2012.