P0420: catalyst efficiency
P0420 means the Bank 1 catalyst failed an efficiency check. Learn how exhaust leaks, sensor response, and engine problems affect the repair decision.
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Warning lights & codes
Understand the warning before choosing a repair.
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P0300: random misfire
P0300 means the engine detected a misfire pattern. Learn what to do if the light flashes and how cylinder data and driving conditions narrow the cause.
P0171: a lean-running engine
Understand P0171, how fuel trims reveal a lean pattern, and why checking for unmetered air can be more useful than replacing an oxygen sensor.
A flashing check engine light
A flashing check engine light while driving can signal a damaging misfire. Know what to do if the car shakes, loses power, or the flashing stops.
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