it was misfiring before this problem
With no information on the vehicle, or details about the symptoms, the issues are impossible to definitively diagnose. A misfire and stalling issue can be due to fuel issues, spark issues, or combustion issues. Fuel issues would arise if you have dirty or clogged fuel injectors, if the fuel filter is clogged, or if the lines are clogged. Spark issues usually arise from a bad plug, wire, or distributor. These are typically an easy and relatively cheap fix. Depending on the amount of miles since you changed the spark plugs last, you may want to start there. When doing so, make sure your spark plug wires are clean, dry, and tight. If the engine is worn down significantly, the compression may be too low to maintain the combustion cycle.
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