Why is the service engine light coming on and the car vibrates when I stop?

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The car blew the radiator a few weeks ago, the mechanic replaced the radiator, water pump and thermostat. I had the car back for one day, had to take it back vibrates and engine light came on. They replaced all spark plugs and wires. I had the car back for one day, service engine light came on and vibrates at stops. Took the car back, why can't they figure out what the problem is? They do the diagnostic thing, come up with numbers but they can't come up with a solution.