As previously stated, erase the codes, and run until the MIL lights up again.
BUT FIRST...check over the basic engine. Plugs and their condition/age. Distributor cap and rotor. Secondary wiring (spark plug wires). Be sure that the engine is running on ALL cylinders. A compression test may also be advised. And be advised:
THERE IS NO SUCH THING AS A TUNE-UP!!!!!!!!!
Look it up in the dictionary. To tune means to adjust for peak performance or efficiency. Everything that was ever adjustable in the "old days" has been replaced by electronics. There are no points, therefore no dwell adjustment. Timing is synthesized by the PCM. No longer adjustable. Mixture, choke, and fast idle settings were part of a carburetor, which has been replaced by injectors. Nothing is left that is "tuneable" in modern engines of the last 25 years or so.
The garage that offers to sell you a "tune-up" is ROBBING YOU!!!