I have a 1999 Subaru Legacy 2.5 4 cyl wagon. Compute code indicated 1 & 3 not firing. What could be problem?

To begin with, check the spark plugs for fouling.

Coil, ignition wire or spark plugs. Have it tuned up.

By compute code, do u mean ur getting a P0301, & a P0303?

Those would be cylinders 1 & 3 misfire codes, not to be confused with cylinders 1 & 3 not firing at all.

That engine uses a NGK Platinum spark plug that has a recommended service interval of 60,000 miles. Due to the heat that is in the spark plug tube, the boot ends often get hard & crack which can cause the spark to jump out of the plug wire end & ground onto the spark plug tube. That creates a misfire & can set ur ''check engine'' light.

It may also be caused by worn spark plugs & or a faulty ignition coil.

Good luck to you!