I have a 99 Subaru Legacy (Outback, 2.5L). The check engine light has come on and the codes indicate Cylinder 3 and Cylinder 4 misfires. The light came on about two weeks ago, and occasionally disapears for a day or two, then returns. I have recently replaced the spark plugs and wires and coil. I know these codes can be caused by numerous problems, but any suggestions as to where to start would be appreciated. Joe

You'll have to get someone to check the spark and injector on cylinders 3 and 4.
Code 303 is for loss of power on cylinder 3 and code 304 is the same for cylinder 4. The most common causes are lack of spark or fuel.
Once the problems are fixed the codes will need to be erased with a scanner or code reader.

I would change it anyway. I would scan it again and see if the knock sensor code is stored.

Any time your check engine light is flashing this indicates an engine misfire that has the potential to damage your catalytic conveter. A diagnostic trouble code has been stored in the vehicles onboard computer that will confirm the engine misfire. Continued driving may ruin your very expensive catalytic converter located in your front exhaust pipe.
Good luck, I hope this answers your question.

I have the same problem!Running on three.I have checked compression,wires,plugs.Changed injecters,plugs.
3 mech.cant figuire it out???

try pulling the plug out and putting a very small dab of loctite thread locker on it and see if this stops the problem.

try opening door and holding unlock button ,it sounds like its in valet mode

when you replaced the plugs , was one of them (cylinder 1 ) dirty and fouled ?
I'm thinking that bad rings in cylinder would give you low compression , and eventualy cause the missfire by fouling up the plug.
There is an inexpensive product that will help with the compression by filling up scratches in the cylinder walls. It is called Restore , and it is one of the only products of its type that I have used , and feel actually works.
Unfortunately , if your rings are broken , or badly worn , nothing in a bottle will fix them.

If your car had a distributor cap and rotor , I would change them too.

Get the P code read at your local parts chain store for free. It sounds like you need to find this out very soon. The code will tell you exactly what part is failing.

Many things can cause a misfire,but basic mechanical checks or tests can be made relatively inexpensively.Compression testing the cylinders with a compression gauge will verify the general condition of the engine's piston rings and valve sealing capabilities.A leakdown test is similar to a compression check except that a leak down test is directed more towards verifying specifically the valve's sealing capabilities.A valve can burn or a bad seal occur between valve and seat...causing a misfire.Valves and piston rings are mechanical in nature,but there are many other possible causes.Vacuum leak,ignition problems,fuel problems,sensor problem,etc etc.Good luck.

any check engine code?