Q: Mercedes Misfire codes Po300 301 302 303

Ruling out ignition or fuel system problems is a lot of work. I usually do it by substituting components from non working cylinders to working cylinders to see if the misfire moves. Misfires on one side of the engine like this could be the result of a bad vacuum leak. So you’ll want to listen carefully for a hiss while the engine is idling. If you suspect a mechanical failure in the engine, you can check that with a compression tester and a vacuum gauge. Doing a compression test can give you an idea about the rings and valves, and with the vacuum gauge a good mechanic can determine if the camshaft is worn. Look carefully at the spark plugs for reddish deposits that might indicate head gasket failures. The control units do fail on these cars sometimes but they are very expensive and need to be coded by the dealer so you can’t by them from a salvage yard. So you want to be absolutely certain before you try replacing one. If you want a professional to check it out, Contact Your Mechanic, they will send a technician to your home or office to check out your car and tell you what you need to do to fix it.