what needs to be fixed when the check engine code 300 for a 2003 x type

P1300 Boost Calibration Fault...whatever that means. Hopefully somebody that knows jag's will help

Or it could be P0300 - random misfire. Try replacing the spark plugs and air filter.

Spark Plugs definately but also manifold gasket should be replaced too as there are located nearby . , air filter leaks need fixing , by changing imt o-rings , breather hose ,and brake booster check valve . After that clear the codes with code reader, if it still comes back , then check fuel filter.

You might also consider cleaning the mass air flow sensor located on the battery side on the intake. Remove the battery cover and disconnect the battery. Then get a screw driver (philips on my 2003 x-type) and remove.two screws on the sensor. The sensor is very near the positive terminal post. Remove carefully and to not touch the small filament. Go to an auto parts store and buy CRC cleaner. That is very important - do not use brake cleaner or anything else. You don't need to disconnect the wire assembly if you don't want. Put a rag underneath the part and spray liberally with CRC and whatever is sticking to the filament will spray off.. Wait maybe 20 minutes for it to dry completely and reinstall. It should be as snug as originally. You might also take this time to clean the negative terminal of any corrosion, using a mixture of baking soda and water and a toothbrush.. Buy some terminal protectors for a couple bucks and reinstall battery connections. Hope this helps. And what Raihan said about the o-rings and hose is advisable. Look also at jaguar forums by googling that.