wont start what else could it be

A,re you saying it cranks good, but, won't start? Or the starter motor won't crank the engine? If it cranks, have helper crank, you visually check for spark at the plugs-- use a gage and check proper fuel pressure-- check fuel injector pulse. If you use a jump box or jumper cables and it starts and runs fine, i'd have to check for voltage drop, somewhere. you could use digital multimeter across battery posts, generic spec for full charge, 12.6 volts. with meter leads still touching battery posts, have helper turn key to crank, note the reading, generally shouldn't drop below 9.6 volts. if it drops way down, that isn't going to work. battery cables are all good? W/o jumper cables or anything, engine cranking, all your systems get battery voltage, engine running, the system get generator voltage, there can be a several volt difference.