bought before xmas,and was anon starter,replaced battery and checked all basic stuff and after it one day cranking and not firing and then another day not even cranking it finally sprung into life...runs and drives great,but engine light on in dash,but only runs and starts when it wants too..when it starts and runs the light is on,then it will just stop either on a run or while on spot,and not start again,just crank and not fire,at this point the engine light will be off,then when it decides too a few days or longer after it will start again and be great untill the next time.....with the light on again! had code read and got p4020(catalist not effeicent to bank 1) reset engine light and guess what? now its off it wont run again...and sometimes not even crank over all decides on its mood. no fuel pump charge on ignition on when its like this and strangely the air con light glows slighty with ignition on when the car is a non starter...yet when its a runner it dont........my head truly hurts!!!! please help!

keith