Truck is a 1996 F150 , 4.9L/5 speed OD ,1/2 ton ! On and off problem , on again ! Scanned PO420 -" Catalyst efficiency low-bank 1" ! Both O2 sensors have been replaced ( 3 months ago ) , throttle body cleaned , IAC valve cleaned , MAF sensor cleaned , new thermostat , new plugs and wires , new distributor cap , new PCV valve , and new fuel filter . Scan list of probable causes are # 1 -Cat conv. defective ( possibly because of # 2,# 3, or # 4 ) . # 2 -engine misfire or running condition . # 3 - large vacuum leak , or # 4 - engine oil leakage into exhaust-valve guide, seals , or piston rings . ( # 2 ) - Engine does seem to have a slight miss and seems to slightly stumble under acceleration and does not have much power . ( # 3 ) - Have already replaced several small vacuum hoses because of tiny cracks and can not find any major leaks ? ( # 4 ) - Engine does not smoke or use oil . It does have one strange habit - the engine temp gauge normally runs ( when at operating temp. ) slightly below the mid point . Occasionally , the idle will hover around 1200 rpms and when this happens , the gauge will show the temp going up top about 3/4 on the temp gauge . That usually happens in slow traffic and tends to get me very nervous ! When the engine idles at it's normal range , the temp seems to stay in the appropriate range . The TPS switch has not been replaced . I live where emission tests are done and this is my month to renew my tags ! The " check engine " light and the cause of it have to be repaired before the end of the month ! I was hoping to fix it myself but may have to take it to a professional if I can't figure it out soon . Any and all help will be truly appreciated !