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 !
Not an expert but if nothing else this will bump.
A few causes, bad cat or exhaust leak or fuel pressure too high or defective wiring to downstream oxygen sensor or bad engine coolant temperature sensor.
check fuel pressure, you may have a bad regulater, and pressure is too high. does your oil smell like gas? if you have a fuel problem, it will burn out the cat. hit the bottom of your muffler with your hand to see if it rattles, like pieces of the cat are caught inside. do this when it's cold though so it don't burn your hand
I had the catalytic converter replaced today and it made a huge difference in the way it runs ! The unit was apparently the original one ( 180,000 miles ! ) and it was disintegrated inside ! The second converter was full of debris from the first one and the muffler shop guy said the second one was the California required unit and not needed in our area ! So it was removed and replaced with a straight pipe . I went straight to the emission testing station and passed with flying colors ! Thanks for the help from all !
The muffler shop guy deals with lots of converter replacement and does his own code scans . He turned the CEL off after finishing the job and so far it hasn't come back on . The thought of shaking the converter to listen for rattles doesn't always work according to the muffler guy . The converter can be semi-plugged , basically stopped up , or contaminated to the point of destroying it's ability to do it's job and not make a rattling sound but still be bad . My particular situation was the exhaust system was properly secured and there wasn't enough movement in the mounting points to allow " shaking " the converter .
Wish I would have seen your thread earlier....P0420 almost always means that you need the Cat replaced. I send everyone to the local muffler shop, they can usually replace any truck cat for around $150 total. They will usually take the cat out, look inside, call me and say "That cat aint plugged up!"....and I'll say, "I know, just replace it, please".
Anyway, P0420 means that the cat isn't doing what it's supposed to do and the O2 sensor right behind it knows that it's flowing too freely and sends the message to the PCM and sets the code.
Personally, I think it's a stupid code and the OEM's (and possibly the govt.) should really loosen up the criteria for the PCM setting the code.