P0420 code Engine light comes on intermittently and the car has a P0420 code? The engine lights resets itself after a couple of days.

Run a diagnostic test to see what codes come up and go from there.

for the code po420, you need to patch the leak first then clear the code and dirve the car one or two days to see the code come back on, if not you just fix the problem. but I bet it is your catalytic converter is bad, the rattle sound is not coming from the engine, it is from the catalytic converter, use a seth pole to listen, the catalytic converter is right infront and it is part of the exhaust mainfold.

Good luck.

Mine is a 2001 Sienna LE. I got the P0420 code and the dealer want to replace an engine control computer for $1600. I've found posts that say you can replace your oxygen sensors, which still costs around $100 each. The P0420 seems to be emissions related. When my check engine light comes on, I run the highest grade, no ethanol gas, and usually within a couple of tanks, the light goes off again. The last time it happened about 2 wks ago, I put in the 93 octane no ethanol premium and a can of Seafoam and within a half a tank, the light went out. Your problem might be more serious, and I don't know what the 1150 code is, but so far that's worked for me.

It's an O2 sensor telling you there is a catalyst problem. It could be the sensor or the converter itself. Have you been using cheap gas? Cheap fuel will do the same effect.

try replacing the fuel cervo... this maybe the cause of faulty run. a backyard auto technician can replace this without charging you much. hope this helps

Try replacing the o2 sensor in the exhaust header or the cat pipe.

It is probably an intermittent fault in this sensor that clears itself due perhaps to chafed cables or a bad plug connection hence no codes are stored.

I would send the car for a smoke test for the emissions leak...Find out where the leak is comming from..Repair it..Reset the codes, then do a drive cycle and see what happens...

yes for sure if it say that is where in malfuction is it is allways best to try that first.
the sensor is only about 35 dollars

A code P0420 may mean that one or more of the following has happened:

• Leaded fuel was used where unleaded was called for
• An oxygen sensor is not reading (functioning) properly
• The engine coolant temperature sensor is not working properly
• Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
• Retarded spark timing
• The oxygen sensors in front and behind the converter are reporting too similar of readings
• That should get you going.
  

PO420 is for catalyst system efficiency below threshold. I have seen this many times on vehicles from 96 up. It is due to the system not burning the fuel at what it considers an acceptable rate. Todays fuels are not the best we could have for internal combustion and sometimes it will set off the light. I tell my wife she is too easy on the car and it's not burning the fuel proper. After a couple days it goes out. It will not harm your engine and should go out again. Fuel system cleaning additive will also turn of light. Good luck and enjoy your Subbie. Greatest car ever Built.