its your o2 sensor. Does it say Bank 1 or Bank 2?
first bank I believe its before your cat.
Rockauto.com has OEM sensors way cheaper than Stealership. Don't use a Autoparts store one, runners don't play well with them.
Sigh. So much wrong here.
It will NEVER say Bank 2 on a 3G 4Runner. It's Bank 1 Sensor 1 for front and Bank 1 Sensor 2 for rear.
The tags are confusing. Look in the top left corner below where it says "Engine Family".
If it says: "...AF, TWC(2), HO2S" it's a Cali.
if it says: "...HO2S(2), TWC" it's Federal 49-state.
P0420 is not strictly speaking, a sensor code and will not indicate Sensor 1 or Sensor 2. It says your cats are registering as below efficiency. This can be caused by one of four things:
1) Bad rear O2 sensor.
2) Bad front o2 sensor (rarely).
3) Exhaust leak any where before the rear O2 sensor.
4) Bad cats. Plugged, physically damaged, or burnt up due to excess unburned fuel coming through.
First check carefully for any exhaust leaks. This is frequently an easy cheap fix.
If your o2 sensors have 75K or more, I suggest you change the rear one. It owes you nothing.
If that doesn't resolve it, someone knowledgeable with a live scanner can help narrow it down for you.
Amazon has the OEM Denso sensors usually for a better price than Rockauto. Only use Toyota or Denso sensors. (same thing). To get your Denso p/n, go to
Underdog Racing Development - Toyota Truck Performance Parts and Accessories | Tundra, Tacoma, 4Runner, FJ Cruiser, Hilux, Land Cruiser | Superchargers, Clutches, Headers, Exhaust, Pulleys, Rotrex, Gauges, Intakes
Don't trust the Amazon applications info. It's hit or miss.
