Hi guys,

Is it possible to diagnose it without even looking at the car? I.e. - I'm thinking what if it's just the O2 sensor located after the cat? I believe there are two O2 sensors: one in front and one behind the cat.

I believe we already replaced the O2 sensor in front of the cat about 30k miles ago. The error code that made us replace this was P0135 "Oxygen Sensor Heater Circuit (Bank 1 Sensor 1).

Can anyone share some thoughts on this?

 

Chances are its the secondary O2 sensor. I'd change that before changing a cat. It's much less expensive and I'd rather be wrong that way.

 

Is it safe to say that the first O2 sensor is the same as the secondary O2 sensor so that I could literally swap them around?

 

No they are not the same. The primary one has 5 wires and the secondary has 4.

Doug

 

They are not the same. Dont go back to that shop, a post O2 sensor could cause that problem. If you dont have to pass emissions, you dont necessarily need to worry about this (but could be an indication of a CAT that is breaking down and could eventually plug up).

 

Guys... It's been on/off. I think I got bad gas or maybe need to replace one of the O2 sensors...

 

I wouldn't be surprised, your cat has probably seen better days. But as others have said it could be the o2 sensor. Have a buddy you can swap with? Try that first and see if it corrects it. If not, your converter is shot.

Jason

 

i would change the sensor first since it's less than 1/3 the price of the cat alone. if it doesn't do it then fork out the money for the cat.

i mean you could change the cat first but if that doesn't do it you'll need to pay for a sensor anyway so why not try your luck at possibly paying much less to solve the issue haha

 

buy a test pipe and o2 sensor spacers. ive got an oem cat with like 65k miles, but its cut out of the pipe.

 

My understanding of this code (P0420) is that the cat is not functioning at OPTIMUM level (meaning less than 95% efficiency) and by the use of the secondary O2 sensor the computer is throwing off code P0420. Doesn't mean that the cat is no good, just not at 'OPTIMUM' level (95% or better).