How do I check my oxygen sensor???

I recently purchased a 2002 Honda Civic LX (141000+ km) from this lady and took it to get a diagnostics test and the code P0420 came up (Catalyst Efficiency Below Threshold - Bank 1).

I am hoping that it is just the oxygen sensors but I need to know the amount of voltage that should come up when you test the sensors and if there is another way to test it without using a flame torch.

Also might need to check the spark plugs...what kind of wrench will I need for that (they sit down so far!!!). Thanks

Ok....if you have a P0420.....get ready to kiss your wallet goodbye!! A P0420 indicates that the rear o2 sensor is switching rapidly, above and below .5 volts. The reason for this is because the amount of oxygen that the catalytic converter is suppose to store is now depleted.

If you had a decent scan tool in which you could graph the front and rear o2 sensors, you'd see the rear o2 switching the same amount of times as the front. It's not supposed to. The rear o2 should stay at slightly above .5 volts or slightly below .5 volts. If you stepped on the gas, the rear o2 voltage should go up to .7 maybe even .8 volts. And when you let off the gas, it should go down to .4 or .3 volts, but only momentarily.

The most common reason that a cat goes bad is because of failure to change spark plugs in a timely manner. Or making multiple short trips from the house to the store, then to your friends house then back to the store, then finally home. During those trips, the cat never got the chance to get fully warmed up and start the process of converting hydrocarbons to oxygen.

I'd say change your spark plugs, replace the cat, and reset the light. Problem is, those cats are expensive because they usually come with a manifold. And use NGK plugs. Your o2 sensor is usually fine, but I've seen a few cases in which the rear o2 was bad from being over-saturated with raw fuel.

Best of luck to you.