Hi, all. I've read the posts about P0420 (Catalyst System Efficiency Below Threshold - Bank 1) and some folks have had success with replacing O2 sensors while others have had success with some fuel additives (voodoo, I say!).

Since an O2 sensor is $60 and replacing a cat is a whole lot more, I decided to take the O2 sensor approach and assumed I needed to replace the down stream sensor.... but I have some doubts. It seems that either one could be the culprit based on what the code really means.

Anyone have thoughts or experience on the topic?

 

There's alot of variables before one can give a valid opinion. How many miles are on the car? How old are the spark plugs? Has the fuel filter been replaced? Have you changed the air filter on a regular basis? Have you ever cleaned the throttle body? These are all things that can affect the quality of fuel burn, which effects the readings from the pre-cat sensor. You might be putting the cart before the horse.

 

You are right. On Monday, I'm going to take the car to the shop to have them read out where the voltages are when the engine goes into closed loop. I'm expecting to see the precat fluctuate quite a bit, but the after-cat should sit around .45 V.

Unfortunately, a lot of the questions you asked I don't have an answer for. I recently got this car so the past is a bit of a mystery. The only thing I know for sure is that it has 69K miles

It's interesting you mentioned the spark plugs because I was just musing on those yesterday. How in the world do you get to them? It looks like the whole intake needs to be removed to be able to take the coils off!

 

Turned out to be the O2 sensor after all. The down stream sensor was reading a -101% difference from the upstream... because apparently that sensor the catalytic converter can REDUCE the amount of O2 in the exhaust by more than possible. It turned out to be an easy fix after using a little-bit-o-magic (liquid wrench) on the threads of the sensor and swapping it out.

 

Oh, one more note for anyone else getting a P0420 error. The problem can be one of a few things:

• Upstream O2 sensor: The upstream sensor may not be switching like it should. If the O2 sensor is dead or very slow to respond, the measurements will very closely match the downstream sensor which is what actually causes this fault.
• Downstream O2 sensor: If your downstream sensor is malfunctioning, it can show too great of a delta as far as measured oxygen compared to the upstream sensor. In my case, it was detecting pretty much NO oxygen in the exhaust where the upstream sensor did find some. This very quickly can confuse the computer.
• Wiring on either O2 sensor: It's possible your wiring got bungled up as you worked on something else under the car, but this will usually present itself with a second error indicating the heater circuit is bad as well (from experience with other cars).
• Catalytic converter: Sorry, it sucks, but this monitor specifically monitors the efficiency of the cat con - if you are sure the two sensors are good, you'll be spending a pretty penny to get this fixed (yeah, you have to drop the frame to get this cat out).