Downstream O2 sensor voltage drops durring acceleration Normal?

2.2 litre 06 Chevrolet/220,000 miles, has had a P0420 code come and go twice in the last few weeks.
There is a slight possibility of a minor exhaust leak 12 inches after of the second O2 sensor. Car runs very well.

I have a simple scan tool but it is not capable of storing live data so I cannot post a graph of what is happening. The up steam O2 sensor fluctuates as normal.

Question is...
I was watching the live data while my wife drove in the city and when under acceleration the voltage from the down stream O2 sensor would drop to .1 volts for a second or two. Normally it was fairly steady at around .7 volts but not when accelerating. Is this normal or the sign of a worn CAT? Faulty down stream O2 sensor?

ALL O2 sensors should go FULL RICH during WOT. If they don't, you have a fuel delivery problem, or a bad O2 sensor.

Someone correct me if I'm wrong.

An Exhaust leak near the O2 sensor could be suspect, too.

When I say accelerate, I mean mild=mid acceleration, not WOT. The second O2 sensor voltage drops to .1 volts at this time.

I have had my hands all around the exhaust and cannot fine a leak but the wire mesh around the flex pipe is partly missing. So I suspect a minor leak.

Hi drunken,

You can check for a exhaust leak by restricting the out of the pipe.

One person can do it for you and you can hear the gases going out through the leak

That's not a bad idea. I was thinking of hooking my shop vac backwards [blowing] into the exhaust pipe but Im afraid of pushing debris into the cylinders. I was really asking if O2 sensor voltage normally drops when applying moderate throttle. I have searched for videos of the live data and I find examples where the voltage signal drops and some videos where the voltage rises. Makes no sense to me.

Also considering performing a propane enrichment test to gauge the condition of the catalytic converter but I don't have the best equipment for that. The P0420 has not come back for a while but if it does, I'll have to investigate further.

Well, i was testing a fixed car and paying attention to your request.

When mid aceleration, when upstream went rich (let's say 0.7v), downstream followed same voltage. Sometimes a little bit lower (let's say 0.5v).

Never downstream went to 0.1v

Thank you for the information. My wife drives the car every day so I don't get many chances to look at it and it's been raining here a lot lately. I'll have to do more examination. Perhaps an exhaust leak.

Drunken Elvis wrote: That's not a bad idea. I was thinking of hooking my shop vac backwards [blowing] into the exhaust pipe but Im afraid of pushing debris into the cylinders.

I'm actually OK with this idea. I use a rubber cone and an air pressure regulator connected to shop air for the same purpose. Very handy for finding leaks without having to run the engine, and for checking repair welds for pinholes.

To answer your first question, it certainly sounds like a catalyst issue. Manifolds and flex pipes are also suspect on that engine.

You need a graphing scanner that you can either watch live or record. You want to record the front sensor and the rear sensor at the same time as well as TPS or speed, some way to mark the location of snap angle changes. What you do is cruise along at 50mph or so and then instantly let off the gas. Watch your O2's at that spot. A healthy cat, the rear o2 will jam itself lean a few MS after the front one. If the cat is bad, they will simultaneously jam lean.

Tyler would you be able to show us exactly what you use for exhaust leak testing and where you got them. I like that idea.

Jasondo wrote: Tyler would you be able to show us exactly what you use for exhaust leak testing and where you got them. I like that idea.

I bought one of these, but any brand will do:

www.amazon.com/OTC-6522-10-Leak-Tamer-Adapter/dp/B00KIL0P0Q

Added an air regulator to the hose end with a worm clamp, and a gauge. Stuff the cone into the muffler end and add some pressure.

One warning: It's possible to add a LOT of pressure to an exhaust system this way, which can 'create' leaks that otherwise wouldn't be there under normal operation. Take care to add a reasonable amount of pressure (5 PSI-ish).

An update to this thread. The car is gone to the great scrap yard in the sky as some 4 legged creature decided life was too long and ran out in front of the moving vehicle. The car died with the check engine light on and a P0420 code stored in memory.

An exhaust leak from the flex joint, a good 12 inches downstream of O2 sensor number 2, was becoming more evident [louder]. I can only assume the P0420 code was the result of this leak. Twice before this car has set a P0420 code when the same flex coupling developed a leak, I had just never seen the 2nd O2 sensor voltage drop so low but that's probably what it was.