I have a 2004 Sebring 2.4 at around 175k miles. A couple of months ago as I was about to head out of town my oil light came on. Luckily it was just the sensor but since it was in the shop I had them also replace the valve cover gasket since there were some broken bolts and it was leaking. It was also time for an oil change so they did that as well.

I got the car back and a few days later blew out a tire so I had a pair put on to keep them matched. Since then my fuel economy has plunged from 23 mpg to 19 mpg. I don't know what caused the drop, though, and am trying to figure it out.

I thought maybe it was the breather hose being cracked that runs from the valve cover to the intake. It's supposed to be part of the pcv system but it isn't the hose that comes off of the pcv valve. It's been cracked for a while but thought it was an easy shot since they would have needed to pull it off to do the work on the valve gasket.

There are other things that may be symptoms but they existed before I took it into the shop so they may be red herrings. The car idles a little rough and occasionally sputters and surges at idle when the rpms dip. There is also a P0420 code but there is no noticeable decrease in performance or power and it's been around off and on for at least a year. From what I understand the upstream O2 sensor voltage graph should look like a sine wave but mine is very irregular. The voltage covers the full range and it samples often enough so I think the sensor is fine. It'll go through stretches for a minute or so where the voltage range is low, say in the .1 to .5 range, rather than .1 to .9 but I'm not sure if that's normal. I was watching in Torque and it read that it switched from closed loop to open loop and back to closed loop momentarily. Again, not sure if that's a normal reading at idle after the car is warmed up. I've also checked for vacuum leaks with some carb cleaner and didn't see any.

Any help would be appreciated. Thanks.

 

So I watched the O2 sensors some more today. I'm sure the cat is gone. At times it seems the rear O2 sensor is where I would expect it, muted fluctuations in the center of the graph. Other times it bounces all over just like the upstream sensor. I'm guessing that means the cat isn't burning off the emissions. The upstream sensor still confused me and will spend time anywhere in the graph. It may show rich for 30 seconds straight straight, it may show lean for 30 seconds straight, it may look normal, or it could be all bouncing over the place. Still trying to chase where my gas is going.

 

I would expect if you are getting readings like that, then the computer is also getting confusing readings and is reacting in a confused fashion. Closed loop operation is going to be more efficient than open loop because the computer is taking actual conditions into account rather than guessing at certain parameters. I would expect that if you are operating in open loop mode your engine will be less efficient and your gas mileage will suffer.

The P0420 code existing for a year or more probably resulted in degradation of the cat from excessive open loop operation. It's risky to ignore fault codes for long periods of time.

 

I haven't been all that concerned about the cat code since it comes and goes and didn't appear to restrict airflow. I assume it's a breakdown of the precious metals and isn't always cleaning well enough to pass. I've looked into replacing the cat but the Haynes manual I have doesn't cover the 2.4 and looking online they seem to come in several shapes and sizes. Is there a diagram online or has someone replaced it before and had a link or part number? Any recommendation?

How can I prevent from dropping into open loop? Replacing the upstream sensor? Is there some kind of vacuum leak?

 

Oxygen sensors are designed to work in pairs. The computer compares the reading from the upstream sensor to that from the downstream sensor to get a handle on how efficiently the cat is doing its job. If one of the sensors goes bad, the comparison is no longer valid. Likewise, replacing only one sensor means that the computer is comparing the readings coming from a brand new sensor with an old one and and the comparison again may be no longer valid. Also, if you read through various threads here you will see that for some reason, people have had bad luck with Bosch sensors in many cases. The point of this is that replacing only the upstream sensor may not solve your problem.

As for the cat itself, contact the people at Rock Auto. They can probably get you a new cat for a much better price than a dealer. I have had good luck with them.

Vacuum leak is always a possibility. The usual manifestation is high idle RPM.

 

I'm not sure how to tell if either of my O2 sensors are bad or not. I'm watching the graphs but I don't know what a good baseline would be to compare them to. I checked my plugs and two of them were very rounded down and worn out so I replaced the plugs, wires, and coil. Power seems a little better but after two tanks I can say economy didn't improve much. Still under 21mpg with mostly highway driving. City has been around 20.