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Catalytic Efficiency Code and Exhaust Leak

I just (this morning) had the flex pipe between the 2 cats replaced on my 2004 Toyota Sienna. It all started with a loud noise from underneath vehicle, then about 2 weeks later the check engine light came on, then about 2 weeks after that I brought it into the shop. I was informed that there was an exhaust leak, and when they scanned for a code, it came up with a “catalytic efficiency” code (I don’t know the specific code number). I assumed that the code was related to the exhaust leak. They wanted to replace the pipe AND the 2 sensors on each end, to the tune of $1,000. I called another shop that I had used in the past to see what they would charge for the same work, and the first question was “why are they replacing everything? They should start with the pipe and see if that solves the problem first”. That made me think that the pipe caused the code to trip. I called the original shop and told them I couldn’t afford the $1,000, and they said they could just replace the pipe, and save me half the cost. They just prefer to do everything at once. After the work was done and the check engine light was still on, I asked about the relationship between the exhaust leak and the codes. They said they were totally unrelated and just a coincidence. They only way to get the check engine light to go off was to replace the 2 sensors.

After that long story, my question is: Are they really unrelated, or did one cause the other? And should I expect that replacing the pipe should keep the check engine light from coming on?

I’m sure there are details I’m leaving out, but any thoughts would be appreciated.

  1. The “catalytic efficiency” code, typically P0420, means the cat converter isn’t working properly.
    Sometimes the converter is bad and sometimes another problem is keeping it from working,
    like excessive oxygen going into it from an exhaust leak.
    Yes, an exhaust leak can suck in air as well as blow out exhaust gases.

  2. If the check engine light was still on immediately after the repair was done that means they didn’t reset the computer.
    The light will reset itself after a few drives or you can disconnect the battery for 30 seconds.
    There’s also a fuse you can momentarily pull that will kill power to the computer, but I don’t know which one in particular for your vehicle.
    I suspect they left it on to encourage you to come right back and get the extra work done.

I advise you to reset the computer and not go back to this shop.
If the light comes back on after a few drives then you’ll know there’s still a problem.
Unless this vehicle has over 100k miles I really doubt the converter is bad.

Thanks for your comments, I appreciate it. I was hoping that the light would reset itself after a few cycles. I’m curious what part of the story led you to advise not to use the shop again? The fact that they would replace the sensors without confirming it was necessary?

Thanks again…

“what part of the story led you to advise not to use the shop again?”

That they would let you drive away with the check engine light still on,
in hopes that you would come back to get the sensors changed.
It seems like a dishonest move to drum up more business.


Many thanks.

I agree with circuitsmith. In fact, when a P0420 code is thrown you’re supposed to eliminate all other possible causes other than the converter itself. One of the things you’re supposed to eliminate is the possibility of an exhaust leak. By all indications I also don’t think you can trust this shop. They either don’t know what they are doing or they are unscrupulous. It doesn’t matter which because you don’t want either. That point is all assuming that all of the info is here.

I wouldn’t disconnect the battery or pull a fuse to clear codes though. That can cause problems, I even if it doesn’t I normally find it to be a hassle to have to reset anything that has been set. The best way to reset it is with an actual code reader/scanner.

Your second shop gave you good advice. An exhaust leak can affect the comparative readings out of the oxygens sensors in a few different ways, depending on exactly where the leak is. And, as already suggested, the amsrt thing to do is fix the known problem first and see if the code disappears. Change the pipe. Post back with the result.