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.