Hello everyone. This problem has a long history and I’ve pretty much exhuasted all ideas on how to resolve it.
In November of 2022, my car starting misfiring on the highway. After the spark plugs were replaced (apparently these were the original plugs, and they were being replaced for the first time at 189k miles… not good on my part), the dreaded P0420 appeared. In an effort to save some money, the downstream o2 sensor was replaced on the small chance it was that instead of the converter. This did not resolve the problem. When the old manifold was taken off the honeycomb inside the cat was visibly damaged, so its good that it was being replaced anyway.
The OEM exhaust manifold with the cat in it was replaced with one made by “Excel Exhaust” in May 2023. However, this too did not resolve the problem. Within the first day of having the new manifold/cat installed, again: P0420.
Take 2 on the downstream o2 sensor replacement, no charge from the mechanic. Again, P0420.
Take 3, upstream o2 sensor this time. Again, P0420.
Take 4, spacers on both o2 sensors. Again, P0420. (I realize this is not the correct method to use spacers in this case- as I’ve learned the top sensor is only A/F and not measuring catalyst efficiency… on to Take 5).
Take 5, spacers on just the downstream o2 sensor. Again, P0420, sometimes but not always. 9 times out of 10 if you clear it, it comes back, the other time the test shows “OK” but then is pending P0420 the next cycle.
My understanding is that the P0420 code is almost always the catalytic converter. The new cat + o2 sensors are all aftermarket.
I just don’t know what to do at this point. I’m considering trying to get one of those 90 degree o2 sensors with a so called mini cat in them, and trying that on the downstream sensor. Another thing I considered is replacing both sensors with OE Denso ones. Final consideration is that the aftermarket exhaust may just be that terrible that even with a spacer the sensor is showing that it just does not work.
Any suggestions for things I haven’t thought of, or ideas on where to take this next?