Driving to dinner to meet the MiL, nephew and his gf who are in town. Normal road driving, 35-45mph.

Hear a loud cracking noise. Couldn’t tell if it was from my car or somewhere else as we had music on.
Then the dreaded check engine light.
Temp 195, trims -3 to +3. So not related to those.

Pulled into a parking lot, open my BT connected OBD app and scan codes.
Only two. The cruise control module that’s been there since I got the car.
And the P0420 code for "Catalyst System Efficiency Below Threshold (Bank 1)” aka O2 sensor.

Cleared the code and drove the remaining 10mins to the restaurant.
On the way home, no issues. Even found a couple of quiet straightaways and out the pedal to the floor.
zero issues.

If I remember correctly the O2 sensors are an inexpensive and very DIY friendly replacement.
Gosh I hope it is lol.

Thoughts?

 

I will bet ya that your cat is going out. Somewhat common problem. Cheapo replacements from RockAuto are cheap, and they are cheap. Good ones are a couple grand from Jag. Hope it IS the rear sensor.

 

I would be careful and not make the common mistake that everybody makes and just throw a sensor at it. P0240 can indicate a bad O2 sensor however I don't think it is as often as people think that it is. Based on your description, it almost sounds like you had some sort of backfire or fuel ignition maybe in the cat or somewhere in the exhaust. Could be a fluke. Could be that you jinxed yourself with your post. Wondering about driving to NY! I don't know that I would also sign off on a bad cat just for the sake of a bad cat. S only go bad usually due to another reason upstream usually a rich condition. However, you said your trends were good so maybe it really was just a fluke. If it were me I would have done what you did. Cleared the codes and just kept going and see if they came back.

 

Moved the trip to the 1st weekend in August, so have time to investigate and resolve.
Gonna drive a bit and track the bank 1 up and downstream to see if there are any signs.
Then give the bank 1 cat the ole tap test.
While it’s up I’ll visually inspect the pipe upstream from the cat for cracks. Never know.
Found the cats at rock auto. Not terrible, about $350 each. DIY job? If there is welding involved, nope.
Anyway, one step at a time.

 

Car is fine to drive if that's the only fault, it's just an emissions check. The car watches the upstream and downstream sensors, and expects a certain amount of change. If it doesn't do what it is supposed to, you get that fault. So either sensor, the cat itself, or leaks can cause the same fault, and the car has no way to tell the difference. O2 sensors get lazy with age and catalyst performance degrades. If you do the O2s, used good OEM quality ones and do them as a set. One new one will read differently than the other 20 year old ones, and may continue to falsely indicate an issue.

 

Instead of buying a new sensor, just swap sides of the rears. The front sensors are the ones that control the air-fuel mix. If the fault follows the sensor, there ya go. If the fault stays on that side, it's the cat.
Those Rock Auto cats are mild steel and kind of small. They 'work', but are not great in appearance nor quality. They do 'bolt up', but may take a wee bit of tweaking to fit well. I got one from my kid's 2007 /R and I had to bash it around and make a slice to get it to get into place. It worked fine and then the OTHER one went. He installed a spacer and then sold it not long ago.