Trying to diagnose getting a 430 Cat efficiency code but, only when towing. I started getting the code about a month ago. Had it looked at by a good local shop since it is out of warranty (2015 95k mi.) They said it needed a new cat but, I could still drive it for a bit since I needed the truck to tow a 7k trailer the next day. The engine ran very rough when pushed at all during the trip and after clearing the code a couple of times it alway came back. I checked the codes myself after the trip and found a misfire on cylinder 4. Decided to replace the plug before trying other things like swapping coils. The cleared code did not come back even though I tried to re-create it.

3 weeks later I was towing again and the code came back without a misfire after an hour on the road. It certainly appears to be load related and does not appear to be the cat. If I am not towing, I can clear the code and it stays off. Any thoughts on diagnosing the real issue? Thanks.

 

what engine?

 

Very likely the catalytic converter starting to go out. You are putting a much heavier strain on the system when towing, which appears to be accentuating the issue. Converters are covered by the emissions warranty for 8 years/80k miles. Take it to the dealer with the light on if you are under the emissions warranty parameters and get it replaced.

Sorry - overlooked the out of warranty statement. Get the converter replaced.

 

Compare the secondary o2 values to the primary. If your affected side readings are swinging as rapidly as the primary that would indicate a catalyst that is not working. Sounds like it is teetering on the edge of efficiency (cat going bad). If you had a long term misfire issue on that bank that could have compromised the catalyst.

 

Good luck getting a converter. Mine are on backorder for 6/17 after being pushed back from 5/25. Assuming it will likely be pushed back more... If it wasn't a warranty repair I'd just get downpipes.

 

oops. 3.5 Ecoboost. Thanks for all the feedback. Will post the outcome when I know.

 

When towing, you have a lot more open throttle.
More exhaust volume flow.
The converter likely is now causing a restriction, hence the roughness.
It has to be replaced in any event.

 

If one plug was bad, why didn't you change all of them?
As one of the post said earlier and I will expand running with a cylinder miss enriches the mixture going to the catalytic converter, shorting it's life or kills it.
Knowledgeable others may expand but an O2 sensor going bad may also be at fault as a long shot.
What about other Catalytic converter test not just a DTC.
Please keep us informed
Good Luck