I changed my tranny oil with the Honda OEM fluid last week.
The next day, the CEL came on.

The code read P0420, which read catalyst efficiency below threshold.

Would changing the tranny oil really cause this?
I read that resetting code by unplugging the battery would solve this if its just a computer problem.

Any other insights on this?

Thanks

 

I had that code on my 00 civic EX, it was because I was leaking/burning oil and it destroyed my catalytic converter.

The way I bypassed it was to put a spark plug defouler drilled out to fit the back O2 sensor between the sensor and the cat, so it gave a cleaner reading and removed teh CEL.

Doubt thats your problem just posting what my situation was with the code.

 

Ah, shoot. Yeah, I was hoping it wasn't going to be a big problem like that.

Everything seems to drive normally though.

 

first of all p0420 is either your 02 sensor or your cat is going bad it doesnt have to do with your tranny, and about the defouler you dont have to drill out something for it your cat already have a second bung for it
i still have that p0420 problem with my ep3 and im waiting for my blox 02 defouler

 

it's literally just irony that the CEL light came on after your tranny fluid change. They are 100% not related

 

yeah, driving back home last night, the light went away
now i'm really confused

 

That's good if the code went away on its own. The ECU has to "cycle" a few times (a cycle consists of driving the car up to operating temp and then shutting it off until cool again) before it doesn't recognize the problem anymore and turns of the CEL.

How many miles are on the car? Might not be a bad idea to replace the secondary o2 sensor if the code comes back again.

 

113,000 miles

light hasn't been on since it randomly shut off.