Alright, I've got a 2016 Fit LX, p0420 occasional p0171.

replaced fuel injectors + fuel pipe
Replaced cat
Replaced MAF sensor
MAF sensor wiring harness was shot, replaced engine wiring harness and warrantied MAF sensor due to being defective
Replaced cat again under warranty, defective replacement cat
Replaced upstream O2 sensor
could this literally just be the downstream sensor?
Unfortunately this has been a very long problem I've had to deal with. Car was running lean and apparently caused cyl1's valves to not seal so engine was replaced between the last MAF sensor and the new warranty cat.

 

Welcome. There is an extended warranty for the injector / rail system on certain 15-16 models, and p0420 is what starts it all.

I know because my injectors & rail were replaced at 88K under warranty. Were your injectors and rail replaced under warranty?

https://static.nhtsa.gov/odi/tsbs/20...08198-0001.pdf

 

Yeah I already know about the fuel injector warranty. I bought the car and p0420 came on the day I bought it. Car had 161,888 on it. So no warranty. Sales department at the dealership paid for the parts for that tsb. They were replaced around that mileage too. I drive a lot, so unfortunately those injectors are out of warranty (20k mi/20mo) and I'm at 200k on the odo
That's the first thing I have listed as replaced as well on this post.

 

It's a high latency issue more than likely.

 

What exactly would that mean. Or do you mean it's just a highly complicated issue

 

https://www.8thcivic.com/threads/inj...lained.189947/

 

Only thing tho is this car is bone stock. So no tuning or messing with any air fuel ratio has been done.

 

Absolutely. If the post-cat O2 sensor is reading but incorrectly, it can cause a P0420. Considering you've replaced the cat twice (only OE design can be used), I'd replace the downstream sensor. If you did replace the cat with a non-OE design, that could be an issue too. Problem is on the cheap aftermarket cats, the converter doesn't move enough volume sometimes/doesn't have enough metals to catalyze exhaust gases and doesn't catalyze enough of the output, causing a P0420.

 

how was the process dealing with the replacement? I'm showing p0420 but none others, will this extended warranty still cover it? The pdf says p0420 "a long with" another code. Just curious, I know nothing about cars at all.

 

The process was straightforward. When I got the p0420 code, I contacted Honda with my car's VIN. They confirmed I was in the serial number range for the extended warranty and advised me to take it to a dealer for diagnosis.

Dealer confirmed the cylinder fail test and replaced the rail and injectors at no charge. While they had everything open, I had them replace the plugs and do a valve lash adjustment - coming up on 90K service anyway.

 

Thank you! I am worried my local Honda Dealer is going to give me the run around as I am not very good at these things, I need as much confidence and info before I go in. Honda mentioned something like they have to check the cylinders to make sure it's eligible, this is what I'm afraid of my dealer saying that's not true when it really is

 

I think you're OK. The procedure outlined in the link above is clear on what has to happen. The cylinder AF test has to fail before they consider replacing the injectors. Once that fails, they replace the injectors and rail, and off you go. There's not much opportunity for a dealer to deviate unless something else is going on.