Hi I've had my 2017 6MT fit for about 6 months, now at 77k miles. A couple days ago it it turned on several warning lights:

The tpms light had been on, pressures are fin i just hadn't gotten around to doing a tpms reset which would hopefully clear that, but the others are new. I bought a cheap code reader and it spat this out:
[code]Car Scanner ELM OBD2
Version: 1.99.0/400990/GP
DTC report
Connection profile: Honda Fit GK3
VIN: [REDACTED]
============================
OBD-IIOBD-II
DTCs: 2
----------------------------
P0304 [0x0304]
Cylinder 4 - misfire detected
No. 4 Cylinder Misfire
Status: Confirmed
----------------------------
P0304(00) [0x030400]
Cylinder 4 - misfire detected
No. 4 Cylinder Misfire
Status: Pending, Confirmed, Warning indicator requested
============================
Engine #2Engine #2
DTCs: 1
----------------------------
P0304(00) [0x030400]
Cylinder 4 - misfire detected
No. 4 Cylinder Misfire
Status: Test failed (current drive cycle), Pending, Confirmed, Warning indicator requested
============================
ABS
No DTC found.
============================
SRS
No DTC found.
============================
EPSEPS
DTCs: 1
----------------------------
U0416(68) [0xC41668] Archive (inactive)
Invalid data received - vehicle dynamics control module
Status: Confirmed[/code]

So it's pointing to a cylinder 4 misfire. I of course immediately thought of the loose spark plug problem with the fits, but i believe that is an issue with the older models right? shouldn't happen on a 2017? A friend told me to check for any leaks in vacuum hoses as that's a common and cheap issue, and what I've read is other than the spark plugs or coil packs will be the most common cause. I will honestly probably end up taking it to a mechanic but wanted to see what i can check myself first, i don't have much car mechanics experience cause this is the first car that's actually mine and i can mess with I'd always driven my parent's before, but i want to learn. Any suggestions?

 

Bad spark plug or bad coil pack are most likely. Common test is to swap the coil pack over to another cylinder and retest to see if the problem follows it over. Same thing could be done with the spark plug although plug problems are rare on this generation Fit.

 

p0304 could be the first sign of fouled injectors. It's one of the codes mentioned in the earlier (2015-16) extended warranty in the link below.

I know you have a 2017. I don't know the status of any extended warranty outside 2015-16. I hope that's not your issue, but it is a possibility.

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

 

Swap coil packs and spark plugs around. Trade plug 4 with cylinder 1 and coil packs with either cylinder 2 or 3. If the code follows the plugs, ok.. time to do spark plugs. If it follows the coil pack, time for a coil pack (Hitachi recommended).

With that said... I wouldn't be surprised if the misfire stays on cylinder #4 after that. The reason being GKs with the L15B1 are notorious for having oil blow by and carbon build up issues that end up clogging the fuel injectors. This affects ALL GK5s not just the ones listed in the service bulletin linked above.

The honest correction would be to walnut blast the intake valves (valve adjustment is optional since you right there), replace (or possibly send off the injectors for cleaning) as well as **REPLACE** the high pressure pump's fuel feed pipe (one time use, can leak fuel and possibly a fire). Then adding oil catch cans to combat the oil blow by and reducing/slowing the chances of this happening again.

 

I had the same thing on my 2017 6MT. It was cylinder 2 misfire. Would come back every 100 miles or so. I fixed it with new injectors.
Its a 6/10 job if you're mechanically handy - cheaper compared to what the dealer charges. Mine fouled at 60k miles

 

I'll bet it's fuel injectors but swapping the coils is good solid troubleshooting...