So our '07 kit base with 205K miles, this afternoon cylinder 3 decided to act up.
The engine was running rough, had a 10 mile drive home and it acted up the whole way: city street, stop/go traffic, highway. There was deffinately loss of power.

After I dug out my ScanGuage2, the Initial code was P0420 (Catalyst System Efficiency Below Threshold). I cleared it, then P0303 comes up over and over. I haven't run it long enough to see if the 0420 gets thrown again.

I put in new plugs and coil packs around 180/190k miles. I started there, replaced the #3 plug, issue was unresolved so I swapped #3 and #4 coil pacs. Issue is still there, although it does take longer to throw the error code.

What should I look at next? The exhaust had a fuel-smell from the cap (in the driveway, door open, hood up. I've suspected the engine has been running lean for a while now (our fuel eco is down to around 30 from 37), I'm wondering if the #3 injector has closed off enough to cause mis-fires and unburnt fuel?

At idle, the air/fuel ratio bounced between 0 and 20, it read a solid 22 at 2000rpm.

Let me know what else I can look at.

 

P0420 is usually the primary o2 sensor or the catalytic convertor that's going bad. I'm new to fits but definitely not new to Hondas or the P0420 code. If it's idling rough and bad gas mileage, I'd guess it would be the primary o2 since that's the one that controls your air and fuel ratio. The o2 would be the cheapest before you buy a new catalytic converter.

 

And for the p0303 code, it's a misfire cylinder 3. If u already replaced the plugs and switched coil 3 to 4 and it's still misfiring on #3, the next thing I would try is switching the fuel injectors just like how u switched the coil pack. Hopefully it's the injectors. If not, I'd try a valve adjustment or a compression test and hope #3 doesn't have a low compression reading.

 

I had misfiring problems in my 06 jazz too. Found out that it's actually my head gasket leaked engine oil and caused the plugs to be wet therefore couldn't fire properly. Do check your engine if there's any sign of leakage. if your engine is covered in grease,it's very likely your head gasket leaking. And if your head gasket is leaking, your flywheel gasket will most likely to be leaking too though flywheel gasket wouldn't affect your misfiring, your transmission oil will lose pressure = no good for gear box. Hopes this helps

 

I would start with the cheap and work your way down the list.

Unplug the negative terminal on the battery to reset the ecu
valve adjustment
new spark plug change
pull the injectors and see if there is buildup
new primary o2 sensor
new coil packs

That should more then likely cure the issue. I would also check the spark plug gap and make sure all the wires are connected securely on the coil packs.

if your fuel mileage is getting worse that means the car is running richer rather than lean.