Misfire woes- O2 sensor?

Hi folks, first post hope for some wisdom.

My car is a 2001 2.5 v6 galant with about 70k miles on the clock. (model EA5AARGER6 engine 6A13)

I've been running with a fault code for a few months: Bank 1 Sensor 1 O2 sensor fail. Was advised that it wasn't a problem if I left it a while to fix, would maybe run a little rich but shouldn't affect driveability. This proved so far to be the case so I left it and ignored the engine light till next service time. Car has driven fine in the intervening months, until...

Recently had the car serviced, and shortly afterwards got the garage to swap out for a new O2 sensor. They advised that after changing it, the fault code came up immediately for Bank 2 Sensor 1 and so they reckoned the other sensor needed doing as well. As the first one had cost £120 just for the OEM part (thats about $200) they suggested I source one myself and they would fit it for free.

All well and good, I got the new sensor online for less than half the price.

Meanwhile in the few days in between getting the new sensor I noticed that the car was hesitating a bit, not from start but at cruising speed when warmed up.

Took the car in and they popped in the new Bank 2 sensor for free. They advised that an operating temperature code had popped up when they tested it, but after clearing it it hadn't reoccurred, but said to keep an eye on it.

Subsequently the fault light came on again the following day. I also noticed that the hesitating/ stumbling/ misfiring at speed had not improved. Indeed over a couple of days it got worse, becoming more obvious at lower speed and even idle and really bad at speed albeit still intermittent. It still only comes in when the car has warmed up. The fault code is coming up with a random misfire on cylinder 5. (P0305)

The garage have subsequently changed the spark plugs but this has not sorted it and they seem to have no clue or inclination to spend more time on investigations and can only suggest taking it to a Mitsi specialist. All I can see are the prospective £££s wasted on extensive diagnosis!

So... any clues? An O2 sensor issue shouldn't really be causing a consistent misfire on one cylinder so I'm thinking its probably something else, but I keep coming back to the fact that the issue only became noticeable after the first sensor was swapped out?!


As an aside, I'm not 100% sure which cylinder is no 5- despite having a workshop manual and googling this extensively I can't find a definitive layout- I think it would be the on the RH side at the rear as I look at it from the front, with the belts on the left:


timing belts 1 3 5
this side 2 4 6
front car

This look correct?

Many thanks for any advice. I'm not really equipped to do much in the way of diagnosis myself but a bit of knowledge from the Galant Hivemind might just help point me in the right direction.

Cheers all.

I have a '02 with a 4G64 2.4L I4 and had a similar issue with the misfiring (Cly 2) and sputtering at idle but not initially any codes with the 02 sensor. Changing the plugs resolved the issue (didn't change the wires) however I still was getting the check light, this time it was in relation to the sensor as well as the thermostat. I didn't change it as it was too much at the time and there didn't seem to be any ill affects like in your case. Then, of course, one day it refused to start up after running for about 20-30 min and took roughly the same amount of time sitting before it would start again. Didn't stress it to much since parking it at work 10 hrs made the wait easy. A few days latter the battery (used) apparently crapped out and a simple jump got me rolling but it did so 3 times in 5 days. Replaced it along with alternator (as it was a victim of the bad battery) but still the start up problem progressively got worse and needed to sit longer and longer (currently 3-4 hrs). I took it to the dealership for a diag ($99 US) and they advised the that catalytic converter (the 1st one) was considerably clogged up and causing what essentially seemed to be some sort of overheating problem and quoted $1200 US (parts and labor) since it's part of the exhaust manifold but they side noted 'off record' to remove it and 'bang on it carefully' until the debris started to fall out.

Not willing to pay that much and to avoid it reoccurring down the line, I've opted to swap it out for a 4-1 header made for the 3G Eclipse and eliminate the 1st cat all together. Haven't done so yet as the part in on its way as we speak. Hope this helps shed some light and i'll update if the issue is in fact resolved from this. Good luck

Hi
Thanks for response (first and only).

Mine eventually fixed by swapping out the original lambda replacement with a new one, having changed out all the sparks, leads, cap etc with no success.

Now runs ok without the service light, still feels rough to me, but have done several hundred miles and it hasn't got worse or better... I wonder if maybe the cat has taken some grief from running with the misfire...

Anyway thanks for responding & good luck with your cat issue.

Just as a thought....when I am trying to diagnose an issue that is for a single cylinder I move everything. I have the 3.o v6 cyl. 1,3,5 are rear so since they require removing so many parts, move it all to the front put the spark plug in 1 cylinder plug wire on another and last item is fuel injector. See where the code goes....what does it follow. Clearly you can't change the distributor location so if it comes up po305....change cap and rotor otherwise the new code will tell you what's up.