2000 Accord EX V6, 133K
I'm having trouble tracking down the cause of some ECM codes I'm
getting. Here's the story so far...
On the way home from work the other day, engine started running rough
and check engine light came on. Being an hour from home with a Honda
dealer just up the street, I stopped in and figured they could help me
determine what suddenly went wrong. They could (for $85! - what
I had them hook up to diagnostics and came back with codes 0302 and
0306 indicating misfires in these cylinders. The tech determined that
I needed plugs (I have 133K on the car). He also did a 'coil swap
test' and that the ignition coils in cylinder 2 and 6 failed as well.
They wanted an additional $340 to change out spark plugs and the 2
coils. I thought that was pretty steep (including the $85 for diag, I
was looking at 425 - no thanks)
I did some research and decided on iridium plugs. I also purchased 2
ignition coils from Autozone. I installed the plugs myself (I didn't
gap, as several sources on internet said that was not necessary with
these plugs). I then installed the 2 ignition coils in cylinders 2 and
6 (from a ignition firing diagram at autozone, they said cyl 2 was the
middle rear and cyl 6 was right front as you're facing the engine. I
disconnected the negative lead from the batt and waited 10 minutes to
reset the ECM. I reconnected and started up expecting it to purr...
Still running rough! About one minute later, check engine light comes
I take it to autozone this time for the FREE diag (I'm getting
smarter), and I get back codes 0300, 0305, 0306. For some reason, cyl
5 now has a problem, cyl 6 still has a problem and I'm not sure what
I'm already $275 into this just on parts and would appreciate some
info on what might be happening.
Assuming your errors are P03xx (the "P" means powertrain), P0300 means a
random misfire (any cylinder). The other codes are misfires for specific
A misfire DTC is not necessarily a spark issue (and often isn't).
Replacing coils is NOT a good idea, and is certainly NOT a substitute
for proper diagnosis.
Misfires can also be caused by lean mixtures. such as may be caused by
intake leaks or clogged EGR ports.
Some vehicles even suffer misfire codes if the battery has been
disconnected and the idle-learn procedure hasn't subsequently been done.
A bad throttle position sensor or crank position sensor can also cause
Have your dealer look up the Feb '02 issue of Honda Service News. There
is an article in there involving your exact problem. Dealers are
supposed to keep a binder with all the HSN publications in it.
Somebody needs to do a proper diagnosis on this car with a proper OBD-II
code reader that is capable of advanced reporting.
Thanks Tegger. I've seen your FAQ in the past, so appreciate the fact
that you know your stuff. The bottom line is that there appears to be
more potential causes than I can deal with as a layman. I was just
going off what seemed to be black and white on the diagnostics info I
got from the Honda dealer - and I quote:
"pulled codes P0302 and P0306 for misfires in cylinders 2 and 6.
Pulled spark plugs and found all plugs to be very worn. Also performed
balance test and found the number 6 and number 2 coils to fail. Needs
spark plugs and coils number 2 and 6, EST = $340".
So to me sounded pretty cut and dried - new plugs and replace coils 2
and 6 - and all things I could do myself for way less $. I was pretty
proud of myself until I started my car and it still ran like shit.
I'm reluctant to bring it to that Honda dealer, as they've raided my
wallet every time I've had an issue. I have a local mechanic I trust
more that I will present with some of the information you noted and
see what happens. I'll post back when the mystery is solved.
There sure are. Engine systems tend to be interconnected, whether on
purpose or not. I don't pretend to be an expert on this.
Worn-open gaps will cause the coils to fire at a higher-than-normal
voltage, which has a tendency to chew up the insulation. This eventually
leads to shorts to ground through the coil body (before the plug gap).
This is plausible, but, I think, unlikely.
Hmm. I wonder what the tech actually did. Did he do "freeze-frames"
using the Honda HIM tester? Did he check any other values, such as fuel
Except that now you're getting a P0300, meaning the ECM can't tie the
"misfire" down to any particular cylinder. This means a global problem
with engine management, and not likely anything to do with the ignition
The problem is one of unfortunate terminology. Unfortunate for those not
versed in the EPA's mandated automotive systems, that is.
"Misfire", as a term, has nothing whatever to do with the spark. All it
has to do with is the ECM not seeing the crankshaft speed up like it
should after a spark event.
Your engine has a firing impulse every 1/3 of a revolution. The ECM
expects that the crankshaft will speed up and then slow down with each
firing impulse, following a particular pattern. All the firing impulses
should make the crank speed up and slow down the same way (within a
range). If some of the impulses result in LESS of a speedup than
expected, the ECM interprets this to mean the mixture has failed to
combust correctly and sets the P030x DTC.
The failed combustion can be anything from incorrect mixture to a stuck-
open valve to failed coils and a number of other problems (as I outlined
in an earlier message).
See here for a good professional explanation of misfires:
So I figured out the reason that replacing the 2 failed coils didn't
fix the problem. User error - I put one of the new coils in the wrong
cylinder! Took it back to Honda and after talking to the mechanic, he
corrected me on how the ignition firing order is laid on on the V6's.
Once I got home, I swapped out the coil I now knew was bad with one of
the ones I had originally taken out. I got lucky in that I picked the
good one, and the car started running right. Reset the check engine
light (pull negative lead off of battery) and the things running like
a top again.
As a treat, I filled her up with some Shell VPower and will throw in a
new PCV and air filter this weekend (with an oil change about a month
Thanks again for the info. I have your FAQ bookmarked - some very
useful information up there.
Somebody pissed in your gas tank. I've had similar codes when I stopped
suddenly at a traffic light. The Honda had been in storage all winter
and there was lots of water in the gas tank. A can of injector cleaner
and some gas line antifreeze fixed it up right away.
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