jlmd

Had my car at the dealer today because I've had a CEL now for a week and the oh so common pulsating brake problem.

Anyway, the cel code was p0300 which is random cylinder misfire. I assumed it was going to be a bad spark plug or wire..something along those lines.

Turned out to be a bad O2 sensor...can that normally cause a misfire? I have a limited knowledge of this area and would like to know. Also, if it were the case, how would the tech diagnose the O2 sensor as being the problem?

Cheers

OH and the brakes...machined down for the 2nd time...according to the amount left on them, one more machining on the fronts should get me to the discard amount...mauahahah only 12000KM left on the warranty...gonna be some heavy/hard breaking to get me a new set of rotors before its up

Jimmys2007CobaltSS/C

iTrader: (1)

i have 3 codes....and the misfire is one of them when i got my downpip w/cat and header put on...i have the engine light on for a week now...and im running lean to

g5mike

find a mountain on a rainy day and start heading down that should speed up the process.

jlmd

I first got the code when I swapped the LSJ parts on...misfire. Dealer never questioned the LSJ stuff being on the car and said it was a bad O2 sensor...

Can an O2 sensor cause a misfire? Anyone?

mike: One way or another, these rotors will be warped again within 12k KM.

Doc

hhmm...

jlmd

hhmm... indeed

Coblasts

I think if your O2 sensor was bad in a way to cause the engine to run lean then yes it could cause it to misfire.
Also the other way( To rich) and the plugs fouling and causing it to mis fire as well.But I am not an expert!
I think that the tech 2 tells them what cylinder,you should ask which cyl for future reference.

Novajoe

random misfire isn't a specific cylinder. It's random misfiring in all of them.

Usually when I get a random misfire code, the first thing I look at is how old the fuel filter is. But chances are that if there was a bad O2 sensor, then there was a code for it. It's not uncommon at all for an O2 sensor to cause some kind of misfire.

g5mike

Ive been runnin this code since the dealership rebuilt my head and none of them canfigure it out,gone to the dealer like 9 times now.

Old Man

First what LSJ parts???

And yes a 02 sensor can cause a P0300 misfire.

A bad 02 sensor can allow the motor to go rich enough that it wont fire a cylinder all the time.
Or lean enough it detonates enough on its own so when the plug actually fires, there isnt enough left to fire.

stonny9

I get the p0300 code everuy few weeks but I dont notice any drivabliity issues and the dealer couldnt find a problem or so they said. They teck did say the the car is running rich and every ss/sc he has seen is running rich.

g5mike

Same here my car does not run any worse when it comes on.

WopOnTour

P0300 series misfire DTCs re seldom caused by a bad oxygen sensor (if at all)
Methinks someone was throwing darts! lol

Providing spark plugs and other secondary ignition sytems havnt failed prematurely, fuel supply (low? or filter?) or basic connection issues at the PCM or CKP sensor are more likely . (It's even possible your bad brake pulsation can be enough to trigger a fail of the P0300)

Does your MIL (CEL) flash On and OFF anytime when you are driving??
WopOnTour

Old Man

That statement is totally in correct and now is becoming common place looking point for mechanics.

Once fuel related issues of pressure, sticking injectors etc and ignition are ruled out, which are the first areas checked into, 2 other ares are now being looked at.

02 sensors and MAF are then being looked at.

02 sensors are being checked for sticking. There suppose to switch back and fourth between low voltages and higher ones. If it intermittently sticks putting out low voltages, the ECM think it needs to add fuel, opposite if it sticks to higher voltage outputs. If a 02 sensor is seen sticking, its bad and needs to be replaced.

i havent looked into the operation of the new MAF's, but the older 3wire ones, once they begin to get a buildup on them. it acts as insulation making them read off and do the same types of things. That was the main reason GM began to void parts of warranties of people running K&N filters. High damp mornings with lots of dew building up on K&N filters, the dew releases with a film of oil on it, hit the heated wires and begin to carbon load the wires up, making them read off.

WopOnTour

Trust me my statements are totally true. I've been working with oxygen sensors and GM engine management systems every day for the last 27+ years.
Thanks for the mini lesson, but I know exactly how they work.

However P03xx series DTCs have really only been around since 1996 and OBDII. These are the result of irregular crankshaft velocity variations as measured by the crankshaft position sensor (CKP) due to a significant misfire or mecahnical disturbance (even a rough road can cause this in a car with a std trans and the reason why most cars have a "rough road detection" algorithm with the help of the ABS wheel speed sensors)
The possible causes of a mis-fire sufficent to set a P0300 are of course numerous (including an erratic MAF) but the need for O2 replacement is more likely the RESULT of the misfire, than the root cause. The O2 sesor is essentially "ignored" by the engine managment system under higher loads (when PE modes take over) so the "lean detonation" as you describe would not occur based on any O2 input. The basic "narrow-band" O2 is essentially most influential at idle and steady state (light-moderate) road loads where it's primary purpose is to provide feedback required to maintain correct combustion stoichiometry (approx 14.7:1) neccessary to insure the catalytic converter is operating most efficiently in it's reduction/oxydation modes.

While it's true that a "pinned" O2 voltage can cause the engine to run extremely rich or lean, this is actually a very uncommon root cause due to the electro-chemistry of the sensor and typically only caused by a circuit fault (short to voltage or ground). Much more common however is a "sluggish" O2 sensor or one that no longer develops correct electrical potential across the zirconium boundary resulting in a "dead" O2 (resulting in a fixed 450mV) Another potential (and common) O2 sensor fault is a failure of the electrical heating element within the sensor.

The big thing to remember is the O2 doesnt techncially measure Rich or Lean, but the actual oxygen in the exahust.(that "might" indicate rich or lean) When a misfire occurs, the missing cylinder not only releases unburned fuel but also a "bubble" of oxygen at atmospheric concentrations (due to the lack of combustion) and the O2 sensor therefore erranty measures this as a false lean condition,(it cant measure the hydro-carbons) so even though fuel might be dripping from the tail-pipe the ECM/PCM "thinks" it's LEAN! The engine managment system therefore reacts by adding even MORE fuel, resulting in an extreme RICH RUNNING engine but with an indicated LEAN on a scan tool (voltages lower than 450mV on a scan tool)

If this was left unchecked for any length of time, the resulting raw fuel passing through the O2 has been known to "poison" the delicate chemistry within the zirconium bulb, destroying the sensor.

When an owner looks at their repair order (after bringing it in for a poor running condition and/or MIL) all they see the "Scan & diagnose P0300 DTC" combined with a "replace Heated Oxygen Sensor" and assume that was the root cause when more often it is actaully more likely to be "the fall-out"
Less informed service advisors/managers attempting to rationalize the repair order with the customer at pickup, might also mislead the owner.

HTH
Regards
WopOnTour

Old Man

I was making the point that and 02 sensor itself, could be the reason for the p0300 once everything else has been ruled out in which the last post you made, confirmed that it could be.

Thats all, nothing more.

WopOnTour

Actually my point was a failed O2 was more likely the RESULT not the cause or "reason" for the P0300
Wop

Novajoe

I can see how the warped rotors could cause the p0300... It would have to be a SEVERE case though for that to happen. I may not be a veteran or anything but I've been working as a tech for close to 3 years and I haven't seen a case of it yet.

Also if an O2 sensor were completely failed(sending 0v) then it could cause the engine to overload the fuel running extremely rich. Now it may not cause a strong misfire but there could be enough fuel left over from the combustion process to make the bcm think there is a misfire. If this is happening over all the cylenders then it sets a random misfire code. This I HAVE seen before.

WopOnTour

Actually if you measure the voltage on a narrow band O2 circuit with a "dead" sensor (or unplugged for that matter) you will read the "bias" or switching voltage of approx 450mV. This reference voltage is sourced from the ECM/PCM , NOT from the sensor. An O2 sensor essentially rasies or lowers this bias voltage depending on the oxygen content in the exhaust.A dead/inactive sensor (or open circuit) as you have indicated, will generally set a different set of DTCs (i.e. P0130 series)
OR
Even if it WAS pinned at 0-volts like you say (i.e. short to ground) it would eventually set a P0171 (excessive lean condition) or a P0172 (excessive Rich condition) if shorted to B+

Depending on the "other" DTCs that are set it MAY result in the P0300 DTC NOT EVEN RUNNING! (see the diagnostic enable criteria for the P0300 aka Conditions for Running the DTC If the enable criteria are not met, the diagnostic does noteven run it's course. Assuming they HAVE been met THEN the "Conditions for Setting" the DTC must occur for the DTC to be flagged as FAIL. Since P0300 DTC is a Type B code, it would have to fail on consecutive trip cycles for the MIL to be illuminated. (Although the MIL MAY "flash" while catalyst damaging levels of misfire exist)
Hence my reasoning for asking the OP if this was noted.

As a reference, here's an excerpt of the text from the GM Service information regarding diagnosis of a P0300.

NOTE: That there IS a recommendation that brake pulsations CAN cause a "false" P0300 and that there is ZERO recommendation during the diagnosis of a P0300 to check out the oxygen sensor in any way shape or form.
There is a good reason for that...
Wop

g5mike

damn Just got back from the dealer(lost count on times) for this code,i tend to get it on damp wet days,when I first wake up and go turn the ignition?????????????????????????they keep comming up with nothing,told to check the o2 sensor,they said they did not check it,because there is no way it could be that.DUMB *****>.should have taken it to dodge.LOL!!

WopOnTour

Actually they were correct! (and therefore likely NOT the dumb ***** that you suspect)
See my posts above.
Wop

ss sleeper

the only code i got is the runnig lean, sometimes it will go an hour on and another on, wierd

Novajoe

that pretty much shoots me down. that's what I get for arguing with a tech for 27 yrs

The question I have is... are those more or less conditions for ALL obd2 vehicles? or just gm? I ask this because I was diagnosing a p0300 on a 02 galant. It was throwing a random misfire code AND a dead O2 code. Ign, fuel, compression, and timing were all great. No drivetrain shaking or anything. Replaced the O2, cleared the codes and ran the car for about 10 min and neither code came back. Even test drove the car up to highway speeds. No more codes.
EDIT: come to think of it I don't remember if the random misfire was a pending code or not. This was over a year ago

wondering if said humidity has something to do with it...

are you modified?

jlmd

Holy jesus...I don't check this thread for a few days and its 2 pages long.

WopOnTour: You seem to know your stuff when it comes to this issue. In case you don't know, I am the OP of this thread...If you say the O2 sensor likely was ruined as a result of the misfire....what would you say caused it?

Some more background info...

- My rotors had some high spots on them....I wouldn't say the warping was severe...just to the point of being annoying.
- I never once threw the code until I installed the LSJ Header/DP/Catback. 5 Minutes after leaving the garage, it threw that code.
- I could always smell unburnt fuel in my exhaust fumes while the CEL was on.

Been driving since last week when it was fixed and it hasn't come on since and the nasty exhaust smell has gone away as well.....SO..based on what you said earlier, the bad rotors usually throw a false misfire code but since I could smell the unburnt fuel in my exhaust, I would assume my code was not a false and it was actually misfiring..essentially ruling out the rotors as the cause. Reading the potential causes you listed above, all my wheels are balanced...it was done not 1000KM prior.

Nothing else listed in your post was worked on or replaced on my car, just the O2 sensor.

So...based on all of that, what else could have caused the problem?

WopOnTour

Dont sweat it dude, not really "arguing" just trying to explain how it works is all...
The "basic" criteria is linked to the Federal Tes Procedure (FTP) and therefore an OBDII "standard" But GM DOES do a few things differently with respect to the misfire accumulators and the CKP velocity table that is used to establish the intensity of the misfire (as this affects the behavior of the MIL (flashing) and Type A/B states)
There's a real good explaination for this in the GM OBDII training texts. Are you a member of the i-atn? If so check out the GM section in the Techncial Resources section
Regards
WopOnTour