Need advice from someone with OBDII knowledge

[slavrenz]

Okay, here goes:

I had a pending P0421 code for the last couple of days, indicating that my front cat is bad. It finally lit up on my dashboard this morning. Well I'll be damned if I didn't just replace the cat a year ago for the same code, and I haven't had any misfires or anything since that would ruin the new cat.

I hooked up my code reader to monitor my O2 sensor readings, and this is what I got (see attached image).

My rear O2 sensor isn't switching at all, which tells me that my cat is in fact good. However, notice in the bottom graph that my fuel trim spikes up briefly to 100% (I was idling the whole time these readings were taken, and nothing was done that would have caused the fuel trim to do this).

My question is does this fuel trim reading indicate a bad front O2 sensor, or an exhaust leak somewhere? Those are pretty much the only 2 possible causes of the P0421 code that I haven't ruled out. The front O2 sensor is only about a year old, but it doesn't seem from the graph that it is switching completely normal (notice the long time in the middle where it sits down near 0v).

Any help would be appreciated.

 

[buttthrax]

Mine just threw that code today. I replaced my spark plugs Sunday and this directly caused my old ass ignition coil to crap out while driving last night and I had to limp home about 1 mile on just two cylinders (not the best thing to do, I know). I replaced the coil today and everything seemed fine but it was running a little hot (I think the thermostat is starting to fail) and then it threw p0421.

I obviously got a ton of unburnt fuel on the pre-cat driving on two cylinders and I read where you wrote in another thread that unburnt fuel will screw our pre-cats up.

So I am also interested in what causes this code.

 

[njaremka]

that graph is useless for this code, since it only reads the conditions when driving down the road. there is an engine speed range, load range, and mph speed range that needs to be met for the codition to be monitored to. letting the car idel will not repreduce the conditions.

get yourself a copy of the shop manual, if you can. VERY helpful with stuff like this. that code more than anything else probably means your pre-cat o2 sensor isnt right. was it replaced with an OEM one, or a generic one? if it was a generic one, i would first check the wiring connections. if the wiring is tip-top, i would try swaping it out for another generic one, or front the money for a new OEM one.

according to the shop manual:
possible causes:
- WU-TWC (front cat) deterioration or malfunction
- exhaust gas leakage
- looseness of front o2 sensor
- looseness of rear o2 sensor
- front o2 sensor malfunction

 

[slavrenz]

that graph is useless for this code, since it only reads the conditions when driving down the road. there is an engine speed range, load range, and mph speed range that needs to be met for the codition to be monitored to. letting the car idel will not repreduce the conditions.

get yourself a copy of the shop manual, if you can. VERY helpful with stuff like this. that code more than anything else probably means your pre-cat o2 sensor isnt right. was it replaced with an OEM one, or a generic one? if it was a generic one, i would first check the wiring connections. if the wiring is tip-top, i would try swaping it out for another generic one, or front the money for a new OEM one.

according to the shop manual:
possible causes:
- WU-TWC (front cat) deterioration or malfunction
- exhaust gas leakage
- looseness of front o2 sensor
- looseness of rear o2 sensor
- front o2 sensor malfunction

I have the shop manual, and a very good knowledge of our exhaust systems and the requirements for the P0421 code. I looked at the O2 sensor readings while idling immediately after driving 5 miles at 65 mph in order to warm up the TWC, so those voltage readings are accurate, and prove that the cat itself is functioning.

I have eliminated loose O2 sensors as a cause, and so that leaves me with a bad O2 sensor or an exhaust leak. The O2 sensor is a direct fit Bosch sensor (I don't deal with that universal wiring crap), and is only about a year old. As for exhaust leakage, I had a leak where my j-pipe and rear cat meet, but there are none others that showed up with a smoke test for leaks. The reason I was asking about condeming the O2 sensor is because I am use to seeing my fuel trim readings bounce between +/- 10% on my scan tool, but I have never seen it spike to 99.2% (which unless I'm wrong, means extremely lean).

I appreciate the suggestions so far, but I need more than just "get a shop manual" and look up the code. I have studied the shop manual extensively, and I have a pretty good knowledge of the exhaust/emission systems on our cars.

What I don't know very well is how to interpret fuel trim. Is it realistic to ever see a value like this? If not, would that indicate a buggy O2 sensor, or something with the actual fuel system. Or would a 99.2% lean number more likely indicate a leak somewhere?

 

[Bigway]

both o2's should be changing if the one behind the cat is not changing that could mean that it is bad. the readings help but dosn't really allow me to see load while driving. i have seen alot of o2's going bad here lately. but that because people here have been useing e85 then changing back again.

 

[slavrenz]

both o2's should be changing if the one behind the cat is not changing that could mean that it is bad. the readings help but dosn't really allow me to see load while driving. i have seen alot of o2's going bad here lately. but that because people here have been useing e85 then changing back again.

I have no issue with the rear O2 sensor - the fact that it's not "switching" tells me that the front cat is working very well. As for E85 ruining O2 sensors, I'm not really sure how to respond - maybe people who don't know not to use this shouldn't be driving cars.

 

[Bigway]

they both should be changing all the time never the same or not moving that means the car is not running

 

[slavrenz]

they both should be changing all the time never the same or not moving that means the car is not running

I'm sorry, but that is not correct. The front O2 sensor is supposed to switch constantly, but once the rear O2 sensor reaches operating temperature, it is supposed to stay almost flat, which indicates that all unburned fuel and excess oxygen ahead of the cat is being chemically altered to provide a consistent output.

 

[buttthrax]

I'm sorry, but that is not correct. The front O2 sensor is supposed to switch constantly, but once the rear O2 sensor reaches operating temperature, it is supposed to stay almost flat, which indicates that all unburned fuel and excess oxygen ahead of the cat is being chemically altered to provide a consistent output.

Yes. This is shown on 1-16-20 of the service manual:

Inversions of sensor 1 have to be at a 2:1 ratio with inversions of sensor two or the p0421 is thrown.


As far as the e85 comment I have no idea, but in GA and many other states they changed the law to force 10% ethanol on us. I have no doubt that this expedites problems with cars that are getting older such as ours.

 

[Bigway]

that is the same thing as what i am saying it should never be flat.

 

[slavrenz]

that is the same thing as what i am saying it should never be flat.

Look at those pictures more closely. Flat is good, bouncing is bad.

 

[Bigway]

I thought the bottom one was for the front and the top one was the rear.

 

[gino]

Re: ethanol in fuel.

Colorado runs 10% in most, if not all, 87 octane fuel to help meet EPA clean air regs. It has never affected the running of my '5 or my wife's ES, both '03.5 models.

E-85 should NOT be run in these engines. Period. The ethanol content is too high.

 

[kudakev615]

what type of replacement cat did you put in last year? was it OE, aftermarket, used? ive seen aftermarket cats go bad within the first year of use pretty often at my job. and could have the fuel trim levels spiked cause the a/c compressor kicked on? was the defrost/ac on while the readings where taken?

 

[slavrenz]

what type of replacement cat did you put in last year? was it OE, aftermarket, used? ive seen aftermarket cats go bad within the first year of use pretty often at my job. and could have the fuel trim levels spiked cause the a/c compressor kicked on? was the defrost/ac on while the readings where taken?

The cat was a Walker catalytic converter (direct fit) with a 25,000 mile performance warranty. The defrost/AC was not on either.

Update: The garage I took it to says it's the rear O2 sensor gone bad. When I tried to ask how this was possible (the shop manual does not list a rear O2 sensor as being a possible cause of the P0421 code) I got the whole, "Well, I've been doing this for 30 years, blah, blah, blah..."

Anyways, I still have my old rear O2 sensor from when I swapped it out last summer, so I'll give that a try and see if it works. If not, I'm just gonna replace my front O2 sensor and go from there.

 

[slavrenz]

what type of replacement cat did you put in last year? was it OE, aftermarket, used? ive seen aftermarket cats go bad within the first year of use pretty often at my job. and could have the fuel trim levels spiked cause the a/c compressor kicked on? was the defrost/ac on while the readings where taken?

But wouldn't they spike in the "rich" direction? I dunno, it still seems to me like a 100% fuel trim reading is excessive.

 

[flash75]

Based on what I have been reading on OBDII theory the short term fuel trim normal range is +- 10%, if it goes to around 20% the long term fuel trim is used to make corrections to bring the short term back to +- 10%. Unfortunately the material I'm reading doesn't say what could cause fuel trim to go to 99%.

Although your oxygen sensor signal from number 2 sensor is basically a straight line I noticed it's riding at about .7 volts. I believe it should be around .45 volts.

I don't know the time length on the graph in your waveforms but I think sensor 1 should varying more than shown in the graph. It seems to move up or down and temporarily stop.

This article may help if you don't already know how to test oxygen sensors.
http://www.pelicanparts.com/bmw/techarticles/JF-Tech/about_oxygen_sensor.htm

I realize this doesn't answer your basic question about why the high fuel trim percentage.

Clifton

 

[slavrenz]

Based on what I have been reading on OBDII theory the short term fuel trim normal range is +- 10%, if it goes to around 20% the long term fuel trim is used to make corrections to bring the short term back to +- 10%. Unfortunately the material I'm reading doesn't say what could cause fuel trim to go to 99%.

Although your oxygen sensor signal from number 2 sensor is basically a straight line I noticed it's riding at about .7 volts. I believe it should be around .45 volts.

I don't know the time length on the graph in your waveforms but I think sensor 1 should varying more than shown in the graph. It seems to move up or down and temporarily stop.

This article may help if you don't already know how to test oxygen sensors.
http://www.pelicanparts.com/bmw/techarticles/JF-Tech/about_oxygen_sensor.htm

I realize this doesn't answer your basic question about why the high fuel trim percentage.

Clifton

Thanks - I appreciate your response. I ended up just replacing the front O2 sensor because the garage I took my car to mentioned that it was switching somewhat slowly. But I live and die by the repair manual - and if it turns out to be that the rear O2 sensor was causing the CEL (when the manual doesn't list it as a possible cause), I'm gonna start rethinking everything else in there.

 

[msp5]

did replacing the front o2 sensor take the code away????

 

[slavrenz]

did replacing the front o2 sensor take the code away????

No, but the garage said that the rear O2 sensor was bad and that wasn't it either. I ended up doing the nonfouler trick and it worked, but now I gotta figure out why my cat is apparently bad when the O2 sensor voltages say otherwise.