posted
My check engine light came on about a month ago while I was driving down to Florida, and I had the code read at autozone. It said something about the catalytic converter, but the guy at autozone said that probably meant the O2 sensor. I didn't know what to think, but I had him erase the code, because I figured it if was a fluke it wouldn't come back and if it came back fast, it would give me a better idea of how often the error occurred.
Well it went about a week without coming back on despite quite a few miles that we drove around down there. But then it came back on when we were driving back to NY. Same circumstances, the car had been driving for a long distance, but shut down briefly to get gas. The check engine light came on shortly after we got back on the highway. Some time on the way up, the light went out again, and it stayed out until last week, when I was driving to Massachusetts. Same thing, I pulled off for gas, and it came on when I got back on the highway.
So today I went into an autozone, I figured I'd just get an O2 sensor and be done with it. The car has 110,000 miles on it, so it wouldn't hurt. But they told me it has two sensors, one before the cat, and one after. Suddenly the code made sense, since the second O2 sensor can determine how effective the catalytic converter is by comparing it to the upstream sensor.
The thing is, I don't know which sensor to replace, or if this really means the catalytic converter is shot.
quote:Heres a link that seems to adress your exact problem.
Close, but it doesn't answer the question of whether the sensor is bad, which gives a false error code, or whether the cat is bad.
What I can get from the car talk page is that the leading O2 sensor is probably good, because it is the one that controls the fuel injection, and there are no codes that indicate the fuel injection is malfunctioning.
Which leaves me with the choice between the cat and the rear O2 sensor. The O2 sensor is easier to replace, but both are pretty expensive parts, if all I'm doing is trying them out.
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Thanks for the link Aspectre. I think all the information I'm looking for is in there, and it points out exactly what I was afraid of; that if I go to any old mechanic I'll likely spend money I don't need to spend.
Now I just have to get the equipment to do the diagnosis myself.
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I was an ASE certified master automobile technician, 18 years ago. The technology has changed, but the industry hasn't. Frankly, I'm not willing to pay for the diagnosis, because I know that flat rate won't pay the mechanic enough to make it worth his while. It's easier just to replace the parts and hope you got the right one. But given that I actually have the time and the skills to make the diagnosis, it's just a matter of having the right equipment.
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From what I understand the forward (pre-cat) O2 sensors fail more often than the rear sensors. They are dealing with slightly hotter exhaust AND a higher sampling rate than the rear sensors, so they tend to wear out first. (This is from memory from some recent reading, not from training as a mechanic, FWIW.)
Glenn, I don't know that I agree with your reasoning, and I'll risk sounding a fool since you're actually the trained technician. My understanding is that the function of the forward O2 sensor is to check that you're getting optimal combustion. Feedback from this sensor guides adjustments to the air/fuel mixture. If it is failing, you MIGHT get a suboptimal air/fuel mixture and this will reduce your MPG and power to some degree. However, I do not think this would lead to trouble codes with your fuel injection system - it is still working fine, at whatever default it resorts to when it's not getting the proper input from the forward O2 sensor.
At least this was my situation. I had the same problem at Checker Auto, where they told me it had something to do with one of the O2 sensors but they couldn't tell me which one.
I bought a cheap USB OBD II reader on eBay for about $30, which came with some free software, and hooked up my laptop to the van through the reader device and was able to pull the specific trouble codes, from there I was able to look up the likely diagnosis online.
I was getting a trouble code from my forward sensor but NO trouble codes related to the fuel injection system.
The two possibilities were failing sensor or a problem with the heater circuit built into the sensor unit, either way replacing the sensor made sense since it was 120k miles old.
BTW I saved about $30 by using the USB reader with my existing laptop instead of buying a stand alone reader device, and it allows the possibility of advanced uses like monitoring the internal systems in real time if I get the right software. (Plus I save the $80 the dealership will charge me to do a simple computer based diagnosis.)
Sad story: I bought the part, went to replace the sensor, but my forward O2 sensor has seized threads and is in a very difficult to reach location (stupid minivan!). I need to apply penetrating oil to try to free it up enough to remove but I don't see how I can get a penetrating oil dispenser up into that area (at least not without dumping some oil on my face). I'm considering cutting a temporary hole in the firewall insulation so I can reach it from inside the cabin, or breaking down and taking it to a shop...would be a shame since I've gone to so much effort to avoid doing that, though.
Edit: aw, geez, I wrote that whole post under the impression that you had a trouble code related to an O2 sensor but unsure which one, but now I see your code is related to the catalytic converter but the Autozone guy is who inferred "o2 sensor" from that. I stand by my argument that lack of fuel injection error codes doesn't mean the forward O2 sensor is good but I can see that doesn't necessarily apply to your situation.
Anyway I think you definitely should make the minimal investment for OBD II reader, whether it's the kind you hook up to a computer or use by itself. So far I've used it twice to figure out why my check engine light is on and it has paid for itself four-fold.
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quote: My understanding is that the function of the forward O2 sensor is to check that you're getting optimal combustion. Feedback from this sensor guides adjustments to the air/fuel mixture. If it is failing, you MIGHT get a suboptimal air/fuel mixture and this will reduce your MPG and power to some degree. However, I do not think this would lead to trouble codes with your fuel injection system - it is still working fine, at whatever default it resorts to when it's not getting the proper input from the forward O2 sensor.
The O2 sensor code often serves as a catch all for other errors. Mazda got so sick of mechanics replacing the O2 sensor when they got O2 sensor codes that they changed the code name to "feedback system." An O2 sensor code will often be the first code when a fuel filter clogs or there is a vacuum leak. Basically anything that affects the air/fuel ratio.
quote: I need to apply penetrating oil to try to free it up enough to remove but I don't see how I can get a penetrating oil dispenser up into that area
The key here is heat. Exhaust systems are not merely rusted, heat has actually changed the dimensions of the parts. When it's an exhaust system, I don't even bother with penetrating oil, I go straight for the torch. It may also be easier to remove the front pipe to get to the O2 sensor, rather than trying to remove it in place.
quote:Anyway I think you definitely should make the minimal investment for OBD II reader, whether it's the kind you hook up to a computer or use by itself. So far I've used it twice to figure out why my check engine light is on and it has paid for itself four-fold.
Yeah, I just haven't gotten one yet. What kind of software do you need to use the PC version? Of course, I don't have a laptop, so maybe I should just get the stand alone version.
edit: whoops, fixed the quotes
[ October 23, 2008, 10:34 PM: Message edited by: Glenn Arnold ]
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Unless you want to move a PC into the front seat of your car, yeah, you'll probably want the stand alone version.
I forgot the name of the software. This is probably it: scantool.net
Or - this just occurred to me - the kid at Autozone probably sees a number code which means nothing to him and a vague description that is programmed into his cheap reader. He goes from the vague description which isn't too helpful. But if you can get him to tell you the actual code, then you can probably do more with it than what he can tell you off the top of his head.
I've had no trouble finding the two or three codes I've read from my van in online discussion forums where often actual mechanics were participating and giving advice.
quote:The key here is heat. Exhaust systems are not merely rusted, heat has actually changed the dimensions of the parts. When it's an exhaust system, I don't even bother with penetrating oil, I go straight for the torch. It may also be easier to remove the front pipe to get to the O2 sensor, rather than trying to remove it in place.
Well, sounds like I'm kinda jacked either way. the exhaust manifold is on the back of the engine and I can only reach the O2 sensor in the narrow gap between it and the minivan's cabin firewall from the bottom, and that's reaching past a couple of things in addition to the pipe that leads down to the catalytic converter. I struggled to get the sensor socket and wrench up in there. I don't think I can get a torch up there and aimed correctly, and that's leaving aside that I wouldn't be able to see what I was torching and whether anything was melting or catching on fire. Same problem with the oil, except in that case I'd just be slopping oil around on more than I intended to and maybe having it drip down on me at the same time. Maybe if I do the oil THEN the torch, I can create an insurance claim situation and forget the replacement! Ha ha. JK.
I can't see what I'm doing up there to remove the pipe either, but maybe I'll check the manual and see if it looks easier than removing the sensor. If it's held on bolts that aren't seized, I might try that.
But if it turns out not to have been my O2 sensor, I'm going to cry.
(I spend about 2 hours changing out my transmission fluid and filter. I save money by trying these things myself, but if I was billing my time at a reasonable rate to myself, I'd hardly be saving money.)
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but I can see that doesn't necessarily apply to your situation. 
