Is there a mechanic in the house?
October 8, 2005 11:14 AM   Subscribe

I drove for about nine months on a bad oxygen sensor. Will my car suffer any long-term ill effects?

The check-engine light (not the emergency one) came on in January. I called a mechanic, who said something was most likely wrong with the emissisions system, such as a bad O2 sensor, but nothing to keep me from driving it. So I didn't get it fixed, and went on my planned road trip that weekend.

Nine months later (yesterday), I finally took care of it. Yup, the O2 sensor. The car now drives noticeably better, with much improved pickup. I hadn't even realized the drop in performance, but now that it's fixed, Wow: Like night and day.

But -- and I was too ashamed to ask my mechanic this -- I'm a little worried about whether the car may have suffered any lasting damage due to my not attending to the problem for so long. I don't drive very much -- I estimate I put about 4K miles on it during the period in question. Should I worry?
posted by donpedro to Travel & Transportation (12 answers total) 1 user marked this as a favorite
Best answer: With a bad O2 sensor, your car either ran rich, lean, or both.

Running lean increases combustion temperatures, shortening the life of rings and such. Running rich sends unburned fuel through the exhaust, and kills catalytic converters. It also fouls spark plugs. You may want to have your plugs replaced (or learn to do it yourself-- it's not usually difficult).

The good news, however, is that neither of these things likely happened to a severe degree. Do you remember your engine ever knocking?

Here's a decent little writeup.
posted by Kwantsar at 11:26 AM on October 8, 2005

Response by poster: Helluva link Kwantsar, thanks. The mechanic didn't say whether i was running rich, or lean, or both. I should probably call him up and ask. I wonder if it's worth taking the car back in & asking him to just look at rings, spark plugs, catalytic converter.

There was never a problem with knocking. I think that would have sent me to the shop immediately.
posted by donpedro at 11:46 AM on October 8, 2005

Best answer: The mechanic wouldn't know whether you were running rich or lean, unless O2 sensors typically fail in one direction or the other (I don't know, but one of the resident gearheds of MeFi probably does).

Rings: Examining your rings is probably an expensive job with little payoff. Check the color of your tailpipe smoke both at startup and when your engine is warm. Keep records of oil consumption. Do you need to add between changes?
Plugs: Almost as cheap to have them replaced as it is to have them checked. If you car is running well, I wouldn't fuss. If they're approximately due for replacement anyways, replace them.
Cat: does it rattle or make other noise when you drive? If not, don't worry. OTOH, are you old enough to remember what uncatalysed exhaust smells like? If so, take a whiff (but don't get too close or prolong exposure!!!) and see if your car smells like that. Also, if your cat is shot (and you live in a state that tests emissions) you'll know next time you get it smogged.
posted by Kwantsar at 11:57 AM on October 8, 2005

If you're in the US, Go to autozone and have them pull the check engine code for you. They bring a little gizmo out to your car and plug it into your OBD2 port to pull them, and they do it for free.

The problem with the CEL (check engine light) is that it doesn't tell you diddly squat, and there could be several codes stored in there indicating other problems (though your 02 sensor is the most likely candidate)

They can clear the codes for you, or you can reset your car's computer and get rid of the CEL by unplugging the battery terminal, and stepping on the brakes a couple of times.
posted by freq at 12:12 PM on October 8, 2005

Best answer: I'd just like to say that while Kwantsar's info is correct, the likelihood of seeing these problems is pretty low. Most modern cars inject fuel based on an air-flow sensor of some kind, which means that the O2 sensor is only used for a little fine tuning. It's highly unlikely you did any damage to your engine by running with a bad O2 sensor... Get new plugs though, they might have gotten a little fouled and they're super cheap to replace.
posted by knave at 12:14 PM on October 8, 2005

Response by poster: That's good to hear. The car is a 1996 Toyota -- modern enough to operate as you say?
posted by donpedro at 1:02 PM on October 8, 2005

My 1984 Audi was modern enough to operate as knave says. Your 1996 Toyota certainly is. And you really should learn to do your plugs yourself-- Toyotas are usually among the easier cars to service.
posted by Kwantsar at 1:07 PM on October 8, 2005

Not making a helpful comment, but hey, who knew Kwanstar could wrench? Good show man! Excellent way to explain that stuff.
posted by snsranch at 4:27 PM on October 8, 2005

Don't be too relieved -- the short and long term fuel trims, which are based on the O2 sensor(s), can modify your fueling +/- 25 percent, even with an airmeter. You still may have shortened the life of your catalytic converter.
posted by rfs at 8:31 PM on October 8, 2005

This vedry thing happened to my partner's car. After it was all fixed, the car started blowing a LOT of smoke, and consuming oil at a vast rate.

We took the car to a mechanic, who diagnosed it as a split ring. He said that before he did anything else, he'd run some cleaner through the car just to see if it might be some carbon deposits.

Lo and behold, it was. Saved us thousands of dollars in repairs. It's probably worth getting something similar done, just to flush all that junk out of the engine.
posted by tomble at 4:47 AM on October 9, 2005

Saved us thousands of dollars in repairs. It's probably worth getting something similar done, just to flush all that junk out of the engine.

Well, don't pay to have someone else put a bottle of injector cleaner in your car!

If you want to do this, here's my suggested method (suggestion was originally put forth by one of the engineer-types who frequents the online Porsche communities):
1. Wait until you're, say, 200 miles from an oil change.
2. Insert one bottle Techron into half-tank of gas. Do not use any other product. Do not use "Pro-Gard with Techron." Use Techron.
3. Drive your car until the tank is as low as you can stand (without running out of fuel).
4. Get your oil changed.

The reason for all this is that
1. The cleaner works best when concentrated.
2. All the crap that gets unbounded from where it shouldn't be either burns off (good) or mixes with the oil (bad).

It could be that the resultant difference between following my advice and ignoring it is negligible, a possibility I freely admit. But doing it my way is no worse than being haphazard about it, and short of a formal scientific test, intuition is usually better than nothing.
posted by Kwantsar at 9:09 AM on October 9, 2005

*Also, FWIW, I endorse Techron because of personal experience. When I once overhauled a car that had some fuel injection problems, the dealer wanted $300 each for new injectors. Faced with a cost of $1,800, I decided to try some other alternatives. I broke out an ultrasonic jewelry cleaner, filled the tub with the cheap STP stuff, and the solvent didn't change at all-- it stayed clear. Then I paid $6 for the Techron, and the solvent practically turned muddy. So I bought another bottle and did it again, and got another basin of mud. After the third batch, the solvent stayed pretty clean, so I figured the injectors were pretty well-cleaned. The car ran much better after that.

So I spent $20 to save $1,800. Hence my rabid brand loyalty.
posted by Kwantsar at 9:23 AM on October 9, 2005

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