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I own a Hyundai elantra 2001, the check engine is on for the longest time and we got around to replacing the oxygen sensor. The check engine light turned off for a while but it came back on afterwards, now the fuel efficiency is low and putting the car into neutral gear makes the rpm skyrocket to 2k and hovers at that point for a couple of seconds. I don't know what could be wrong but my guess would be that the catalyzer converter is getting old, has anyone had the same problem? Please help.
So.. Either you're leaving all the important bits out of this post.. Or you saw a light and blindly replaced an O2 sensor.
I'm hoping for the former.
There are at least 2 O2 sensors, and the vehicle being 17 years old.. Wouldn't be unheard of for either both to be bad, or you've replaced the wrong one.
The code will generally tell the problem. If it's a P0420 standalone.. That's a good indication of the cat. Though, the fact that you're seeing no real performance issues and haven't mentioned a horrible rattle coming from the exhaust.. Indicates to me that it's probably not the cat.
That one O2 sensor was a problem in the beginning, after it was replaced everything went smoothly from that point, the code i received was a dealers code of some sort and i have no idea what to do with it.
Oh my word - SAME THING is happening to me. Also a Hyundai Elantra, but mine is a 2005.
Got a check engine light last weekend. Turned in my car and it was determined that an O2 sensor was bad. In addition to replacing that I did two other things that needed repair (transmission mount and power steering flush).
Three days after I picked up my car, I had the same sudden throttling and revving - it was very scary and I wasn't sure what was happening. Check engine light came on a moment later. Took it to the mechanic immediately and he said the code he pulled was related to the throttle control. $400 repair for part and labor.
Thought I'd update in case anyone else looks for this issue.
I ended up not having to do the $400 repair. I had a couple days of driving my car before my mechanic was able to do the repair. My car never had the issue pop up again after he cleared the code after his initial diagnostic.
My mechanic determined that the likely cause was that with the O2 sensor replaced, the air/exhaust mix was rebalanced but that the part of the engine that handles that mix as well as the throttle control likely was "choked" by the adjusted mix as a part of the O2 repair. The exhaust got "caught" in the throttle control, causing the hyper rev and throttling issue, but that once the exhaust was able to clear out, it went back to normal.
Sure enough, it is been almost a week since the initial sudden throttling and it has not repeated itself since. So glad I have an honest mechanic who was unwilling to do a repair he didn't feel was necessary...
Thought I'd update in case anyone else looks for this issue.
I ended up not having to do the $400 repair. I had a couple days of driving my car before my mechanic was able to do the repair. My car never had the issue pop up again after he cleared the code after his initial diagnostic.
My mechanic determined that the likely cause was that with the O2 sensor replaced, the air/exhaust mix was rebalanced but that the part of the engine that handles that mix as well as the throttle control likely was "choked" by the adjusted mix as a part of the O2 repair. The exhaust got "caught" in the throttle control, causing the hyper rev and throttling issue, but that once the exhaust was able to clear out, it went back to normal.
Sure enough, it is been almost a week since the initial sudden throttling and it has not repeated itself since. So glad I have an honest mechanic who was unwilling to do a repair he didn't feel was necessary...
I think what he's saying is that he didn't reset the trims.
When you have an O2 sensor out, the computer is trying to compensate for the bad data you're getting from the O2 sensor, either restricting or supplying too much fuel. You have 2 different trims, one for each bank (4 cylinder is generally a single bank, 6 and 8 have 2 banks) a short term and a long term. your total fuel trim is the short and long term added together.
So, if you have a short term of 0.5 and a long term of -15.6.. You've got a total trim of -15.1, which, if I'm not mistaken, is adding fuel to the cylinders.. A negative number means rich if I remember. I think this one is opposite of what you'd generally think.
The long term will adjust itself fairly quickly, and I've never encountered the surging that you described by not resetting trims, though I haven't ever had my hands on or in a Hyundai. It probably would have been wise for him to reset the trims, but I know a good number of mechanics won't do that. I think some vehicles you can't reset the trims independently.
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