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Discussion Starter · #1 ·
Hi guys,
My car is Hyundai Accent 1.5L automatic 2000.
When I was driving in the rain suddenly the "check engine" light went on.
I went to a grage and checked with a computer the problem and got 2 issues:

- P0135 O2 sensor heater circuit performance
- P0420 Catalyst System Efficiency Below Threshold (Bank 1)

I checked the voltage on the oxygen sensor and got 0.4-0.6 volts when the engine was hot(I waited about 10 minutes).
Is that a normal range for the sensor?
This is te location of the sensor I checked:


I also tried to reset the computer by disconnecting the minus from the battery and when I did that the alarm went crazy, how can I reboot the computer?

Thanks :grin:
 

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Sorry, can't help with the alarm problem...except to say that hooking it back up would be my first step. Once you get that part solved, see if the car shows the same codes a second time. If it does, then I would do the following. BTW this advice is based on the assumption that all of your wiring harnesses are in decent shape and that your car hasn't got any "gremlins"...Basically, I'm seeing a horse shape and thinking horse, not zebra.....so no promises, but this is how I would proceed if it were my car.

P0135 refers only to the state of the heater element inside the 02 Sensor, (or the heater section of the harness of the sensor) it's triggered when the resistance of the heating circuit is too high (usually indicating a burnt out heating element, less often, a bad harness). So, the voltage you're measuring is more or less a moot point, because it's unrelated to the heating element. (the voltage is the computers reference for adjusting fuel/air mix after the 02 sensor is at operating temp, the heater helps get it to operating temp faster)

Long story short. Replace the Sensor, and reset the computer. At the least, this will solve your P0135. **unless your harness is bad**

P0420 is probably being triggered by the malfunction of the heating circuit in the Bank 1 O2 Sensor, if so, replacing it will correct the condition. So after you replace the front O2, reset the computer (disconnect the battery for 10-15 minutes) and run the car. No Check engine light, no problem. If it is not being triggered by the malfunction in the Bank 1 O2 sensor heating circuit, then it is likely you have a small exhaust leak between the front and rear O2 Sensors. Check carefully for a carbon trail all around the exhaust between and at the two sensors, have a friend blip the throttle and look, listen and feel for leaks. If no exhaust leaks are present, then either the Bank 2 O2 sensor is also bad (just not bad enough to trigger a code yet), or the catalytic converter itself has reached the end of it's life.

Keep in mind that 02 sensors have a limited lifespan (usually around 100k miles)
Running with bad 02 sensors will cause your car to use extra fuel, that extra fuel ends up unburnt and gets dumped into your catalytic. Then your catalytic has to work extra hard to burn the leftover fuel, and it's lifespan is significantly shortened....resulting in a P0420 (the computer reads the difference in exhaust quality before and after the Cat, compares the two readings and determines whether or not the cat is working properly...or you just have an exhaust leak between the two sensors.)

Good Luck.....
 

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Discussion Starter · #3 ·
Wow first of all, really thank you for the explained response.

Which O2 sensor should I replace? the one that is placed in the picture I've added or the one below the car?
I think its the one below because like I said the "check engine" light came up in a middle of a drive in the rain and I got into some puddles..
 

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try this…kinda easy way…disconnect the +ve terminal from your battery for about 5 to 10 min…or else try pulling the ECR fuse for the same length of time…
 

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First, did disconnecting and reconnecting your battery make the check engine light go out (at least briefly)? If so, when it comes back on read the codes again. If it codes for the same then.....

P0135 definitely refers to the heater circuit of the bank 1 circuit 1 O2S (the front one...., yes, the one in your picture). Extra info....Bank one would mean either the front or drivers side Bank of cylinders if your car were a V6,8,10. However, in your case, having an in line four, you have only one Bank of cylinders anyway.

Circuit One refers to the position of the O2 Sensor in the exhaust pipe. Circuit One is the first one from the engine (pre-catalytic, also called upstream), Circuit 2 would be the second sensor from the engine(or post catalytic, also called downstream). So the code is defiantly referring to the sensor in your picture. If it were the Bank 1 Circuit 2 sensor, the car would throw a P0141...the code for the sensor under your car.

Going strictly by the P0135 code, if you replace the O2 sensor in your picture, it should clear the P0135 code. It is possible that the harness to the O2 sensor is bad (that would throw the same code), but in my opinion, that is less likely.... and if your car's got over or near 100k on it, then replacing the O2 sensor isn't a total waste of money.

I'm running with the assumption that the fact that it was raining when the code occurred was simply a coincidence.....because by now you've reset the code and had it come back, (hopefully it wasn't raining and I say that just to reinforce my point), but even so it's not throwing a code for the sensor under your car....but I've digressed a bunch.......anyway....

If you're not willing to take a small change on a misdiagnosis, you could test the harness to see if 12v is making it to the sensor. I'm not sure what the test procedure is for the harness on your year accent (2003 uses a 4 wire sensor, and pin 4 is the heater 12v+), but I'm sure a little googleing could lead you to a description of the "pin-out" configuration of your front O2 sensor......just check that the harness is providing 12v to the proper pin to activate the heating circuit and if it is, then you can reasonably conclude that the heater circuit in the sensor is bad. If you don't get 12v there, then your harness is bad. Incidentally, I've probably swapped out 15 O2 Sensors on various cars with heater circuit codes, and it's always cleared the code....it's never been the harness yet. As for that P0420, I did have one car with no exhaust leaks, that ended up needing a cat to clear that code.....usually that code is set by a leak or bad O2 sensor though....

Personally I'd just swap the sensor in your picture, disconnect the battery to reset the codes, hook it all back up, go for a drive, and wait to see if the P0420 comes back.

Please keep me up to date on where you're at with this, I'd like to see it through....but sometimes I flake and don't get on the computer for a few days.....sorry...and again, good luck.
 

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Discussion Starter · #7 ·
Problem solved(I really hope so).
I disconnected the whole alarm mechanisem and then disconnected the minus out of the battery and waited.
When I connected everything the light of "check engine" was gone..
I guess some water got into the sensor and therefore the computer showed the sensor code.
I also think that the catalyst system code was showed because the sensor is the computers way to know how polutive the car is and therefore if the catalyst is working properly or not.
Meanwhile I don't get any errors for two days so I hope that was the problem.

The funny part is that when I went to a garge they said that both the sensor and the catalyst sytem will cost about 700$..so I feel lucky :grin:

Thanks guys :)
 

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QUOTE (AccentUser @ Dec 18 2010, 05:26 PM) index.php?act=findpost&pid=381248
Problem solved(I really hope so).
I disconnected the whole alarm mechanisem and then disconnected the minus out of the battery and waited.
When I connected everything the light of "check engine" was gone..
I guess some water got into the sensor and therefore the computer showed the sensor code.
I also think that the catalyst system code was showed because the sensor is the computers way to know how polutive the car is and therefore if the catalyst is working properly or not.
Meanwhile I don't get any errors for two days so I hope that was the problem.

The funny part is that when I went to a garge they said that both the sensor and the catalyst sytem will cost about 700$..so I feel lucky :grin:

Thanks guys :)
Good work, I'd look for another garage the next time you want or need to take your car to one & it does pay to do a bit of research - garages that prey on peoples lack of knowledge are not uncommon.
 

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I'm also getting the p1123 code all of the sensor readings came back normal except the second o2 sensor. I replaced it and still have the same code. So if I've done all of the above and replaced the catalytic converter what should I try next? I tried checking the power to the sensor at idle and it showed 11 volts (it was dark and raining so the reading might not be 100% accurate). Is it possible the new sensor is bad? My uncle is out of town with his labtop so I don't have a way to check the sensor readings this time
 

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I yoinked this off another forum, but I agree.....P1123 is long term fuel trim too rich......

" , then start at the air inlet tube, checking for obstructions or leaking vacuum hose., if ok, then
inspect exhaust sytem for leaks, manifold, cat, muffler. inspect for faulty o2 sensors, if there ok, then
inspect evap canister purge control soleniod for operation, and ability to HOLD a vacuum., if ok, then
inspect fuel pressure . if you have a fuel pressure tester, hook it up and measure fuel pressure with the engine running(49.8psi). "

One of the people responding said that a new MAF fixed their problem.....but I wouldn't run out and buy one without checking the other stuff first.....Oh, btw.....is your air filter clogged?
 

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I have wondered about the MAF sensor.. I've replaced it three times in the last year (thanks goodness it has a warranty) the last time I replaced which was October I also replaced the filter. I guess I'll look for leaks next... I rebuilt the engine of this car last spring and checked for leaks then but there's probably been enough time to develop one. I also replaced the catalyic converter flex plate and purge control soleniod about the same time but with the luck I'm having with this car I guess I'll be looking into that possibility also. Thanks for your help I'd considered a exhaust leak but not a leaking vacuum hose.
 

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Wow.... if you're burning up MAF's like that, I'd be looking for intake leaks and/or running down the wiring harness looking for another cause.....unless....are you running a K&N.....I've heard of people fouling their MAF's by over oiling their air filters.... just sayin'
 
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