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Thread: OBD2 codes explained

  1. #1
    DonTom
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    OBD2 codes explained

    Here is some info. that many don't understand about OBD2 codes. The codes we can read with our cheap OBD2 readers are the "P" codes such as P0123 or P1234.

    The "P" means it's a power train problem, smog related.

    More expensive scan tools, can read "B" (Body) or "C" Chassis as well as "P". Also, perhaps "U" (undefined or other).

    But the first digit, as the "0" in the P0123 means something too. The "0" means it's a generic code, and that code is for any make vehicle.

    But when the first digit is a "1", as in P1234, the "1" means the code is ONLY for that vehicle and may mean something else with the same code in another vehicle. IOW, only look up the codes that start with "1" when dealing with your make of vehicle, as any other vehicle's information will most likely give you incorrect information.

    A cheap OBD2 reader can be purchased on E-Bay for as little as $25.00 (including the shipping!). These cheap ones are very helpful when you have a "check engine" or "Service Engine Soon" warning light on, especially when you live in an area that has smog tests. However, it sometimes does little good to know the code when you don't have the factory service manual to explain what to look for.

    A cheap OBD2 reader can be found here:

    http://cgi.ebay.com/ebaymotors/OBD2-...spagenameZWDVW

    But IMO, anybody who has the slightest interest in repairing their own vehicle (when newer than 1996) should own a OBD2 reader, even if in an area where there are no smog tests, as it can show some small problems before they are noticed and cause symptoms. For an example, it might be nice to know that your catalytic converter is jamming up before it gets so clogged that the performance drops off and MPG drops way down.

    You can find what the OBD2 code means on the web, such as at:

    http://www.obd-codes.com/trouble_codes/

    -Don- San Francisco

  2. #2
    Vulture of The Western World Eric's Avatar
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    Re: OBD2 codes explained

    Good info, Don - thanks!

    (Although I am grateful I don't have to fool with "codes" on my cars.... ;D)

  3. #3
    DonTom
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    Re: OBD2 codes explained

    "(Although I am grateful I don't have to fool with "codes" on my cars.... )"

    Didn't you mention driving around in one of your vehicles with a "check engine" light on, because of a bypassed catalytic converter?

    BTW, there should be a way to fool the sensor so you don't get any check engine light. That way, you would know if you have a new OBD2 problem while driving.

    Depending on the design, it might be possible to simply remove the O2 sensor that is AFTER where the CC was and ground it to the chassis, away from all pipes, with the wire still on. IOW, move it to a location where there are no emissions.

    An even better way would to use a resistor to simulate the "no problem" condition to fool the OBD2. But I don't know what a normal "no problem" value would be for the resistor without working on it myself.

    If either of these things are done, you won't have a check engine light warning until you have a different new problem.

    BTW, you can still check for new problems with an OBD2 reader without changing anything. Just ignore the first code and see if you have more codes after.

    -Don-

  4. #4
    MikeHalloran
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    Re: OBD2 codes explained

    Sorry, you can't mimic an oxygen sensor with a grounded wire or with a resistor.

    Its normal response is a square-ish wave of irregular period, phase- locked to the corrections that the ECU makes. I.e., at some threshold O2 value, the sensor's resistance changes so that the voltage at its ungrounded terminal goes from a low-ish value to a high-ish value. I think it normally wobbles between ~0.2 and 0.8 volts. Here's a plausible page:

    http://www.picotech.com/auto/lambda_sensor.html

    I think the ECU continuously samples the voltage with an analog input, and makes decisions based on both the voltage level and the timing of changes in the voltage.


  5. #5
    DonTom
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    Re: OBD2 codes explained

    "I think it normally wobbles between ~0.2 and 0.8 volts."

    I think you're confused with the main O2 sensor and that's not what we're talking about here.

    The O2 sensor that's after the catalytic converter is only used to see if there's excessive gasoline in the exhaust. It only cares about two states, a "no" (low) or "yes" (high) unlike the main O2 sensor that keeps the A/F mixture at 14.7 to 1 when in closed loop.

    IIRC, without the load of the O2 sensor, the supply voltage is a lot higher than 0.8 VDC and a resistor to ground should be able to lower it to 0.4 VDC and keep the ECM happy, for this O2 sensor (but not for the main one, of course).

    It's not pure DC in normal operation, but the lower DC should keep the ECM happy.

    -Don- (Reno)





  6. #6
    Vulture of The Western World Eric's Avatar
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    Re: OBD2 codes explained

    Quote Originally Posted by DonTom
    "I think it normally wobbles between ~0.2 and 0.8 volts."

    I think you're confused with the main O2 sensor and that's not what we're talking about here.

    The O2 sensor that's after the catalytic converter is only used to see if there's excessive gasoline in the exhaust. It only cares about two states, a "no" (low) or "yes" (high) unlike the main O2 sensor that keeps the A/F mixture at 14.7 to 1 when in closed loop.

    IIRC, without the load of the O2 sensor, the supply voltage is a lot higher than 0.8 VDC and a resistor to ground should be able to lower it to 0.4 VDC and keep the ECM happy, for this O2 sensor (but not for the main one, of course).

    It's not pure DC in normal operation, but the lower DC should keep the ECM happy.

    -Don- (Reno)



    Let's try it and see what happens!

    When I have a little spare time, maybe this weekend, I will try grounding the after-cat O2 sensor per your recommendation and report the results here.

    I've had no cat on this truck for about two years now, fyi -


  7. #7
    DonTom
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    Re: OBD2 codes explained

    "When I have a little spare time, maybe this weekend, I will try grounding the after-cat O2 sensor per your recommendation and report the results here."

    Grounding the lead will NOT work because it will give you a code for a shorted sensor. OBD2 checks for opens and shorts in all sensors.

    It will have to be a resistor to drop the DC voltage to a few tenths and I have no clue of the value resistor to start with, without working on it myself. If you have a large selection of resistors, you can experiment.

    But for a technophobe, the easiest thing to do is simply get the OD2 sensor moved to a place where there's nothing to detect and then reset your check engine light.

    IOW, still use the same O2 sensor but NOT in the pipe. Remove it from the pipe, put the single wire back on it, ground the outside of the sensor by the shell of it being on the frame of the car (anywhere that's grounded). You can leave it like that, or you can measure the DC voltages on the sensor (only a few tenths of a volt) with it outside the pipe and then use a resistor to drop to the same voltage in place of the O2 sensor.

    But remember, in OBD2, removing the problem will NOT get rid of the check engine light (on most codes) without a reset. If you don't have an OBD2 reader, try disconnecting the vehicle battery for about 15 seconds.

    I don't see how this could not work. After all, you should not have much gasoline to detect outside the pipe!

    You will then be using the O2 sensor as a resistor as there won't be much of anything to sense.

    -Don- (Reno)

  8. #8
    Vulture of The Western World Eric's Avatar
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    Re: OBD2 codes explained

    Quote Originally Posted by DonTom
    "When I have a little spare time, maybe this weekend, I will try grounding the after-cat O2 sensor per your recommendation and report the results here."

    Grounding the lead will NOT work because it will give you a code for a shorted sensor. OBD2 checks for opens and shorts in all sensors.

    It will have to be a resistor to drop the DC voltage to a few tenths and I have no clue of the value resistor to start with, without working on it myself. If you have a large selection of resistors, you can experiment.

    But for a technophobe, the easiest thing to do is simply get the OD2 sensor moved to a place where there's nothing to detect and then reset your check engine light.

    IOW, still use the same O2 sensor but NOT in the pipe. Remove it from the pipe, put the single wire back on it, ground the outside of the sensor by the shell of it being on the frame of the car (anywhere that's grounded). You can leave it like that, or you can measure the DC voltages on the sensor (only a few tenths of a volt) with it outside the pipe and then use a resistor to drop to the same voltage in place of the O2 sensor.

    But remember, in OBD2, removing the problem will NOT get rid of the check engine light (on most codes) without a reset. If you don't have an OBD2 reader, try disconnecting the vehicle battery for about 15 seconds.

    I don't see how this could not work. After all, you should not have much gasoline to detect outside the pipe!

    You will then be using the O2 sensor as a resistor as there won't be much of anything to sense.

    -Don- (Reno)
    Roger all that; will do! ;D

  9. #9
    DonTom
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    Re: OBD2 codes explained

    "Roger all that; will do!"

    Perhaps you should have done it a while back!

    Do you check your MPG often? Both before you removed the CC and after?

    I am not sure about this, and it may depend on the make of vehicle, but if you have a "check engine" light on because of excessive emissions are detected, you may be stuck in open loop, meaning your MAIN O2 sensor is not doing anything at all. You might not notice any difference in performance, but you most likely will in MPG when at a steady freeway speed.

    IAC, it's best you get rid of that check engine light and then there's no doubt it's running as it should.

    When stuck in open loop, you run a bit rich when at a steady speed.

    -Don-


  10. #10
    Vulture of The Western World Eric's Avatar
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    Re: OBD2 codes explained

    Quote Originally Posted by DonTom
    "Roger all that; will do!"

    Perhaps you should have done it a while back!

    Do you check your MPG often? Both before you removed the CC and after?

    I am not sure about this, and it may depend on the make of vehicle, but if you have a "check engine" light on because of excessive emissions are detected, you may be stuck in open loop, meaning your MAIN O2 sensor is not doing anything at all. You might not notice any difference in performance, but you most likely will in MPG when at a steady freeway speed.

    IAC, it's best you get rid of that check engine light and then there's no doubt it's running as it should.

    When stuck in open loop, you run a bit rich when at a steady speed.

    -Don-

    Gonna try and git 'er done today.. will report back!

  11. #11
    DonTom
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    Re: OBD2 codes explained

    Quote Originally Posted by Eric
    Gonna try and git 'er done today.. will report back!
    Well, Didja do it yet?

    -Don-

  12. #12
    Vulture of The Western World Eric's Avatar
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    Re: OBD2 codes explained

    Quote Originally Posted by DonTom
    Quote Originally Posted by Eric
    Gonna try and git 'er done today.. will report back!
    Well, Didja do it yet?

    -Don-
    Well, sort of!

    The second 02 sensor (downwind of where the converter used to be) proved impossible to remove. So I snipped the wires to see whether that would have any effect. It didn't.

    I assume that's because the open wires aren't receiving whatever "info" the sensor was supposed to transmit. I may just buy another 02 sensor and hook it up to the wires. But I kind of don't want to waste $30 on the stupid sensor - and get no change for my money.

    The truck runs great; has run great for years with the dumb "check engine" light on.

    What a PITAS these damn emissions/electronics are. All because of too damn many people!

  13. #13
    DonTom
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    Re: OBD2 codes explained

    "The second 02 sensor (downwind of where the converter used to be) proved impossible to remove. So I snipped the wires to see whether that would have any effect. It didn't."

    Did you use an O2 sensor removal tool on a half inch socket wrench?

    Now you have a new code in the OBD2 memory, the code for an open in a secondary O2 sensor. Won't hurt anything and your old code of excessive emissions will still be in there too, but is no longer an active code.


    "I assume that's because the open wires aren't receiving whatever "info" the sensor was supposed to transmit. I may just buy another 02 sensor and hook it up to the wires. But I kind of don't want to waste $30 on the stupid sensor - and get no change for my money."

    [size=10pt][color=blue]Sort of. Opens as well as shorts are tested for in all OBD2 sensors by the PCM and either will give a check engine light.You now have an open.

    [size=10pt]Do you have an ohmmeter? You might be able to measure a resistance (engine NOT running) across the O2 sensor that you removed the wire from and then try a ten cent resistor of the same value in place of the O2 sensor. I think that secondary O2 sensor simply acts as a resistor when it sees no problem. After all, it wants to see NOTHING after the CC.

    "The truck runs great; has run great for years with the dumb "check engine" light on."

    Yes, but I made an experiment with my 1988 Biretta (but was OBD1). I removed the MAIN O2 sensor and drove it that way for about 100 miles. It ran perfectly, but the MPG dropped down enough to notice. I used to average 25 MPG on the freeway. It went down to 21 MPG when the O2 sensor was removed and returned back to 25 MPG after it was reconnected.

    The question here is does your PCM stay stuck in open loop when you have such a check engine light? If so, it's just like removing the MAIN O2 sensor. However, I think an open is less likely to put it stuck in open loop than is a reading of excessive emissions.

    When in open loop all sensors except the O2 sensor are used. The F/A ratio will vary and most likely will almost never be the ideal 14.7 to one. When in closed loop, it's the opposite. Only the O2 sensor is being used to keep it exactly 14.7 to one.

    When the engine is cold, or when you accelerate, it will be in open loop anyway (in any vehicle), so you notice no difference with or without a (main) O2 sensor. When you cruise is when there's the big difference and that's only noticeable in MPG and in smog.
    -Don-
    [/size]

  14. #14
    Vulture of The Western World Eric's Avatar
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    Re: OBD2 codes explained

    Quote Originally Posted by DonTom
    "The second 02 sensor (downwind of where the converter used to be) proved impossible to remove. So I snipped the wires to see whether that would have any effect. It didn't."

    Did you use an O2 sensor removal tool on a half inch socket wrench?

    Now you have a new code in the OBD2 memory, the code for an open in a secondary O2 sensor. Won't hurt anything and your old code of excessive emissions will still be in there too, but is no longer an active code.


    "I assume that's because the open wires aren't receiving whatever "info" the sensor was supposed to transmit. I may just buy another 02 sensor and hook it up to the wires. But I kind of don't want to waste $30 on the stupid sensor - and get no change for my money."

    [size=10pt][color=blue]Sort of. Opens as well as shorts are tested for in all OBD2 sensors by the PCM and either will give a check engine light.You now have an open.

    [size=10pt]Do you have an ohmmeter? You might be able to measure a resistance (engine NOT running) across the O2 sensor that you removed the wire from and then try a ten cent resistor of the same value in place of the O2 sensor. I think that secondary O2 sensor simply acts as a resistor when it sees no problem. After all, it wants to see NOTHING after the CC.

    "The truck runs great; has run great for years with the dumb "check engine" light on."

    Yes, but I made an experiment with my 1988 Biretta (but was OBD1). I removed the MAIN O2 sensor and drove it that way for about 100 miles. It ran perfectly, but the MPG dropped down enough to notice. I used to average 25 MPG on the freeway. It went down to 21 MPG when the O2 sensor was removed and returned back to 25 MPG after it was reconnected.

    The question here is does your PCM stay stuck in open loop when you have such a check engine light? If so, it's just like removing the MAIN O2 sensor. However, I think an open is less likely to put it stuck in open loop than is a reading of excessive emissions.

    When in open loop all sensors except the O2 sensor are used. The F/A ratio will vary and most likely will almost never be the ideal 14.7 to one. When in closed loop, it's the opposite. Only the O2 sensor is being used to keep it exactly 14.7 to one.

    When the engine is cold, or when you accelerate, it will be in open loop anyway (in any vehicle), so you notice no difference with or without a (main) O2 sensor. When you cruise is when there's the big difference and that's only noticeable in MPG and in smog.
    -Don-
    [/size]
    Ach - all this stuff wears me out!

    Adjusting a carburetor is interesting and fun; worrying about resistors and ohms and stored memory and codes is neither interesting nor fun.

    I'll just keep on driving it as is. Maybe I'm losing an MPG or three; I don't really care about that, frankly. The money I'd waste on a new 02 sensor and whatever else I'd need to diagnose/repair the system would eat up any such minor improvement anyhow.

    The hell with computers!

    PS: When this truck dies I plan to acquire an old, pre-computer F100 or F150. The only area where these trucks are noticeably inferior to new trucks is the absence of an OD transmission - which can be easily remedied and without a single godamn computer, "sensor" or "code" to wory about! ;D

  15. #15
    Vulture of The Western World Eric's Avatar
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    Re: OBD2 codes explained

    "Did you use an O2 sensor removal tool on a half inch socket wrench?"

    No - the thing was heavily rusted and the "nut" part was corroded to the extent that a tool like that would only have rounded off what remained. So I used a large vise grip; if that doesn't break it loose, nothing will. It didn't slip off, but I was at the point where any more force applied would have likely bent the pipe, leaving me with more work/expense to deal with. So I just cut the wires.

    God-damned computers!

  16. #16
    DonTom
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    Re: OBD2 codes explained

    BTW, why did you remove the CC in the first place? Was there a problem with it?

    These days, you're better off leaving in the CC alone. The car is designed expecting it to be there and there are no improvements when it is removed in an OBD2 system. In fact, your performance can drop off when it's removed.

    Read this for some more related info.

    http://autorepair.about.com/library/a/1d/bl408d.htm

    -Don-

  17. #17
    Vulture of The Western World Eric's Avatar
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    Re: OBD2 codes explained

    Quote Originally Posted by DonTom
    BTW, why did you remove the CC in the first place? Was there a problem with it?

    These days, you're better off leaving in the CC alone. The car is designed expecting it to be there and there are no improvements when it is removed in an OBD2 system. In fact, your performance can drop off when it's removed.

    Read this for some more related info.

    http://autorepair.about.com/library/a/1d/bl408d.htm

    -Don-
    It rusted off and I didn't want to buy a new one; also, my thought was that the truck would run better without it and probably last longer...

  18. #18
    mrblanche
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    Re: OBD2 codes explained

    A large pipe wrench will get out rounded bolts, if you can get at them.

    I have heard that those types of items can be made to last forever, practically, but then when they DO fail, you have the problem you've experienced. By lessening the life span a little, they're usually still removable.

  19. #19
    DonTom
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    Re: OBD2 codes explained

    "Adjusting a carburetor is interesting and fun; worrying about resistors and ohms and stored memory and codes is neither interesting nor fun."

    That depends on how technophobic one is. I would rather play around with the codes and resistors than play with a carb.

    BTW, it's okay to be a tecnophobe, but you don't have to be so damn proud of it! ;D

    -Don-

  20. #20
    Vulture of The Western World Eric's Avatar
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    Re: OBD2 codes explained

    Quote Originally Posted by DonTom
    "Adjusting a carburetor is interesting and fun; worrying about resistors and ohms and stored memory and codes is neither interesting nor fun."

    That depends on how technophobic one is. I would rather play around with the codes and resistors than play with a carb.

    BTW, it's okay to be a tecnophobe, but you don't have to be so damn proud of it! ;D

    -Don-
    Someone has to rage against the machine.....!

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