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Old 04-13-2012, 09:43 PM
thcardoc thcardoc is offline
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P0141 DC IV (Easy one)

1998 Chevrolet Cavalier 2.2l

The check engine light came on and the owner retrieved a code P0141. Google diagnostics said to replace the downstream O2 sensor. The sensor was replaced, and the code cleared and the light stayed off for the rest of that day, and for two days later. On the third day the light came back on, and the P0141 was set again.

There are a number of possible causes for this scenario, so there isn't just one correct answer. Identify one of the possible causes and explain how you would prove the fault.
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Old 04-14-2012, 01:12 AM
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Current was going to the check engine light so I remove the bulb. Problem solved.
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Old 04-14-2012, 05:42 AM
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Originally Posted by Daniel Wood View Post
Current was going to the check engine light so I remove the bulb. Problem solved.
Car now fails it's next emissions test because the lamp is inoperative, with the additional problem added now there are two to fix.
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Old 04-14-2012, 02:24 PM
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Doc, it was a joke!
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Old 04-14-2012, 02:43 PM
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Doc, it was a joke!
I figured that, but as you see some jokes don't always play well no matter how you smiley them. On the serious side, the last customer who suddenly found out that she had lamps disabled on a car she had bought really struggled to deal with the cost to make her car right. We got her through it over some six months by doing a little at a time but these things weigh heavily on us too.

Now back to the challenge. Identify a failure in the heater circuit for that downstream O2 sensor and how you test and prove the failure and the fix.
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Old 04-14-2012, 05:30 PM
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Scenario #1: The old O2 failed. The new O2 worked fine when installed, but later failed. Just bad luck, you got a bad O2.

Scenario #2: The ECU reported a bad O2, but there was nothing wrong with it. There was a wiring issue. When you replaced the old one, you wiggled to wires and it appeared to have fixed the problem. Unfortunately when you wiggled the wires you got a good signal again, but now the wires are not working again. Most likely a corroded plug.

The fist thing I would do is test the old O2 and see if it is still working. Seems I have read you can heat them up with a torch and use a volt meter. I would have to check on that.

Scenario #3: The old O2 was bad. The new O2 just died a needless death. There is something wrong with the car that killed both O2 sensors. I haven't a clue what could do that other than maybe the exhaust is getting too hot.
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Old 04-15-2012, 05:54 AM
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Originally Posted by olddog View Post
Scenario #1: The old O2 failed. The new O2 worked fine when installed, but later failed. Just bad luck, you got a bad O2.
The code is pretty specific, the circuit that is failing is for the heater inside the sensor plus the wires and fuse that carry power for it. Many make the mistake of pulling the code and throwing a sensor at the car when the problem can be anywhere in that circuit.

The routine that should be followed for any code is to read the code description, and then the code set criteria. The test that the computer runs requires that the car be started cold with the engine temperature below 115f. The computer then measures the amount of time that it takes for the downstream sensor to become active. If it takes too long, then there is a problem with the heater circuit. The test must fail twice in order to generate a trouble code and turn the light on. In the scenario presented replacing the sensor has not made any difference at all. It could be defective but it's more likely that the failure is outside of the sensor. Many will rely on trouble trees to guide them to a failure such as this and one of the first things the trouble tree will do is have you disconnect the sensor and test for power and ground for the heater in the sensor. Someone who goes to a schematic first would likely test the fuse in the drivers side fuse box for the heated sensor right away.

Last edited by thcardoc; 04-15-2012 at 05:55 AM.
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Old 04-15-2012, 01:03 PM
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Honestly, I never looked up the code. I just went by what was in the first post and had no idea is was the heater not working.

A bit off subject:
An ODOT employee was telling me they had a truck that would sound a bell every half hour. Dealership couldn't fix it when it was under warranty. Did it for 3 years. They hired a new kid. He found the light bulb for the seat belt was bad, and fixed it.
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Old 04-16-2012, 04:51 AM
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Originally Posted by olddog View Post
Honestly, I never looked up the code. I just went by what was in the first post and had no idea is was the heater not working.
I have to laugh cause that is so real world, pull the code and since it has something to do with an O2 sensor change the part. If that doesn't fix the car then it must be a defective part so change it again.

When I am teaching classes I work at getting the students to take a disciplined approach to diagnostics. These challenges all come directly from real vehicle failures and outline the steps one has to take in order to be efficient as well as achieve a fixed the first time result. One such answer for this challenge is:

Raise vehicle, and disconnect the down stream O2 sensor. With the key on test for power on the brown wire, and there is no power. The next step is to check all of the fuses in the drivers side dash fuse block and the 10 amp O2 HTR fuse is found to be open. (blown)

Now do you just replace the fuse and hope for the best, or should steps be taken that ensure no other faults are present at this time? There are some who will say slam a fuse in it and if it doesn't blow your done. Others will take a more disciplined approach and use a fuse buddy and measure the current through the circuit while on a road test watching for correct operation of the sensor with a scan tool and for spikes in the current flow that could indicate a chafed wire. The replacement of the first O2 sensor without any testing set the stage where the chance to prove that maybe the heater had failed and took the fuse out is gone. Getting to diagnose this first could also have proven that maybe there was nothing wrong with the heater in the sensor at all and that the fault is veery likely external of the sensor.

The O2 sensor heater circuit is simple series circuit. An open at any point shuts down the whole circuit. The fuse of course is only one of the points that the circuit could be open. You could lose a connection at any point in the power side, the ground side, or the sensor itself so that all have to be confirmed. Each scenario should reflect a responder choosing the location that the open circuit could occur and what the steps were to prove it as I did here with the fuse.


Quote:
A bit off subject:
An ODOT employee was telling me they had a truck that would sound a bell every half hour. Dealership couldn't fix it when it was under warranty. Did it for 3 years. They hired a new kid. He found the light bulb for the seat belt was bad, and fixed it.
There is always a lot more to these stories than meets the eye. I suspect the dealer techs "wouldn't" fix it instead if couldn't. A chime that goes off every half an hour is something the tech would be fixing essentially for free because "flat rate" doesn't pay for him/her to investigate for that period of time. Then even if they somehow were approved time, they get in the truck with a scan tool to monitor the vehicle systems, and fasten the seatbelt like they are supposed to and start driving around waiting for the chime to go off and guess what, it never does because the seatbelt is fastened. Now maybe they could notice key on/start-up that the seat belt fasten MIL doesn't bulb check, but the most likely reason to notice it's missing would be that the kid was driving around with the belt unfastened and saw that the MIL should have been on, when it wasn't.

My routine would have been to have the customer identify the sound of the chime by commanding them one at a time with a scan tool. Then command a cluster self check, again with a scan tool so that I could confirm that all of the lamps operate.
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