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Old 11-21-2012, 08:46 AM   #1 (permalink)
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Default Lean Codes & O2 Sensor data

I've got a '99 Taurus wagon with the DOHC engine I bought used a year ago. I've got 128,000 miles on it. Used my scan tool before purchase and found P0171 & P0174 lean codes. I found the culprit being the engine vacuum bulkhead connector that also feeds the brake booster - the plastic connector was cracked at the body. The replacement from Ford was substantially thicker there. The lean codes have returned again & I'm looking to replace intake manifold gaskets as it's also time to change plugs and the upper manifodl needs to come off to access the rear plugs. I have a computer program called AutoTap that reads live data and my 02 bank 2 sensor 2 reads a constant 1275mV which should fluctuate between 100 and 1000mV. The bank 2 sensor 1 ahead of the cat fluctuates as it should. I changed out the sensor and the new one reads the same. I verified the correct sensor was changed by unplugging the sensor and the live readout was 0mV and went back to 1275mV when plugged in. Starting the engine cold and watching the sensor output results in a 0 reading for the first 10 seconds or so and then the value just goes up to 1275mV.

The trans was recently repaired (torque converter lockup at idle killing the engine) along with the alternator replaced and the only code received in the last 100 miles of operation is a Bank 2 Sensor 2 (for the O2 sensor) malfunction. One other thing, the scanner indicates the MIL should be on and it's not yet the light works when the key is turned on. Same for the fuel trim codes when they were received. Can the ECM be programmed to do this by an unscrupulus shop (this was a used car!)?

Could a bad cat cause this? Any other places to look for a vacuum leak?

Last edited by tgproios; 11-21-2012 at 09:03 AM.
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Old 11-21-2012, 10:42 AM   #2 (permalink)
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Welcome to TCCA.

I'll see if I can get things started.

The bank 2 sensor 2 is the down stream/after cat converer O2 sensor & if the cat converter is doing its thing, the after cat O2 sensor reading will be fairly steady. It won't switch like the upstream/before cat O2 sensor. The high Mv reading is high/indicating rich, so I wonder if the bank-2 cat is bad, or if the lean problem is with bank 2 & the computer is adding fuel to bank two injectors to try & compensate to bring the fuel trim/air/fuel ratio back to normal/14.7:1????

Not sure what to make of the CEL/SES light not turning on, its not burned out as you say it lights up for its self check at KOEO.
You could try disconnecting something else, like the IAC, TPS, MAF sensor & see if the computer will sense that & turn the trouble light on.

Some thoughts for pondering, keep us posted on your trouble shoot.
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Old 11-21-2012, 11:45 AM   #3 (permalink)
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Check the u-shaped pcv hose directly under the throttle body.
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Old 11-21-2012, 04:03 PM   #4 (permalink)
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One other thing I'm going to try is to switch the downstream O2 sensors to verify the wiring is intact. I should see Bank 1 Sensor 2 go high when the switch is done and Bank 2 sensor 2 switch between high and low values.

What should the long term fuel trim values be? I don't have the computer which has the data with me now but I can supply tonight. Wouldn't that be an indicator of rich/lean conditions?
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Old 11-21-2012, 05:44 PM   #5 (permalink)
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Yes, the O2 sensor reading should never be that high. The actual voltage is probably higher (1275 mV is probably the maximum value it can measure). So first thing I would do is check the sensor voltage at the sensor. Then check it at the PCM connector. If you measure a high voltage (e.g 5 or 12 volts), then you have power shorting into the wire somewhere. If the voltage reads normally (0-1 volt) at the PCM, then I would think about PCM replacement.
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Old 11-22-2012, 01:14 PM   #6 (permalink)
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Quote:
Originally Posted by behlinla View Post
Yes, the O2 sensor reading should never be that high. The actual voltage is probably higher (1275 mV is probably the maximum value it can measure). So first thing I would do is check the sensor voltage at the sensor. Then check it at the PCM connector. If you measure a high voltage (e.g 5 or 12 volts), then you have power shorting into the wire somewhere. If the voltage reads normally (0-1 volt) at the PCM, then I would think about PCM replacement.
Voltage itself means NOTHING on the O2 sensors. What your looking for
is the transition waveform. If you have a live OBD2 scanner, you can see
the waveform live. If you see NO transitions, you have issues. A lazy sensor can cause lack of sharp transitions. When measuring voltage
with no O2 sensor connected on a high impedance DVM, you might
see a phantom voltage that is not really there.1200 MV = 1.2 volts.
You will see 12 volts on the O2 heater lines until the car warms up.
This is a 4 wire sensor. Two wires for the heater, two for the sensor.
On my scanner with good wave forms, the waveform peaks at .7 to .9
volts or so.


The key words in your original post was had the tranny worked on.
If someone was NOT careful and disconnect the O2 sensor before dropping
the subframe, the wires may have been pulled and the harness damaged.
You would need to trace the O2 wiring from the sensor to the PCM to
confirm this.

bob
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Old 11-22-2012, 07:03 PM   #7 (permalink)
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Quote:
Originally Posted by soundu View Post
Voltage itself means NOTHING on the O2 sensors. What your looking for
is the transition waveform. If you have a live OBD2 scanner, you can see
the waveform live. If you see NO transitions, you have issues. A lazy sensor can cause lack of sharp transitions.
Voltage does matter if you're evaluating the health of the O2 sensors. It's true the PCM only cares about cross counts (voltage crossing 0.45 volts), but if the sensor has a bias it's not going to respond properly. So the max and mins are almost as important as the switching rate and you have to look at both when you're examining a sensor.

Because they hardly produce any current, O2 sensors can only be measured with a high-impedance circuit anyway. The OP was using a scan tool for his measurements. From an electrochemical standpoint, there is no way an O2 sensor can produce more than about 1 volt under normal conditions, no matter what kind of meter you're using. The only time I EVER see readings above 1.0 volts on a Ford (which does not uses a bias voltage) was with a power wire shorted into the sensor wire.

I also believe the transmission swap is probably the culprit. They might have damaged a wire or connector so the 12V heater power is shorting into the signal wire.
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Old 11-24-2012, 10:05 AM   #8 (permalink)
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OK Folks, the codes were there before the tranny was worked on. I've done some additional research looking for a replacement direct fit cat in case I need it and came across Eastern Catalytic's website. They have some great info there. Apparently a healthy cat will have a varying O2 sensor voltage on the upstream side and a STEADY voltage on the downstream side like PawPaw advised. My doenstream side seems high on bank 2. Bank 1 downstream is switching high & low which indicates a bad cat. Still have the code for Bank 2 Sensor 2 Circuit malfunction and an evap system large leak code which I believe is from the fuel filler neck which is rusted and I've patched with RTV several times. Time to buy a new one - are the sedan & wagon fillers the same? I'm having a herd time finding one specific to model at Rock Auto Parts. Same with the direct fit cat - the DOHC engine has the AX4N trans yet the parts list AX4S trans. Any help here?

Thanks to all with their answers thus far
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Old 11-24-2012, 11:13 AM   #9 (permalink)
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OK, good feedback, yup you need to properly fix/replace the leaking rusted fuel filler neck, as thats corrupting things. While doing that, disconnect the battery B- cable to wipe the KAM corrupt fuel trim tables, clear the codes & let the computer know a repair has been made, so it'll begin to build new uncorrupted fuel trim tables.

After the repair, do the cold & warm idle relearn routine, so the computer can relearn its idle strategy, as it too gets wiped with a battery disconnect. EDIT: Idle relearn proceedure. http://www.taurusclub.com/forum/1358165-post4.html

Then drive it some & see if any more codes are set. If the after cat code returns, then you can move it higher up on your replacement suspect list.

More thoughts for consideration, let us know how it goes
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Last edited by pawpaw; 11-24-2012 at 11:28 AM.
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Old 11-24-2012, 12:26 PM   #10 (permalink)
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Quote:
Originally Posted by tgproios View Post
OK Folks, the codes were there before the tranny was worked on. I've done some additional research looking for a replacement direct fit cat in case I need it and came across Eastern Catalytic's website. They have some great info there. Apparently a healthy cat will have a varying O2 sensor voltage on the upstream side and a STEADY voltage on the downstream side like PawPaw advised. My doenstream side seems high on bank 2. Bank 1 downstream is switching high & low which indicates a bad cat. Still have the code for Bank 2 Sensor 2 Circuit malfunction and an evap system large leak code which I believe is from the fuel filler neck which is rusted and I've patched with RTV several times. Time to buy a new one - are the sedan & wagon fillers the same? I'm having a herd time finding one specific to model at Rock Auto Parts. Same with the direct fit cat - the DOHC engine has the AX4N trans yet the parts list AX4S trans. Any help here?

Thanks to all with their answers thus far
A couple of notes:

Beware of thinking the downstream sensor has a steady state voltage.
It does not. It varys depending on driving conditions in a more gentile
rolling manner. If you have a OBD2 Live scan tool, you can observe
this waveform.

Now if you see the same sharp transitions you see on sensor 1, that
usually means the cat is dead.

A word of caution here. If your harness has been damaged and
you have no connection to the secondary sensor, you may still
see a phantom voltage/waveform on the scan tool.

ON my 97, i have the bank one sensor 2 disconnected. On my scan tool,
i am seeing a sharper transition waveform on it. Obviously, its not
the sensor since no sensor is connected. Its a phantom voltage induced
into the wire harness. The key thing here is the level. This phantom
voltage only shows about .3V peak level on the scan gage. Thats part
of the clue that its not what it appears to be.

If your secondary sensors wiring has been damaged, and your reading
a low peak value on the post cat sensor, don't be so fast in condemning
the cat

Did you have any cat issues before the trannie work? if not, there may
be your smoking gun of cause and affect.

Be my guest if you want to replace your cats, but its an expensive
and potentially nasty job to do yourself if you have 15year old fasteners
and studs rusted so they won't move.

The aftermarket cats are marginal at best in many cases. And the OEM
level ones will make you think twice if its worth doing on an older car.

If i were you, i would make sure you covered all your diagnostic bases
before you bought cats.

It can be hard enough to remove the post cat O2 sensors. I have broke
and trashed at least two working on my bulls. You have little room to
work on the one under the floor pan even with the special bent handle
sensor remover you can buy. Sometimes, you need to heat them up
with a torch to get them to break loose before your strip the hex head
on the sensor
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