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REPLACING OXYGEN SENSORS. |
STEP 1. |
The following outlines the procedure for
replacing oxygen sensors (pre-cat on OBDI cars
and both pre- and post-cat on OBDII cars). The
procedure was based on a 99.5 Jetta GLS VR6
(MKIV), however, it should be similar on other
models and years. Please do these procedures at
your own risk and be ready to make small
adjustments while doing them. Also, please be
observant while removing parts so that they go
back together correctly.
If you are replacing an oxygen sensor because of
a diagnostic trouble code (DTC) and are not sure
what sensor the code is referring to,
Bank1-Sensor1 is the pre-cat sensor and
Bank1-Sensor2 is the post-cat sensor on an OBDII
car.
For a MKIV car with a 12v VR6 engine, the part
#s for the pre- and post-cat oxygen sensors are
021-906-262-B and 06A-906-262-Q (replaces
021-906-262-C), respectively. If you have
another model/engine, you'll need to obtain the
appropriate sensors for your car. Please verify
the part #s with your parts supplier before
ordering any parts. I am not responsible if you
purchase the wrong parts for your particular
car.
I purchased the above oxygen sensors from
http://www.1stvwparts.com. The pre-cat sensor
('B') was $28.99 and the post-cat sensor ('Q')
was $15.86 (all prices were quoted online on
8/20/03).
Finally, the time required to replace the oxygen
sensors is varied and depends mostly on how long
it takes to loosen or 'wake up' the threads on
the old sensors. If the old sensors come out
relatively easily (not likely if they've seen a
lot of miles), you should be able to replace
both sensors in an hour or so. Most likely, it
will take some extra time to remove the old
sensors - I'd plan on spending at least one hour
per sensor. If you plan on replacing both
sensors, you can try and save some time by
performing the steps for loosening the threads
(steps 15 through 17) on both 'old' sensors at
the same time.
VW TECH BY: Gary Thompson |
The replacement of the oxygen sensors requires
that the front end of the car (at least) be
raised, either on ramps, on jack stands or on a
lift. If you have ramps or access to a lift
(lucky!), use them appropriately (much simpler,
faster and safer than placing the car on stands)
and proceed to step 5. If you do not have ramps
or access to a lift, but have experience jacking
your car up and placing it on stands, do this as
you normally would and proceed to step 5. If
not, you can consider using the method I used
below.
WARNING: DO NOT ATTEMPT TO PUT YOUR CAR ON
STANDS IF YOU ARE NOT COMFORTABLE DOING SO. IF
NOT DONE CORRECTLY, THE CAR MAY FALL, POSSIBLY
CAUSING SERIOUS INJURY OR DEATH. ONLY WORK NEAR
OR UNDERNEATH A CAR THAT HAS BEEN PROPERLY
SUPPORTED ... DO THE FOLLOWING PROCEDURE AT YOUR
OWN RISK!!!
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STEP 2. |
STEP 3. |
Jack the car up using the pinch rails near the
edge of the underside of the car. The pinch rail
is indicated by the yellow arrow in the picture
below and is the place used by the stock VW jack
for raising the car. Make sure that the jack cup
is centered on the jack point on the rail
(indicated by the small indent on the lower side
valence just above the yellow arrow). It is a
good idea to place a thin piece of wood or hard
rubber between the jack cup and pinch rail to
help distribute the load and protect the
underside of the vehicle from being marred. If
you look closely at the picture, you should be
able to see the jack on the far side of the car
underneath the pinch rail. Note the 6" x 6"
piece of plywood sitting on top of the jack cup. |
The jackstands should be placed directly below
the major frame rails of the car, indicated by
the red arrow in the picture above. Make sure
that the stands are placed towards the very
front of the frame rails - this is where they
are strongest. If you place the stands too far
back from the front edge, you run the risk of
crushing the frame rail. Don't get too close to
the curved front edge of the rail, however, as
there is a risk of the jack stand slipping and
the car falling. I placed the jackstands just
below the small hole in the bottom of the frame
rail (see picture above for approximate location
of stands relative to the front of the frame
rail). This seemed to me to be the best
compromise between safety and rail strength.
Again, it is a good idea to place a protective
barrier between the jack stand and the frame
rail - I used a double-thick piece of regular
cardboard. |
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STEP 4. |
STEP 5. |
Slowly lower the raised side of the car onto the
jack stand. Repeat the above procedure (steps 2
and 3) on the other side of the car.
WARNING: MAKE SURE THE CAR IS WELL-SUPPORTED
BY THE JACK STANDS BEFORE DOING ANY WORK NEAR OR
UNDERNEATH THE CAR!!!
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The location of the catalytic converter (the
oxygen sensors are usually threaded into it) is
indicated by the white arrow in the picture
above. Similarly, the location of the plastic
box that houses the oxygen sensor lead harness
connectors is indicated by the green arrow in
the picture above. |
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STEP 6. |
STEP 7. |
Slide under the car and locate the black plastic
box that houses the oxygen sensor lead harness
connectors, shown in the picture below. It is
located just to the inside of the main frame
rail and along side the catalytic converter. |
Remove the two (2) 10mm plastic nuts (indicated
by the yellow arrows in the picture above) that
secure the box to the underside of the car. |
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STEP 8. |
STEP 9. |
The picture above shows the sensor lead harness
connectors inside the black plastic box. The
harness connectors are attached to the box by
tabs on either side of each harness connector.
Slide the appropriate harness connector(s) off
of the tabs to disconnect it from the box (this
can be difficult to do - the fit is very tight).
Note that the harness connector for the pre-cat
oxygen sensor (6-wire) is larger than that of
the post-cat oxygen sensor (4-wire). |
Separate the appropriate harness connector(s) by
pulling up slightly on the retaining tab
(indicated by the yellow arrow in picture above)
with a screwdriver or other small, thin tool and
sliding the two halves apart.
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STEP 10. |
STEP 11. |
Remove the protective
heat shield from around the sensor leads by
unsnapping the snaps indicated by the yellow
arrows in the picture above (there are
approximately 4-5 snaps). |
Remove the appropriate
sensor lead(s) from the retaining clips on the
underside of the car. One of the clips is
indicated by the red arrow in the picture above. |
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STEP 12. |
STEP 13. |
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Now comes the fun part -
removing the old sensors from the threaded bungs
on the catalytic converter. The removal of the
old sensors is often difficult due to the fact
that the threads on the sensor seize inside the
bung and are difficult to loosen or 'wake up'.
This is especially true for sensors that have
seen a lot of miles and seems to occur even when
anti-seize is used on the threads prior to
installation. The following steps will describe
one way to loosen the threads on the old
sensors. I'm sure there are other methods of
loosing the threads on the old sensors. If you
know of a way that works, go ahead and give it a
try. If you've never done this before or have no
idea where to begin (other than straining to
turn the old sensors and possibly hurting
yourself), give the following method a try. |
The loosening of the
threads on the old oxygen sensors is much easier
if you use the three items shown in the picture
below. These items are a can of penetrating
lubricant (PB Blaster, which is shown in the
picture, is excellent), a small butane torch or
other concentrated high heat source and a
hammer. It is not necessary to use all three
items to loosen the old threads, but using the
three together in repeated sequence can greatly
reduce the time and effort needed to get the old
sensors out. |
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STEP 14. |
STEP 15. |
Locate the oxygen
sensor(s) on the catalytic converter. If you
have an OBDII car, there should be two sensors.
The pre-cat sensor is closest to the front of
the car and the post-cat sensor is closest to
the rear. The sensors should be located either
on the sides or on top of the catalytic
converter. The removal of the sensors is much
easier is they are on the side of the cat.
Unfortunately, the sensors on my MKIV VR6 are on
top of the cat (not sure if this is normal or
not). There is very little space up around the
converter, making them even harder to remove.
The pre- and post-cat oxygen sensors on my MKIV
VR6 catalytic converter can be seen in the
pictures above (the protective heat shield
should already be removed from the sensors leads
- the pics were taken out of sequence. Sorry.) |
Begin loosening the threads by spraying the PB
Blaster or other penetrating lubricant into the
crack between the sensor and the bung, as
indicated by the blue arrow in the picture
above. Spray enough lubricant to surround the
threads, but not too much or it may start to
drip into the catalytic converter. It doesn't
matter if the old sensor becomes saturated with
the lubricant, but it's probably best to keep as
much as possible from actually collecting inside
the catalytic converter. As long as you don't go
crazy spraying the lubricant, you shouldn't have
to worry too much. Give the lubricant a few
minutes to do it's job.
WARNING: PLEASE BE EXTREMELY CAREFUL WHEN
USING AN OPEN-FLAME SOURCE NEAR OR UNDERNEATH
THE CAR!!! THERE IS NO POINT IN HAVING WORKING
OXYGEN SENSORS IF THE REST OF THE CAR IS A BIG
PILE OF ASH, METAL AND MELTED PLASTIC!
ATTENTION: IT IS A WISE IDEA TO TEST THE
FLAMMABILITY OF THE PENETRATING LUBRICANT BEFORE
PERFORMING THE NEXT STEP!!!
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STEP 16. |
STEP 17. |
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Next, heat the bung on
the catalytic converter with the torch for a
minute or two. The bung is indicated by the red
arrow in the picture above. The heat will cause
the bung hole (please keep you comments to
yourself) to expand, making it easier to remove
the sensor. Try and only heat the bung and not
the sensor itself, so that the sensor threads do
not expand too (the sensor will get hotter due
to conduction, but not nearly as hot as if the
torch were directed at it intentionally).
WARNING: IF THE TORCH OR OTHER
CONCENTRATED HEAT SOURCE IGNITES THE PENETRATING
LUBRICANT (THIS DID NOT OCCUR WITH PB BLASTER),
THEN STOP IMMEDIATELY!!! |
Finally, lightly hit the
nut area (not sure what to call it) of the
sensor (indicated by the green arrow in the
picture above) with the hammer a few times to
help loosen the threads. Ten or so hits should
be good. Don't do all the hits in the same
place, but instead spread them around the
circumference of the sensor.
CAUTION: IF YOU USED A TORCH OF
OTHER CONCENTRATED HEAT SOURCE TO HEAT UP THE
BUNG, BE CAREFUL NOT TO BURN YOURSELF WHEN
PERFORMING THE NEXT STEP. THE AREA WILL BE VERY
HOT!!!
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STEP 18. |
STEP 19. |
Using an oxygen sensor
wrench (yellow arrow in the picture above) in
conjunction with a socket wrench, attempt to
remove the oxygen sensor from the bung by
turning it counter-clockwise. If you do not have
an oxygen sensor wrench, you can also use an
open-ended 22mm wrench to do this. As a last
resort, use an adjustable wrench. If you do, be
careful that he wrench does not slip. You may
have to apply some significant torque to get the
threads to finally break free - please be
careful not to hurt yourself. If the sensor does
not turn, you can actually try tightening it
slightly (turn it clockwise). Supposedly, this
is an old mechanic's trick for breaking threads.
Then try and loosen the sensor again. |
If the sensor still does
not break free after all that, repeat steps 15
through 18 until it does. It may take a couple
of cycles (it took 3-4 for my sensors) to
finally get the sensor out.
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STEP 20. |
STEP 21. |
The picture above shows a
new post-cat sensor along side the one I removed
from my car. Surprising, the old one doesn't
look too bad considering it was in use for 137k
miles. The sensor actually still worked fine - I
only replaced it as part of preventative
maintenance and because it was so cheap. The
same applies to the pre-cat sensor I removed
from my car. |
Before installing a new
sensor in the bung on the catalytic converter,
make sure to apply some anti-seize to the
threads of the new sensor. If you purchase an
OEM VW sensor, it should come with anti-seize on
it already. If your sensor didn't come with the
anti-seize already applied, purchase some which
is safe for oxygen sensors (silicone-free) and
apply a little to the sensor threads. Be careful
not to get any near or in the holes on the end
of the sensor. This could potentially damage the
new sensor.
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STEP 22. |
STEP 23. |
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Using a paper towel or
thin rag, wipe out the threads of the bung to
remove any excess penetrating lubricant and then
thread the new sensor into the bung. Tighten the
sensor to 37 ft-lb (50 Nm). I didn't use a
torque wrench to tighten the sensor - I just
made it medium tight by feel. |
If you are replacing both
sensors on an OBDII car, remove and replace the
'other' sensor by following steps 15 through 22.
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STEP 24. |
STEP 25. |
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If you have finished
removing and replacing sensors, secure the
sensor leads, reinstall the protective heat
shield, reconnect the harness connectors and
reinstall the black plastic box that houses the
harness connectors by following steps 7 through
11 in reverse. |
If the car was raised
during the above procedure, lower the car using
the appropriate steps for the lifting method
used.
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STEP 26. |
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Since the learned long
term fuel trim values in the ECM memory depend
in part on the condition of the oxygen sensors,
it is a good idea to reset the values if you
changed the pre-cat oxygen sensor (the post-cat
sensor only monitors catalytic converter
efficiency - it has nothing to do with fuel trim
/ engine control). To reset the fuel trim
values, either disconnect the (-) battery
terminal for approximately 15 minutes and then
reconnect it (remember to have your radio code
handy) or erase fault codes in the engine module
(even if there are none) using a VAG-COM. The
car may drive crappy for a little while since
all the fuel trim values are now zero and other
learned parameters have also been erased. These
values and parameters should return to normal
relatively quickly and the car should run
normally after that occurs. |
