loc of oxygen sensor

97 blazer 4wd, engine 4.3 liter w
code: po155 ho25 htr circuit bank2 sensor 1
which one and where is it?
thanks

Passenger side manifold.

if it is code p0155 do both o2 sensors need changed? I was told the 02 xtreme v6 has 4...2 before and 2 after...or does that code go to one specifically...thanks

passenger side manifold O2 sensor.

Always remember sensor 1 is always the precat sensor. Bank 1 is always the side where #1 cyl is. Sensor 2 is after the cat. If there are 2 sensors here the same rule as the precat. Sensor 2 Bank 1 is the side where cyl #1 is. I say it that way because Ford #1 is on passenger side, GM #1 drivers side.

Always remember sensor 1 is always the precat sensor. Bank 1 is always the side where #1 cyl is. Sensor 2 is after the cat. If there are 2 sensors here the same rule as the precat. Sensor 2 Bank 1 is the side where cyl #1 is. I say it that way because Ford #1 is on passenger side, GM #1 drivers side.

Bank #1 is driver's side.

Bank #2 is passenger side.

B1S1 - DRIVER'S SIDE MANIFOLD
B2S1 - PASSENGER SIDE MANIFOLD

B2S2 & B1S2 - SAME SENSOR BEFORE THE CAT.

B2S3 & B1S3 - SAME SENSOR AFTER CAT

B1S1 is precat
B1S2 is after cat
B2S1 is pre cat
B2S2 is after cat
Sensor 1 on each banks tells ecm what engine is doing
Sensor 2 on each banks tell ecm how well cat is working
Sensor 1 voltage will vary from 0 to 1 volt with .5 being the middle, the voltage should vary from above and below the halfway point. This is call cross counts.
Sensor 2 will stay close to .5 if the cat is working good.

http://auto.howstuffworks.com/framed.htm?parent=question257.htm&url=http://www.wps.com/LPG/o2sensor.htm
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Check it out

B1S1 is precat
B1S2 is after cat
B2S1 is pre cat
B2S2 is after cat
Sensor 1 on each banks tells ecm what engine is doing
Sensor 2 on each banks tell ecm how well cat is working
Sensor 1 voltage will vary from 0 to 1 volt with .5 being the middle, the voltage should vary from above and below the halfway point. This is call cross counts.
Sensor 2 will stay close to .5 if the cat is working good.

Bank #1 is driver's side.

Bank #2 is passenger side.

B1S1 - DRIVER'S SIDE MANIFOLD
B2S1 - PASSENGER SIDE MANIFOLD

B2S2 & B1S2 - SAME SENSOR BEFORE THE CAT.

B2S3 & B1S3 - SAME SENSOR AFTER CAT

Those are the true locations.

You don't account for sensor #3? Where is it in your example?

I will repeat here also for everyone.

"Tell me this then, where is the Bank 1 Sensor 3 then? Or where is Bank 2 sensor 3?"

You know that is a good question. Considering I have yet to see a sensor 3 as of yet. And my GENISIS Scanner 2002 domestic, 2003 Asian.

Now why it so hard to understand bank 1 is always the side where cyl 1 is located. So bank 2 must be the other side.

Sensor 1 either bank monitors the engine.
Sensor 2 either bank monitor the cat.

Sensor 3, well like i said got me there.

But if there is a sensor 3 I am going to guess in the pipe, must be after 1 & 2.

Sensor 1 will be in manifold or the pipe just after manifold but before the cat.

Sensor 2 will be in the rear of cat or in the pipe just after the cat.

Now you could at times only have 1 sensor 2 because of single exhaust.

But if they use 2 cats on the car then they will use 2 sensors, one in or behind each cat.

Now I know that the new car are making changes with o2 sensor and people are going to cry when they have to pay the newer prices. Since I have an Auto Repair Shop I don't work much on the 2003 and up, only because most are still under warranty with the dealer.

I continue my training with EAST TRAINING,& CTI.
And my employees.

After doing some looking I have found your answer. Follow the link .
http://www.asashop.org/autoinc/june2002/techtips.cfm

I will repeat here also for everyone.

"Tell me this then, where is the Bank 1 Sensor 3 then? Or where is Bank 2 sensor 3?"

You know that is a good question. Considering I have yet to see a sensor 3 as of yet. And my GENISIS Scanner 2002 domestic, 2003 Asian.

Now why it so hard to understand bank 1 is always the side where cyl 1 is located. So bank 2 must be the other side.

Sensor 1 either bank monitors the engine.
Sensor 2 either bank monitor the cat.

Sensor 3, well like i said got me there.

But if there is a sensor 3 I am going to guess in the pipe, must be after 1 & 2.

Sensor 1 will be in manifold or the pipe just after manifold but before the cat.

Sensor 2 will be in the rear of cat or in the pipe just after the cat.

Now you could at times only have 1 sensor 2 because of single exhaust.

But if they use 2 cats on the car then they will use 2 sensors, one in or behind each cat.

Now I know that the new car are making changes with o2 sensor and people are going to cry when they have to pay the newer prices. Since I have an Auto Repair Shop I don't work much on the 2003 and up, only because most are still under warranty with the dealer.

I continue my training with EAST TRAINING,& CTI.
And my employees.


Bank 1 is driver's side
Bank 2 is passenger side.

There is a Sensor 3.

What are the codes P0143-P0147 for then? That is right, they are for SENSOR 3!!!!!!

They are for the post CAT O2 sensor on 1996+ 4.3L engines.

I really don't know why you are arguing this, you say it doesn't exist but every shop manual and repair guide has them for our trucks.

Let me see if I can clear this up. You guys are talking about different things.

rksnc is talking about a normal oxygen sensor system, with three sensors and single exhaust pipe. This is the same one that I have and everyone else that I have seen. This system includes:

One sensor just below the driver's side manifold - Bank 1 Sensor 1
One sensor just below the passenger side manifold - Bank 2 Sensor 1
One sensor just after the cat - Bank 1 Sensor 2 (The proper way to refer to it) or Bank 2 Sensor 2 (although not the proper way to refer to it this is sometimes used.)


BlazerLT is talking about a system that some of these Blazers are rumored to have. (I have yet to see one, but it has been mentioned on here several times) The system he is talking about would consist of 4 oxygen sensors (still single exhaust):

One sensor just below the driver's side manifold - Bank 1 Sensor 1
One sensor just below the passenger side manifold - Bank 2 Sensor 1
One sensor just before the cat - Bank 1 Sensor 2 aka Bank 2 Sensor 2
One sensor just after the cat - Bank 1 Sensor 3 aka Bank 2 Sensor 3

All that being said...I believe the rumor about some Blazers having 4 sensors is just that...a rumor. The only systems (to date) that I know of having 4 sensors would be those with dual exhaust. These systems consist of the following:

One sensor just below the driver's side manifold - Bank 1 Sensor 1
One sensor just after the driver's side cat - Bank 1 Sensor 2
One sensor just below the passenger's side manifold - Bank 2 Sensor 1
One sensor just after the passenger's side cat - Bank 2 Sensor 2


I should add... the references to which side of the engine is bank 1 and bank 2 is based on GM design.

Changed one o2 sensor and had the po155 code...is there a trick to resetting the check engine light...because it is still on....i guess if that doesnt work it means another sensor was out to?? not sure....thanks

You can reset the light by either clearing the code with a scan tool or disconnecting the battery for ten minutes.

Changed one o2 sensor and had the po155 code...is there a trick to resetting the check engine light...because it is still on....i guess if that doesnt work it means another sensor was out to?? not sure....thanks


humm thats not good news we disconnected the battery for a little while and the light didn't go out...must mean another sensor was out..changed the passenger side before and that must not have been the correct one.

The code is for the one just below the passenger side manifold. The code is actually for the circuit, so if the new sensor didn't help you need to inspect the wiring.

i guess that means another sensor was out..we had the battery off for a while and it didnt rest the check engine light...trying to get off cheap and do one doesnt look like it worked...darn it...

So after changing and disconnecting the battery the light was still on..this is the 02 xtreme...went to autozone and the guy is like it will take 50-100 miles to reset and i said even after taking the battery off and hes like you never disconnect your battery you will mess your computer up...so i just left...went to the other auto store and they said 200-350 miles for it to reset when the computer goes through its function...but then said the print out from those scanners might not be right...said that might not be the correct code that something else could be wrong..said taking the battery terminal off should reset it unless newer cars have a memory backup...so im not sure what the correct story is now...im sure hoping it will reset...

No, the computer does not take any miles to reset.

It take 250 miles to relearn everything.

If you have a check engine light still, you have a problem.

Try this to clear the computer.

Turn the key to the on position but do not start.

Remove the TWO ECM fuses in the fuse panel.

Turn key off.

Wait 30 minutes to be safe.

Insert fuses again.

Start and check to see if the light is gone.