Oxygen Sensors - Clarification please?

[Kalwren]

Hi folks,

I need some help with my oxygen sensors. My computer is throwing codes for sensors being bad, both banks.

I called Advance Auto parts and the guy asked me if it was the upstream or downstream sensor. I have no idea the difference between the two of them.

I'd like to replace my sensors as they're likely as old as the truck so it's probably time. My engine is running like crap.

Can you tell me:

1) Whats the difference between an upstream and downstream sensor?
2) Do I need upstream or downstream sensors when I purchase replacements?
3) Where are the sensors located on the engine / exhaust manifold? What do they look like?
4) Are there 2 sensors or 4 total? (2 each side??)
5) Do these need to be removed with a wrench, socket or specialized tool?

Thanks in advance. I'd appreciate some clarification. I've never changed an O2 sensor, so I'd like to gather some info before I change them on Friday.

 

[cr_the_machinist]

1) They all behave the same in a general sense. The only difference they may have if its a wide or narrow band sensor

2) If you pick up a good multimeter you can test the resistance of the O2 senors on your system.

3) There's one after each exhaust manifold and one after the cats

4) 3 total

5) Heat up the threaded collar on the exhaust pipe and I usually use a box end wrench to take out the sensor.

 

[fastpakr]

You can test the O2 sensor circuit pretty easily with any analog voltmeter. On your '94 there are only two O2 sensors, just the one on each bank. Downstream sensors appeared on '95+ models.

 

[Bob Ayers]

You can test the O2 sensor circuit pretty easily with any analog voltmeter. On your '94 there are only two O2 sensors, just the one on each bank. Downstream sensors appeared on '95+ models.

I agree, the O2 sensor is tested by watching the voltage swing around stoich.
You CANNOT test an O2 sensor by measuring resistance. An oscilloscope
works the best.....

 

[SoLongStang9499]

Upstream and downstream refer to if they are before or after the cat, upstream is before, downstream after. If your getting bank 1 and 2 codes, its the upstream sensors. Bank 1 is drivers side o2, bank 2 is passengers side o2. Good luck

 

[Bob Ayers]

Upstream and downstream refer to if they are before or after the cat, upstream is before, downstream after. If your getting bank 1 and 2 codes, its the upstream sensors. Bank 1 is drivers side o2, bank 2 is passengers side o2. Good luck

WRONG! BANK 1 is passenger's side, BANK 2 is driver's side.

If you are getting codes for O2 sensors in both banks (lean codes) then chances are you have another problem like a vacuum leak that is affecting both O2 sensors, instead of it being the O2 sensors.

 

[Donnie strickland]

Bob is correct, on both counts.

Kalwren, can you tell us exactly what codes you're getting? What's the number(s)?

And yes, you only have 2 sensors, both are upstream. One on each bank.

 

[Kalwren]

Super informative reply! Thank you thank you!!

Upstream and downstream refer to if they are before or after the cat, upstream is before, downstream after. If your getting bank 1 and 2 codes, its the upstream sensors. Bank 1 is drivers side o2, bank 2 is passengers side o2. Good luck

 

[Kalwren]

Thank you again for the clarification! I appreciate the help!

Yes, I'm getting codes for both banks being lean.

Yes, Its likely a vacuum leak - but I cannot find it.
I've poked around under my hood on all the vacuum lines I can find with a propane torch and the engine doesn't change it's idle.

It idles fine. When putting a load on it (starting from a stop) the engine stumbles. it never dies though. I shove in the clutch and the RPMs come back up and I limp it along until its rolling. Once it's rolling, it's not a problem.

WRONG! BANK 1 is passenger's side, BANK 2 is driver's side.

If you are getting codes for O2 sensors in both banks (lean codes) then chances are you have another problem like a vacuum leak that is affecting both O2 sensors, instead of it being the O2 sensors.

 

[SoLongStang9499]

A leaking intake manifold gasket can also throw lean codes on both banks. And dont tell me im wrong on that as well, i just replaced it 2 weeks ago for this exact same condition. If you exaust the vacuum leak idea, keep an eye on your coolant.

 

[Kalwren]

I had this problem two months ago and had the intake manifold gasket replaced, to the tune of $600. Ouch.

I've taken my truck back to the same mechanic twice to have him find the issue and he's unable to duplicate it. I'm able to duplicate the problem every time I drive the vehicle.

So now I'd rather just find and fix the problem myself. Paying the exuberant shop fees is something I'd like to avoid if it's fairly simple to fix.

I was fine with the intake manifold gasket replacement, I wouldn't want to have to take the top of my engine off.

A leaking intake manifold gasket can also throw lean codes on both banks. And dont tell me im wrong on that as well, i just replaced it 2 weeks ago for this exact same condition. If you exaust the vacuum leak idea, keep an eye on your coolant.

 

[Kalwren]

I'm getting codes 181 and 189.

181 - Adaptive Fuel rich limit reached at part throttle, bank #1
189 - Adaptive Fuel rich limit reached, bank #2

I've replaced the intake manifold gasket.
New Throttle Position Sensor
New Engine Coolant Temperature Sensor (this threw a code last week but not since it's been replaced)

Bob is correct, on both counts.

Kalwren, can you tell us exactly what codes you're getting? What's the number(s)?

And yes, you only have 2 sensors, both are upstream. One on each bank.

 

[Bob Ayers]

I'm getting codes 181 and 189.

181 - Adaptive Fuel rich limit reached at part throttle, bank #1
189 - Adaptive Fuel rich limit reached, bank #2

I've replaced the intake manifold gasket.
New Throttle Position Sensor
New Engine Coolant Temperature Sensor (this threw a code last week but not since it's been replaced)

Are you using a K&N air filter? If you are, clean the MAF sensor, go to an OEM paper air filter, and reset the PCM.

 

[Donnie strickland]

According to the shop manual, for either of these codes, you should check, in this order:

Fuel pressure.
Fuel injector.
Air intake.
Manifold Absolute Pressure (MAP) sensor.
Fuel injector electrical circuit.
Fuel filter.
Vacuum.
Air filter.

The oxygen sensors are not mentioned.

Just by chance, have the oxygen sensors been disconnected and reconnected lately? I wonder if perhaps their connectors might be crossed (i.e., the right-side connector is plugged into the left-side sensor, and vice versa). I have seen this occur before.

 

[Sasquatch_Ryda]

No MAP on a 4.0L