Which Oxygen Sensor Is It?

There are many inquiries online about which oxygen sensor to change. Oxygen sensor failure codes are very common on a lot of vehicles. With all of today's vehicles having at least two oxygen sensors and many having three or four of them, it can be a little confusing as to which one is causing the problem.

Before we get into which sensor is which, we need to have a little discussion about oxygen sensor fault codes. There are several different types of oxygen sensor fault codes. Here are just some of the most common ones:

P0135 "Oxygen Sensor Heater Circuit Bank1 Sensor 1"
P0141 "Oxygen Sensor Heater Circuit Bank 1 Sensor 2"
P0147 "Oxygen Sensor Circuit Bank 1 Sensor 3"
P0152 "Oxygen Sensor Voltage High Bank 2 Sensor 1"
P0159 "Oxygen Sensor Slow Response Bank 2 Sensor 2"
P0171 "Oxygen Sensor Lean Sensor 1 Bank 1"
P0172 "Oxygen Sensor Lean Sensor 1 Bank 2"
P0174 "Oxygen Sensor Rich Sensor 1 Bank 1"
P0175 "Oxygen Sensor Rich Sensor 1 Bank 2"

There are many more possible oxygen sensor codes, but I only listed these to make my point. Many times the oxygen sensor code is NOT caused by the oxygen sensor itself. "Lean" or "Rich" oxygen sensor codes (i.e. P0171, P0174) are usually caused by something other than the oxygen sensor. Something is wrong, causing the engine to run lean (not enough fuel or too much air) or causing the engine to run rich (too much fuel or not enough air). In these cases, replacing the oxygen sensor will not fix a thing. (That is, unless you are trying to fix your bank account from having too high of a balance!) The new oxygen sensor will just set the same code as the original one. This is because the oxygen sensor is not CAUSING the problem, it is only REPORTING the problem.

High voltage codes (like P0152 above) can be caused by the oxygen sensor wires being shorted to another wire inside the wiring harness. Sometimes these codes are caused by bad grounds where some other component is trying to ground through the oxygen sensor circuit. Again, replacing the oxygen sensor will not fix this! In short, the problem needs to be diagnosed before running out and buying an oxygen sensor.

Just because a fault code has "Oxygen Sensor" or "O2 Sensor" or "O2S" in its description does not necessarily mean that an oxygen sensor needs to be replaced. Many do-it-yourselfers believe that all there is to fixing the car is to hook it to the "magic box", collect the fault codes and replace the parts the computer tells you to replace. There is nothing further from the truth.

Fault codes only point you toward which SYSTEM is failing. The system must be diagnosed to find the CAUSE of the failure. If this is not done properly, it will only result in wasting a bunch of your money. This is what you were trying to avoid by doing it yourself!

So, after reading all of the above, if you think you still want to replace an oxygen sensor, but don't know which one; here is how to figure it out:

Oxygen sensors are always numbered like this:

Bank 1 Sensor 1
Bank 2 Sensor 1
Bank 1 Sensor 2
Bank 2 Sensor 2

Some manufacturers use a kind of shorthand that reads different, but means the same thing:

Sensor 1/1 or O2s 1/1
Sensor 2/1 or O2s 2/1
Sensor 1/2 or O2s 1/2
Sensor 2/2 or O2s 2/2

Bank 1 is always the side of the engine where cylinder #1 is located and, of course, Bank 2 is the opposite side.
On a 4 cylinder engine, there is only one bank and it is always referred to as Bank 1. The exception to the 4 cylinder rule is on certain 4 cylinder engines (specifically, some Toyotas) there are two catalytic converters used. In this case, Bank 1 sensors will still be in the pipe for the catalyst that is connected to cylinder #1 and Bank 2 sensors will be in the other one.

Sensor 1 is always the "upstream" sensor (the one located BEFORE the catalytic converter).
Sensor 2 is always the "downstream" sensor (the one that is located AFTER the catalytic converter).
Sensor 3 refers to the ONLY "downstream" sensor where there are two sensors before the catalyst and only one after the catalyst. On very few vehicles the reference to this reads "Bank 1 Sensor 3".

If you do not know where cylinder #1 is, then you need to get a diagram of the firing order for your engine. Just post a question on FixYa.com and make sure you give the YEAR, MAKE, MODEL, and ENGINE SIZE of your vehicle and one or more of our experts will be happy to tell you how to find cylinder #1.

- DTTECH
ASE Certified Master Automobile Technician


Also check out this article by dttech: What Else Could Be Wrong?