P0156 is back


Veteran Poster
Sep 22, 2023
Richmond, VA
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Alright, some of you may remember that I had this code last spring. Bank 2, sensor 2, oxygen sensor rear/left.

Back then, all I did was reposition the connector which kept hitting the underbody. The code never came back (till now), and I eliminated an annoying rattle in the process.

Fast forward to today:


Looking at the connector, everything looks fine. I doubt I'll get away with any wire wiggling this time.

I went ahead and ordered two NTK sensors which are actually on sale for $35 a piece at Advance Auto Parts with free shipping. The wires on those sensors are stupidly long, though I guess front/right may need it.

Depending on my motivation when they get here, I'll either replace both fronts and move one front sensor back, or I'll only replace the offending sensor. I won't touch rear/right because that simply looks like a pain.

While I'm waiting for the sensors, I have two questions:

How do I get out the sensor? It seems seized. WD-40? Heat?

Why didn't this code come back sooner? I'm confused about that because this sensor always read differently than all the others. See below (top row):


This sensor also behaves differently when cold: it reads 1.275 V immediately after starting the cold engine; the three other sensors read 0 V until warmed up. Could that be some polarity issue?

And finally, the offending sensor looks different. Here it is:

Here are the other three.



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I use an open-end wrench to pop them loose, then unscrew them by the wires.
Replace the front two, and move them to the rear. Look at the bad one; it's probably covered with egr goo.
Go buy an O2 sensor socket, something like this

If it is rusted in there good, you are likely to round off the flange using just an open end wrench. As for a penetrant, wd40 is pretty weak sauce, and won’t do much. I use this stuff at the shop, it is expensive, but really effective, and a little bit goes a long way.
Heat will also help, but sometimes that can cause the threads to strip out, so be careful with that. Usually I will get it nice and hot, crack it loose, then let it cool down, spray it with the triflow, then slowly work it back and forth till it comes out.
Bank 2/sensor 2 finally out.

Someone remind me: why would it be better to replace both fronts + move those to back instead of just replacing this?

And anybody want to "read" this sensor?

It looks to be running a little rich. As for why not to replace just the downstream sensor, it could cause a catalyst efficiency code. Basically the sensor is always switching between rich and lean, and the way it tells that the cat is working is by looking for the downstream sensor to switch slower than the upstream. The problem is that as sensors get older, they slow down, so if you have a brand new fast sensor in the rear, and an old slow sensor in the front, the computer could interpret that as a failed cat.

Personally what I would do to start is switch both rear sensors to see if the code moves to the other side.
I'll add this into the mix. I know you have a V6 car so maybe it won't matter. But I have run into several of the V8 cars where the oxygen sensor wires get hot and basically melt together. We've had to pull the engine harness off and un wrap it to find the affected area and repair it. That said typically when this happens it causes multiple O2 sensor codes.
It is a good bet that they are original.
On the subject of swapping the fronts to the back...

Connector doesn't fit due to these teeth. New ones don't have teeth.

Should I just shave them off? Or is this an indication that the front ones are different?

The first sensor pictured is a bosch sensor. throw it away. Get two new ntk sensors for the front, move the other ones, and it will go away. Never use bosch o2's or spark plugs on our cars. They just don't work for long.
Only og ford o2's have the keyed plugs.
I started to mention how I do mine, but I realize, mine aren't in the factory locations exactly; they got moved on the new exhaust, to make them easier to deal with. And I added two more bungs... I keep them capped off, but I can add the wideband in either pipe.
A wideband o2 sensor is used when you're writing a tune; it makes it easier to tweak in. There's no reason to leave it in, if you're not changing anything, so I remove it between uses. The sensors are expensive, and our cars oil foul them, unless you've done valve stem seals.

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