O2 sensor question: which is before cat, which after

[j_Zer0]

I need to replace one of my O2 sensors, and it looks like they are different. One is the before cat sensor and the other is the after cat sensor. I need to replace the one closer to the driver's side (the one with the grey clip). Turns out there is a big difference in price. Wow.

Can anyone say for certain which I need to get: before cat or after cat? Thanks a lot.

[PantsDants]

The one before the cat is actually an AFR sensor and costs a lot more. The one after is a standard O2 and costs less. I believe pre-cat is the gray plug and post-cat is black.

[tomaszjanczak]

PantsDants is correct. Before cat is gray, after cat is black. Buy them at RockAuto.com, much cheaper than at the dealer but they are still genuine denso products. When I bought them a few months ago they were both $90, now it seems they raised the price of the pre-cat to $101. It's easy to find a 5% off code online for rockauto as well.

[mad_sb]

Originally Posted by j_Zer0

I need to replace one of my O2 sensors, and it looks like they are different. One is the before cat sensor and the other is the after cat sensor. I need to replace the one closer to the driver's side (the one with the grey clip). Turns out there is a big difference in price. Wow.

Can anyone say for certain which I need to get: before cat or after cat? Thanks a lot.

How do you know you need to replace a sensor? I am asking because typically you already know which you need to replace by the time you make the call that you need to replace one.

Bank 1 sensor 1 is up stream (pre cat)
Bank 1 sensor 2 is down stream (post cat)

Please explain why you think you need to replace a sensor and perhaps we can help.

[j_Zer0]

Originally Posted by mad_sb

How do you know you need to replace a sensor? I am asking because typically you already know which you need to replace by the time you make the call that you need to replace one.

Bank 1 sensor 1 is up stream (pre cat)
Bank 1 sensor 2 is down stream (post cat)

Please explain why you think you need to replace a sensor and perhaps we can help.

Thanks for the offer. I installed a header and noticed I had terribly high LTFTs (19% at idle). After receiving a suggestion to check the seals on my gaskets and O2 sensors in another thread, I discovered that one of my O2 sensors was indeed not screwed in all the way. When I pulled it out to investigate, I found that about half the threads were basically destroyed. I'm assuming this happened when I unscrewed it from the stock manifold. I don't think it's fixable, but a pic is below.

[mad_sb]

Ahh, gotcha. I always let the exhaust cool completely before removing a sensor, and then give it a squirt of liquid wrench or pbblaster before cracking it loose. Putting them back in, i always put a dab of copper anti seize on the threads.


Now, the sensor you pulled out with damaged threads... was it the closest to the front of the car or the closest to the back of the car? Front is the primary or bank1 sensor1 or pre cat. Closest to the rear is bank1 sensor 2 or post cat.

Make sure to clean the threads on the manifold before installing the new sensor.

[j_Zer0]

Originally Posted by mad_sb

Ahh, gotcha. I always let the exhaust cool completely before removing a sensor, and then give it a squirt of liquid wrench or pbblaster before cracking it loose. Putting them back in, i always put a dab of copper anti seize on the threads.


Now, the sensor you pulled out with damaged threads... was it the closest to the front of the car or the closest to the back of the car? Front is the primary or bank1 sensor1 or pre cat. Closest to the rear is bank1 sensor 2 or post cat.

Make sure to clean the threads on the manifold before installing the new sensor.

It's the one closer to the front, so it's got to be the upstream like you and the others said. I think I pretty much did everything right: let things cool, used penetrating oil, used an O2 sensor socket, etc. Just seems more delicate than I thought and maybe I pulled at a bad angle when getting it out of the stock manifold....dunno. I'll definitely clean the threads (maybe use a thread chaser + anti-seize) on the header because I can see some metal shavings on them.

[PantsDants]

FWIW, I've heard penetrating oil can ruin O2 sensors, I've not experienced that though. Also, it's a pretty common problem that the sensors are cross-threaded in the exhaust manifold from the factory, so it probably wasn't your fault.

I had a similar problem with an aftermarket header and ended up over-tightening the pre-cat sensor and ruined the threads. I scored a used replacement on eBay and ran a thread chaser through the header bungs. If your threads are as bad as mine were, you're in for a bit of a workout: Use liberal amounts of anti-seize, get the longest wrench and breaker bar you can find, and get cranking! Screw it down about a quarter turn, back it off an eighth, repeat a few times, unscrew completely and apply more anti-seize, etc. Be careful as you go because the thread chaser can get really hot. You should be able to easily hand tighten the sensor and torque it to spec when you're done. Good luck!

[Twinning]

Can anyone speak to the best approach to replacing the upstream O2 sensor?

Is the BRZ like the WRX where I can access it through the wheel well?

basically I'm looking for the most effective, easiest way to get to the damn thing lol.

[wparsons]

Originally Posted by Twinning

Can anyone speak to the best approach to replacing the upstream O2 sensor?

Is the BRZ like the WRX where I can access it through the wheel well?

basically I'm looking for the most effective, easiest way to get to the damn thing lol.

You can see the plug for the upstream one if you look under the air box. It's close to the middle of the car, so not sure going in through the wheel well is the best approach.