Mix and Match Catalytic - Which Sensors?

[nickb21]

I did some searching, but wasn't really finding the information I wanted, so I'm looking for a few opinions. Hopefully I won't catch too much flak, especially with all the related posts going on lately.

 

Anyways; I picked up a new cat for my '02 OBW. It's nice, low mileage, and minimally rusty (wohoo NC). I cleaned it up and threw some high-temp paint on the flanges (the main reason I'm replacing my current one).

 

The cat is off of an '05 Impreza 2.5, the sensor bungs are in the same places as my current cat. My current gas mileage is 'normal', hasn't changed much over the years, I average 25-27 depending on trip. (1-2 less in winter).

 

My questions:

Do I bother replacing the A/F sensor (front O2)? I'm thinking yes, with 180k it might be a bit lazy by now. If so I'm thinking that I should get the A/F sensor that matches my car (OE of course) since that's all pre-cat. (Oddly enough there are a half-dozen sensors listed for 00-04 models.)

 

More importantly, I need to replace the rear O2, it is physically broken. It's a four wire unit. I was thinking of going with a Denso or Bosch 'direct-fit' piece. Any preference there? (I'll price an OE one as well). I'm also wondering if I should get a sensor spec'd for my car or for the Impreza. There are a bunch of cat part #'s listed between these models that are all the same physical dimensions, which makes me question sensor choice.

 

What are your thoughts?

 

[idosubaru]

Match sensors to the ECU - which hasn't changed, so 02 Legacy O2 sensors.

 

@180k they are tired and old.  If you're planning on another 100K it's worth it to do it now while it's all been apart and you'll probably incrementally recoupe the costs.  If you're limping it along or that cash is better kept in the pocket, then who cares as long as your gas mileage is good and no CEL.

 

Brand, I think Bosch is the generally preferred Subaru unit.  You can wire in or direct fit, either way is easy enough if you're comfortable doing some wire splicing, it's easy and done off the vehicle so nothing tricky about it.  I used to favor Subaru on newer cars I'm starting to see less reason for it on O2 sensors.

 

If the break is a wire, you could splice that back together too.

 

May be a bunch listed because there are H6 sensors listed which may be different.

[nickb21]

Awesome, thanks! Definitely a bit confusing even when you exclude the H6's, seems every year + auto/manual has a different listing for sensors and cats for the same physical size (2 different ones listed for MY02). Some of which is apparently the "U5" emissions stuff.

 

Good point, I do have the pigtail/wires for the rear O2, so I could solder/heat shrink 'em up. I'm back and forth on the A/F, looks like it'll be $100, I'll probably just bite the bullet. I've just started considering an upgrade to a newer OB, but would feel dumb for not replacing 1 sensor when it would be easy to do so.

 

Now to finish sourcing bolts, anyone know off-hand what size bolts go through the end of the y-pipe and attach the cat section? Guess I should do spring bolts too.

 

Thanks again!

[nickb21]

In case someone finds this thread. (from nasioc) Front flange (Y-pipe to front cat) bolts are:

 

 44015AC000 (2pcs) & 902370010 (2pcs)

Bolt is M10x40mm 1.25 pitch (guess length/pitch just have to be close, since it just goes into a matching nut on the Y flange)

 

Off to fastenal.

[Fairtax4me]

Before ordering sensors check the shape of the connectors on your current sensors and match the shape to the replacement sensors.

 

Pay close attention because the differences can be hard to spot right away. The different sensors you're seeing can be either plain O2 sensors or A/F Ratio sensors, (usually CA emission spec.) similar in shape and appearance, but they work totally differently. If your car needs an AFR sensor and you splice in a regular O2 sensor it will toss your fuel economy out the window. So check carefully to find out which type of sensor you need first.

[nickb21]

Thanks for the info! I had decided to just re-use the A/F sensor, but it was a major PITA to get out of the original cat. Apparently 11 years of NJ salt doesn't help. I had been spraying it with PB blaster for about 2-days, hit it with my MAPP torch, and used a 1/2" O2 socket with a 2' breaker bar...

 

Well here are the results, ripped some of the threads totally off.   It'll sort of start to thread into the new cat, but I imagine it's just gonna cross-thread the heck out of everything (if it even goes in).

 

Any way to fix this, or am I getting a new sensor now anyway?

 

Edited August 15, 2013 by nickb21

[Fairtax4me]

It looks like it might be fouled anyway. If the ash looking stuff on the tip is yellow-ish its silicone fouled and should just be replaced anyway.

I wouldn't risk ruining the new cat with it, the few threads that are left may be weakened enough that they could strip off and the sensor blows out of the cat.

[nickb21]

Yea, you're right, figured that is where I would end up. Ah well, at least my local OE parts source is reasonably priced.

 

Tip doesn't look yellow, at least to my eye, but I would hate to 1/2 rump roast this and have it blow out of the cat.

 

I'll post some more pics later of the project.

 

Thanks!

[nickb21]

Ok, updates. Most of the work is done. What. A. Day. I'll post pics and a write-up at some point, added a few things to the project which didn't help my timeline.

 

Currently only 1 major issue. I decided to pickup a new donut, the old one wasn't in awful shape, but I thought it made sense. I also picked up a spring-bolt kit. Problem: I have the spring bolts (NAPA/Walker 5192 or something like that) as tight as they will go, and there is still an exhaust leak around the donut. Donut is centered. It is a NAPA donut (no good?).

 

I'll measure in the morn, but there is prob 1/4" between the cat flange and the mid-pipe flange. Never ends!!

[ocei77]

When I encountered this problem (don't remember which Suby I was working on), I just left the springs out and tightened the nuts.

No more leak.

 

O.

[Fairtax4me]

No go on these style bolts. They have a shoulder that you can't tighten past.

This is a typical issue with the aftermarket spring bolts. I got a set from rock auto made by Bosal that worked fine, but their donut gasket was too large. Had to get a dealer gasket.

If you need bolts in a pinch go to the dealer, but expect to fork over some $$ for them.