O2 sensor: 4-wire better than 3-wire?

[Vanguard]

When looking at oxygensensors.com and some other online vendors for oxygen sensors (Walker and Bosch, both OEM and universal), all they list for my '92 Subaru Legacy non-turbo is a 3-wire unit. 2 wires would be for the heater element and 1 wire is the sense line. So they must rely on the case screwing into the exhaust and follow the exhaust back for the ground connection. Except the exhaust system is rusty so I'm not sure how good that ground path would be. I think in the 4-wire unit that one of them is the ground wire.

 

What would be a good method to attach a ground wire to the O2 sensor? Could I solder onto its casing, or would the heat needed to perform the soldering result in damaging the part? Or is grounding back through the exhaust system okay?

[frag]

I was asking myself the same question so I made a test. I connected one lead of my ohm meter to the sensor body and the other one to the engine ground. Zero ohm between the two so I concluded the exhaust system ground was OK.

[blitz]

Correct, the extra wire is a dedicated ground. That's a good thing for the reason you mention (rust).

 

You didn't specify which configuration you removed from the vehicle (how many leads on the connector plug?).

 

If the sensor that your taking off is a four-wire, you shouldn't have any problem finding a four-wire exact replacement.

[blitz]

Correct, the extra wire is a dedicated ground. That's a good thing for the reason you mention (rust).

 

You didn't specify which configuration you removed from the vehicle (how many leads on the connector plug?).

 

If the sensor that your taking off is a four-wire, you shouldn't have any problem finding a four-wire exact replacement.

[Vanguard]

I forgot to look at the wiring for the O2 sensor when I was last under the car (to secure the rattling heat shields). I took a look at the O2 sensor to see where it was, and it looks pretty easy to get at. When I went to online vendors, all of the ones that I checked only had 3-wire units for my year and model. The dealer wants $129 for their Subaru-branded O2 sensor, I can get a Bosch OEM for $89 or universal for $69, and a Walker universal for $46. I'll probably go with the Walker universal unless someone has some real evidence in difference in quality and compatibility. The local NAPA Auto Parts store carries the O2 sensor, universal only, for $69 but I don't know which brand (and I can get the Walker univeral for cheaper from oxygensensors.com even with the shipping).

 

I don't mind using the universal and slicing (and soldering) on the old connector. I'm very handy with a soldering gun/iron. I don't know if the O2 kit comes with a silicon "bandaid" to cover the splices to weatherproof them. If not, I'll have to get some high-temp silicon and heatshrink tubing to seal the splices.

 

I've heard that sometimes the wire coloring doesn't match. In case that happens (and the colors don't match those mentioned in the instructions, if any are included), what resistances should I be measuring across the heater wires and between them or the unit's casing to the sense wire (so to identify which wires are for what based on resistance)?

 

How long are O2 sensors supposed to last? My maintenance schedule says nothing about replacing it. When looking at the specs for the online vendors, I don't see any duration spec. I think it is the original O2 sensor in my car which has 130K on it now (but the Check Engine light has been coming on erratically probably since turning past 100K). I recall one user saying that they last only 30K, but if that were true that they were so short-lived then I would think they would listed for replacement in the maintenance schedule.

 

When I used the black test connector on the ECU to read the error codes, I got 22 (knock sensor), 32 (purge valve), and 35 (oxygen sensor). When I went through the test procedure for the green connectors, I got only 35 (oxygen sensor). So I'm first targeting the O2 sensor for replacement. I hear it comes out easier with a hot exhaust.

[Olnick]

O2 sensors often weaken or fail around 100k. Yes, they are easier to remove when hot.

 

When I did the 4-wire mod I got a Walker universal from oxygensensors.com, it was a Saturn sensor. Came with solder loaded crimping tubes and heat-shrink tubing. Had pretty thorough instruction sheet.

 

Do a search--topic was covered in great depth a while back, including the 4-wire sensor model number.

 

Good luck.

[Lawsonmh15]

I'm currently doing my Pre-Cat O2 sensor. It looks like you guys are WAY over paying for the universal O2 3-wire. I paid $20 for the brand new Bosch Universal at autozone. At that price, it wasn't hard for me to justify $80 to have it installed. Was going to do it myself, but requires exhaust (baked bolts/rust) removal. Not that patient.

 

MY QUESTION:

 

Is the pre-cat (3-wire) the ONLY O2 sensor that affects mileage/performance?

I thought I had read that the front was the only one that had affect, but when I talked to the dealer they said both were equally important. However, that was from a lady at the parts counter who admitted she wasn't a tech. TIA.

 

Lastly, I want to replace the post-cat, but am wondering if a 3 wire can be adapted to function the same. If so, what wire is left out? I'd imagine the ground. If this has been successfully done, please let me know, pics would be greatly appreciated.