Oxygen Sensor Mayhem 98 Legacy

[burmzie]

1998 Legacy GT

220,000km

 

 

Alrighty here we go.

 

So for about 6months my engine light has been on. YEah yeah I know. So I had the codes read and it told us that it was the fuel trim mech. I was told that it was the rear O2 sensor. I just ignored it. THen as I was pulling through a drivethru, the engine began to kick me in the rump roast. RPM's through the roof, then almost stalling. Luckily I was near my mechanics, and wrestled it over. He replaced the rear O2 sensor, and all was well for a day. Then the front sensor took me for a bucking bronco adventure, and back to the shop. Replace front O2 Sensor. All was well for about 3 days. Then the engine light came back to visit. Replced the rear O2 sensor again (under warranty). The light went on a brief vacation. But it came back. I have been so pissed about it that I just let it go. Last week my brilliant mechanic told me that he thinks that the cat is blown and that is why the O2 sensor doesn't work properly. THis week the car was stalling out and hesitating doing 100kph on the highway. Now there was alot of water on the road, so I figured that my mech had not sealed the O2 connection properly ( water electricity BAD) . Turns out it wasn't. Real problem is that the coupling coming out of the wiring harness at the engine is a wire plug. Someone has taken the two blue w/yellow stripe wires (which I assume are the heater wires) and spliced them together to have both running into a single wire. This has effectively rendered the other end (going into the O2 plug) a three wire connection. Problem O2 sensor is 4 wires. Question here (after all of that), would this lead to a O2 sensor failure? If you doubled these lines could they blow the sensor or am I in deeper trouble than I think. Thank you kindly to everyone that helps.

 

Lane:banghead:

[brus brother]

Are you using Suabru or aftermarket parts? This may explain some of the wirigging.

[burmzie]

Are you using Suabru or aftermarket parts? This may explain some of the wirigging.

 

I am using a OEM on the front and a replacement on the back. The problem seems to be that the actual wire leading from the interior, has been spliced together. The orginal looks to be a 4 plugger, they have just spliced the Blue/yellow into one wire, making it a three wire on the O2 end. O2 is a 4 wire.

 

Thanks

Lane

[Wayne Boncyk]

I'm not quite sure that I get what your config is, but let me take a stab at a comment anyway...

 

My understanding is that 4-wire systems have the heater circuit isolated from the O2 sensor body, but 3-wire systems use the body of the sensor connected thru the exhaust system as the circuit return for heater current. Bottom line - if what I say is true, any system rigged as you say probably has the 2nd heater wire cut - not jumpered together. If it were so that the two wires were joined to one, then I'd expect the sensor heater cct to be shorted out. That could/should cause all kinds of havoc with your ECM.

[XSKI712]

My 1999 legacy has given me that code twice (PO420 i believe). both times i've just had it reset and all has been well. My local subaru garage said its nothing to worry about, as they can be sensative to the smallest things (getting splashed with water, etc...). WHen i had the code read at auto zone, the guy who did it said something about putting a tank of premium in it, then going out and "opening up." I of course have no objection to this, but wonder what his reasoning for it might be?

 

-kevin

[skibumm100]

He works at AutoZone. He's just guessing. If you go back and say it didn't work, his next suggestion might be to stand on one leg, rub your tummy and hum the Star Spangled Banner.....then try it again.

[burmzie]

I'm not quite sure that I get what your config is, but let me take a stab at a comment anyway...

 

My understanding is that 4-wire systems have the heater circuit isolated from the O2 sensor body, but 3-wire systems use the body of the sensor connected thru the exhaust system as the circuit return for heater current. Bottom line - if what I say is true, any system rigged as you say probably has the 2nd heater wire cut - not jumpered together. If it were so that the two wires were joined to one, then I'd expect the sensor heater cct to be shorted out. That could/should cause all kinds of havoc with your ECM.

 

Thanks for the response. Basically they have spliced 2 wires together into one in the transfer harness that runs from the main wiring harness to the O2 harness. The harness that is the main harness has 4 wires. The end that plugs into the O2 sensor only has 3 wires. Within that wire is the splice. From what I see, it looks like the blue/yellow wires. Regardless, the dealer wants $80.00 for a three foot piece of wire. Don't want to buy it if this is not the problem.

 

Thanks again.

[burmzie]

My 1999 legacy has given me that code twice (PO420 i believe). both times i've just had it reset and all has been well. My local subaru garage said its nothing to worry about, as they can be sensative to the smallest things (getting splashed with water, etc...). WHen i had the code read at auto zone, the guy who did it said something about putting a tank of premium in it, then going out and "opening up." I of course have no objection to this, but wonder what his reasoning for it might be?

 

-kevin

 

Hey Kevin,

 

Thanks for the response. My mech told me the same thing until my car failed because of the sensors that were faulting. How's your mileage?? As my CEL was on, my mileage went to hell. 85km on 20 litres of fuel.( I am sure someone can do the conversion) It's like I am driving a Hummer. Watch it. These things turn nasty quickly then screw everything up, as I am finding out right now.

[SuperSubaru]

3-wire sensor = two heater, one signal, sensor body ground to exhaust

4-wire sensor = two heater, one signal, one harness ground (less noise, cleaner signal)

 

With the two heater wires spliced together pre-sensor, then essentially your heater would never heat up, your ECM would see very weird resistances in that circuit and could through things for a loop. The sensor itself is probably fine, it's how your ECM is reading both the signals that is probably causing havoc. If you can see the splice and you could probably fix it yourself, but pick up a manual and verify what the wires are supposed to go. Make sure you use a weather tight seal, otherwise you will probably end up doing it again.

[burmzie]

3-wire sensor = two heater, one signal, sensor body ground to exhaust

4-wire sensor = two heater, one signal, one harness ground (less noise, cleaner signal)

 

With the two heater wires spliced together pre-sensor, then essentially your heater would never heat up, your ECM would see very weird resistances in that circuit and could through things for a loop. The sensor itself is probably fine, it's how your ECM is reading both the signals that is probably causing havoc. If you can see the splice and you could probably fix it yourself, but pick up a manual and verify what the wires are supposed to go. Make sure you use a weather tight seal, otherwise you will probably end up doing it again.

 

Thanks for the response. Unfortunately I can't fix this one. They have broken the post off the sensor end. There are only 3 posts. It pisses me right off that someone has mucked around with such important wires. I really hope that this fixes problem. Does anyone think that this would effect anything else?? My mech told me that the rear cat needs to be replaced. He diagnosed it by tapping it, and getting a second opinion from another mech that came over and concured after tapping it again.

 

Thanks,

 

Lane

[nipper]

Thanks for the response. Unfortunately I can't fix this one. They have broken the post off the sensor end. There are only 3 posts. It pisses me right off that someone has mucked around with such important wires. I really hope that this fixes problem. Does anyone think that this would effect anything else?? My mech told me that the rear cat needs to be replaced. He diagnosed it by tapping it, and getting a second opinion from another mech that came over and concured after tapping it again.

 

Thanks,

 

Lane

 

go to a junkyard and cut the harness off another car, then SOLDER the connections into yours.

 

nipper

[XSKI712]

Hey Kevin,

 

Thanks for the response. My mech told me the same thing until my car failed because of the sensors that were faulting. How's your mileage?? As my CEL was on, my mileage went to hell. 85km on 20 litres of fuel.( I am sure someone can do the conversion) It's like I am driving a Hummer. Watch it. These things turn nasty quickly then screw everything up, as I am finding out right now.

 

I wish i could figure out my mileage. I forgot to reset the trip odometer after i filled up, so I have no clue. This was the first time the CEL came on for about 5 months, same code like I said. I'll let it go til it comes on again, then i'll get it checked out. And by the way, i didnt put too much stock into what the autozone guy said, no worries there.

 

-kevin