need some ammo for my mechanic

[iayers]

99 OBW MT, 130k. Just had a local reputable Subaru mechanic replace my clutch, replace a leaking oil seperator plate, cut off the vibrating heat shields, and do the rear main seal. He appears to have done these tasks successfully, but:

 

When I picked up the car from the service, the mechanic said that my Check Engine Light was on, and that I need a new O2 sensor. The CEL was NOT lit before I dropped off the car, 100% for sure. I had Advance Auto pull the OBDII code: P0135, O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1).

 

Here's what I've done based on reading previous posts... Monitored gas mileage for about 500 miles, it is 27 mpg, pretty good. I've unplugged the battery to reset the codes, but the CEL relights as soon as the car is turned on. At highway speeds, sometimes the CEL flashes for a short periods of <10 sec.

 

For you all: What could the mechanic have screwed up? What can I do myself to check for errors? Is there a wiring harness or something I can check for continuity? Could cutting off the heat shields damage some nearby wiring or something? Do these sensors go bad at my mileage?

 

Any insight greatly appreciated... the car is overdue for inspection so I gotta fix this asap.

[RallyKeith]

Well, to replace the clutch the mechanic either had to move the motor or the trans. If he just moved the motor front, he had to un-bolt the exhaust from the motor, and most likely un-plug the front O2 sensor. If he pulled the trans, then he most likely dropped the entire exhaust and thus un-plugged both. In both cases there is a chance that the plugs/wiring could have been messed up. If the exhaust was removed completely then anything is fair game. It could have been dropped on the floor causing shock damage, or it could have been kicked/ stepped on while on the ground. Hard to say, but there is some chance the mechanic could have create the problem.

 

I'd suggest to the mechanic that they may have caused it in their work, and see if they will replace it under good will and hpefully only charge you for the part itself.

 

Keith

[ferret]

This is an Internal Heater in the O2 sensor. Unless they damaged the wire at the connector end, I would chalk it up to coincidence. Have the connector checked both on the vehicle harness end and the sensor end. I doubt that they removed the sensor from the CatCon during their service.

 

My 02 Forester had this failure (Heater element open) twice on the front O2 sensor and once on the rear one. These were 3 of the 7 times those sensors were replaced in the 62K I owned that car, and one of the many reasons I replaced it after the 60K warrenty ran out.

[iayers]

Where is the front O2 sensor? Is it something that I could inspect the wiring if I could get under the car?

 

Is it easy to access/replace the front O2 sensor myself if I bought one? Is there a notable difference in quality among the Subaru OEM model and other brands like Bosch, AC Delco, Walker... ?

 

Thanks to RallyKeith

[RallyKeith]

If you open the hood and stand along the passanger's fender the wire and the plug for the front O2 sensors runs along the engine/transmission where they mate. My 96 Legacy has a square like gray plug. The actual sensor is located just behind where the half shaft comes out of the transmission on the passangers side. It is relatively easy to replace, but exhausts are known to get rusty and sometimes the sensors don't come out easily. It is however acessable from under the car.

 

Keith

[idosubaru]

i'd begin by asking nicely knowing that they definitley removed and disconnected this joker, it's impossible not to when working on cars. but realize that mechanics aren't necessarily responsible for old-warn out parts even if they damage them upon removal. if they broke a rusted bolt or part when removing then it's partially the condition of the bolt and not so much their part. so in that sense the O2 sensor could have been more sensitive than a new one and gotten damaged during normal repairs which i wouldn't necessarily call their fault. a nice move on their part would be to inspect it for you for free. should only take them a minute to determine if it's wiring, harness or sensor related.

 

check the O2 sensor wiring from the sensor all the way up as far as you can follow it. since it is located so low and runs up the motor it is possible for the insulation to be exposed and shorting against something metal as it snakes up the back of the motor. seen this once.

 

who knows maybe they just forgot to plug the wiring harness back in? i know for a fact that older subaru's run just fine without any O2 sensor connected all. when dropping the exhaust it's common to forget about the O2 sensor and have it rip the wiring out when you pull the exhaust out. so i'd check that too.

 

let us know how they respond, situations like this let you know what kind of people you're dealing with.

[Meeky Moose]

when i did my engine swap i forgot to plug mine back in, then i couldn't find the damn wire until igot under it and found it wedged down in the steering rack, lol..

 

at any rate, i bet he just forgot to plug it back in.

[gbrand]

Yeah, the fault indicates the heater ciruit is open, such as if the plug was left off, or shorted, unlikely but wires could have gotten pinched.

 

Your sensor probabaly has 3 wires, 2 for the heater and one for the signal. If you unplug the sensor and measure the 2 heater wires(on the sensor side) with an ohmeter, it should read about 6 to 12 Ohms or so. If so then your heater sensor is fine and look at the wiring for damage. My guess is as mentioned previously the sensor was just not plugged back in.

 

Even if it is pure coincidencal problem, since you just spent a decent chunk of change with this guy he should cut you slack on the repair.

[RallyKeith]

I just realized that the wire is supposed to be routed through a little metal hold back that is attached to the bottom bolt of the transmissoin. Since the wire runs up right by the half shaft the wire needs to be held back through this thingy or it could rub against the half shaft and wear through.

 

Keith

[richierich]

If you pull of the exhaust without disconnecting the wire, it will pull out of the socket. If you resert it in there is a possiblity that he got the wires in the wrong spot, or one of them is not push in correctly.

[iayers]

Wow, so, with all of your help, I resolved the problem, and saved myself a bunch of money.

 

It turns out that the mechanic must have not unplugged the front O2 sensor when he dropped the exhaust out. Richierich hit it on the head. While the mechanic re-plugged in the sensor's wiring harness after doing the job, he didn't realize that he damaged the harness. One of the wires had pulled out of the wiring harness, creating the open heater circuit fault. I just jammed that mofo back in.

 

A simple fix, but all of your posts really made it easy. The mechanic apologized when I told him that I fixed it myself... he offered me a free oil change... but that doesn't add up to the $200 he was certain I needed to pay to get my O2 sensor to work! I think this should have been an obvious fix for a mechanic who's worked with these cars before...

 

Had Advance Auto Parts clear the CEL codes, and I got the car inspected yesterday. Anyway, thanks a lot everyone- I think I oughta donate some $ to this board.