Subaru Outback O2 Sensor - PO130

Driving along you look down at the dash board and notice that the little check engine light is on.  What a sure way to spoil your day.  Immediately dollars signs flash as you wonder how much this is going to cost you.  If you do the work yourself and are careful with your diagnostics, not very much.  In my case it cost 62 dollars and a 6 pack of beer.  Our Subi is a 2000 Legacy Outback 2.5 Auto with 150k.

A few weeks ago I had such an event occur.  Since the car is newer than 96, it has an OBD II (On Board Diagnostic) system.  I drive home and plug it into my code reader , and out pops a PO130 code.  Cross references an on-line manual which states that I have a Front Oxygen Sensor circuit malfunction.  From some on-line reading on Subaru Fourms, circuit malfunction often means a short in the circuit, usually caused by water in the plug connection. Well that makes sense as right before the code was thrown I was getting the family truckster sideways in an parking lot with 3 inches of  slush on the ground.  So I cleared the message and waited for the car to dry out.  One thing to mention is that when the check engine light came on, it was steady and did not blink.  This means that you have an error message you should get it checked out.  If it was flashing it means that you are doing damage to the car, stop driving. 

A few days later the car develops a stumble.  Press on the gas and it barely runs.  Pull over and take a look at the engine.  Yep it looks like an engine.  Rev it a few times and everything is back to normal.  The check engine light never came back.  Over the next couple of days this stumbling issue happens a few more time.  It feels like the engine isn't getting gas.  I check the basics around the engine.  Gas in the tank, air intake not obstructed, fuel injectors ticking, spark plugs sparking, spark plugs look good, O2 sensor plug dry and secure, all oil levels full, no blown fuses  Over the next few days it also has several more stumbling episodes, and it throws another PO130 code which I clear out.  The last few instances was really rough,  engine even made popping sounds typical of super lean (low fuel) conditions, like the sputtering right before you run out of gas.  Somehow I must not be getting gas. 

I got a fuel pressure gauge and "T" it into the fuel line just down stream of the fuel filter.  Before unhooking any fuel lines you want to bleed off the pressure to minimize the amount of fuel you spray.  Leaving the car over night usually does it.  But pulling the fuel pump fuse or relay and cranking the engine till it dies is much quicker.  Unknown to many is that behind the knee panel compartment is a fuse bank. I removed the whole panel, but all that is required to be removed is the little flip out compartment. Locate and remove the 15 amp fuse at the top left.

The greatest diagnostic tool ever made is the Internet.  So I hit all the usual Subi Forums asking for advice.  Fuel pump, fuel pressure regulator, mass air sensor, coolant temp sensor, transmission temp sensor, and bad gas were allsuggestion.  At this point I had a bad O2 sensor codes appear twice now.  Why didn't I just change out the O2 sensor.  Well I was under the impression that other codes told you you had a bad O2 sensor (sensor reading out of range), and if the circuit was really bad, it would constantly have the check engine light on, not occasionally (every150 miles).

Before I go any further let us talk about O2 sensors and what they do.  Your car needs a mixture of fuel and air to burn.  Back in the carburetor days the carburetor mixed the fuel and air.  Since carbs only mixes the fuel and air to one ratio without consideration for varing conditions, fuel injection was developed.  The amount of fuel the injectors spewed is based on the amount of pressure in the line and the duration the spray nozzles are open.  Input signals from the Mass Air Flow/Pressure sensor determines the duration of the spray.  The O2 sensors read how much fuel is left unburnt in the exhaust and fine tune the mixture.  When one of the sensors is not working it sends the fuel map into a safety mode, erroring on the side of burning more gas than is needed (rich).  Running rich is safer cause the car will still run and you do not run the risk of detonation (blowing up your motor with a too lean of a mixture-this is a whole other topic).  This is why it waste gas when you have a check engine light on.  This is also why I figured a known O2 sensor issue will simply cause the car the run rich and not generate such a shortage of fuel condition, and thus not a result of the PO130 error message. 

Often times cars have two O2 sensors or more.  This can add a bit of confusion, as some are desginated as bank one and two and others as front and back.   In some "V" configured engines an O2 sensor might be present to monitor the left bank of cyclinders and the right.  In newer cars there is often O2 sensors located before and after the catalitic converter, its purpose is to monitor the condition of the cataletic converter.  I assume that it is possible to have a combination of such sensors and have four seperate O2 sensors for your car.  Our Subi has two sensors to monitor the condition of the cats, and does not have sensors to monitor the left and right bank.

The problem with my stumbling problem was that it was intermittent.  This makes diagnosing the issue difficult as it would not do in the comfort of my garage, only on the open road.  If it was my fuel pump or fuel pressure regulator I wanted to confirm it before I replaced it.  Nothing worse than replacing a part only to find that it was fine and not related to the issue.  This I call the "machine gun" way of repair; fire a bunch of bullets and hope to hit the right thing.  A working fuel pump will generate a constant and adequate fuel pressure.  If the pump is dying I would be able to tell with a gauge. 

With the fuel pressure gauge hooked up I drove it around the neighborhood for 30 minutes and no issue.  Dang.  The next day I drove it to work and finally the car started to stumble.  The fuel pressure was higher than spec and steady.  The fuel pressure regulator's job is to lower the fuel pressure to the specified amount. A vacuum actuate diaphragm restricts the amount of fuel which passes.  This is also how the FPR varies its regulated pressure.  As you stomp on the gas, less vacuum is available in the intake manifold, thus reducing the amount of fuel pressure reduction.  To check the FPR, pull the vac line.  The pressure should rise.  There should also be no fuel dripping out of the vacuum port, as that would indicate a ruptured FPR diaphragm membrane. 

I was now at a loss?  Do I start checking other sensors like the forum members suggested.  While driving home from work the car developed a stumble again.  Pull over and the check engine light comes on and the stumble went away.   The code read PO130, and  I did not clear it. I then proceed to drive the car for 50+ miles with out a hiccup.  I think I finally figured it out.  PO130 means that the ECU has received a grossly out of range signal from the O2 sensor for a steady duration of time.  Though often it means something is awry with the circuit, it could also mean that the sensor is completely dead.  An error message is then generated and the ECU no longer looks to the sensor for fuel mixture tuning.  When the error code was deleted the ECU still considered the sensor input (though grossly wrong) to mix the fuel, and did so incorrectly, causing a stumble.  The car sensing that it was not running properly, reverted back to a preset safety fuel mixture. 

Ok now that I concluded the source of my error, I needed to do some actual repair.  First I sourced the part.  Bosch has been known to make quality OEM replacement parts.  Bosch oxygen sensors are often times specified by the auto manufacture and used as original equipment.  I found a direct replacement part at the local Napa Auto parts store for about $60.  There are other cheaper versions of the sensor which require you to cut and splice your original plug connector.  No thanks. 

Now to remove the sensor.  First locate the connector plug on the passenger side of the engine. It has a light grey colored safety clip on it. Remember how the clip is attached cause it is tricky to get back on correctly.  Undo the clip and pull apart the connector.  Undo the wire keeper tab that is located a bit further down.

Most O2 sensor are a standard size, 7/8".  However it is not something that you can remove using a standard socket.  Due to the wire you need to use a tool similar to a flared nut tool.  I guess for removing the old sensor you can simple cut the wire off and use a deep socket.  Using a open end wrench only grabs the hex in two points and easily rounds out the hex.  The idea is to clear the wire but to engage more than two of the hex points. 

Though the subi uses a boxer engine with two separate banks, it uses only one O2 sensor for it fuel mixture reading.  The sensor is located at the "Y" portion of the exhaust as the two header pipes meet.  Because of the location I was not able to get my O2 sensor socket in.  Maybe if I had a crows foot type socket I could wiggle it in place.  But I did not so it was time to drop the exhaust. 

Of course in order to drop the exhaust the first thing that needed to be done was to drop the skid plate.  Next I undid the six exhaust header nuts.  The exhaust comes down but not quite far enough. I needed to go to the next exhaust hanger and undo the bolt.  Once the bolt is removed the hanger has a lip which the exhaust mounting bracket hangs from.  Lift the exhaust up past the lip and to the side to fully release the exhaust.  Be sure to have something support the exhaust.  The flex joint located downstream is not capable of support to forward section of the exhaust.  There is also the wires from

 the rear O2 sensor that will snap if required to hold the weight of the front exhaust portion.  Gently wiggle the exhaust when you lower it onto your support as the O2 sensor and wire stick through a heat shield and gets hung up.  As you can see in the picture I supported the exhaust on the catalytic converter with a block of wood on a floor jack.

Once you get to this point the rest is easy.  Undo the bad front O2 sensor and replace. Once removed it was obvious that the O2 sensor was toast, as it rattled with a broken part inside.  In the picture you can see the small threaded hole, or bung, that the O2 sensor fit in right at the "Y" portion of the exhaust.

Put the car back together again,  clear the error code, and be happy once again.   This is the part where you drink the six pack of beer to celebrate.