Oxygen (O2) Sensors

What do they do?

Brett posted on PPBB:

The O2 sensors do just that, sense the amount of O2 in the exhaust gas relative to the amount of O2 in ambient air. Perfect combustion of a perfect mixture of air and fuel (around 14.7/1 air/fuel ratio) leaves behind only CO2 and water as products of combustion. All the oxygen gets consumed in the combustion and combines with all the carbons and hydrogens. If there is not enough fuel (lean mixture), then all the fuel gets burned leaving some oxygen left over. Conversely, if there is too much fuel (rich mixture), then all the oxygen gets burned leaving behind extra hydrocarbons (fuel). Now an oxygen sensor outputs a voltage between 0 and about 1 V depending on the difference between the amount of oxygen in the exhaust and the amount of oxygen in normal air. If there is a lot of oxygen in the exhaust (lean mixture condition), the sensor outputs close to 0 volts. Conversely, if there is no oxygen in the mixture (rich condition), then the output is close to 1 V. These O2 sensor voltages are read by the computer. This is the feedback loop that tells the computer how the engine is performing with regard to air/fuel mixture. It's impossible for the computer to hold the exact perfect air/fuel mixture constantly, so the way mixture control is designed is for the computer to continually adjust the mixture from very slightly rich to very slightly lean and back again using feedback from the pre-cat O2 sensors. This means that the pre-cat O2 sensor signal will oscillate back and forth from high to low to high to low voltage as the computer adjusts the mixture. In a normal running engine at idle the signal goes from low to high voltage and vice versa about every 1 second, with a transit time from low to high (or vice versa) being about 200-300 milliseconds. This transit time is important because as an O2 sensor ages, the transit time gets longer, and eventually it can get too long such that the computer will call it a malfunction and signal a check engine light and fault code for a slow responding O2 sensor. O2 sensors need to respond to mixture changes quickly so that the computer can keep up with the proper mixture adjustments.

So the bottom line is that the pre-cat O2 sensors should oscillate between about 0.2 to 0.8 volts regularly (about every 1 second at idle) in a healthy engine.

The post-cat O2 sensors are identical to the pre-cat O2 sensors (same voltage outputs). They are there only to monitor the performance of the catalytic converters. So, as discussed, the pre-cat sensor signals are oscillating between 0.2-0.8 volts. Once the exhaust gasses pass through the catalytic converter, most (all, in theory) excess fuel (hydrocarbons) will be combusted thus reducing hydrocarbon emissions. The cat uses oxygen in the exhaust to combust the fuel. So what you end up with in the exhaust after passing through the cat is a gas mixture that is reduced in hydrocarbons and reduced in oxygen relative to the mixture entering the cat. The post-cat exhaust gas mixture should be CONSTANTLY low in oxygen if the cat is doing its job of burning excess fuel. Therefore, the post-cat O2 sensor signal should be a constant lower voltage reading (not oscillating). So, if the post-cat O2 sensor is seen to oscillate just like the pre-cat O2 sensor, that means that the post-cat sensor is seeing the same gas mixture as the pre-cat sensor meaning that the catalytic converter isn't doing its job of burning excess fuel. The computer monitors the post-cat sensor and compares it to the pre-cat sensor. If the signals are similar, it assumes the cat is bad.

What they look like after 50k miles

The arrow below shows where you wedge a screwdriver to pry apart the rectangular kind. Note the black wire is on the side of the clip.

There are two types of connectors, oval in the early versions and rectangular in the later versions.

Now here is a picture of the rearmost sensor with the screwdriver inserted.

You do this to the one side and slide out the locking clip (not removing it).

Part Numbers from the Porsche PET

1997 986 Boxster 2.5L


1997-1999 986 Boxster 2.5L


2000-2002 986 Boxster 2.7L or 3.2L

986-606-127-01 2000-2002 in front of the starter catalytic-converter

986-606-128-01 2000-2002 after the starter catalytic-converter

2003-2004 986 Boxster 2.7L or 3.2L

986-606-227-00 2003-2004 in front of the starter catalytic-converter

986-606-228-00 2003-2004 after the starter catalytic-converter

2003-2004 986 Boxster 2.7L or 3.2L

986-606-226-00 2003-2004 no detail on where used

It is odd that the model year breakpoints don’t quite match up in terms of a part number from Porsche equal to the same part from Bosch after 2003 and that there are 3 part numbers listed on page (Illustration) 202-05 of the PET for 03 on. Anyone who can shed some light, please email me.

If I had a 2003 or 2004 Boxster, I'd remove the O2 sensor in question, look for the Bosche part number stenciled on the part and order that as these parts are frequently not returnable.

Part Numbers from Bosch

1997-1999 986 Boxster 2.5L

13806 Oxygen Sensor - OE Type Before Catalyst

13806 Oxygen Sensor - Universal Type After Catalyst

15738 Oxygen Sensor - Universal Type Before Catalyst

15738 Oxygen Sensor - Universal Type After Catalyst

2000-2003 986 Boxster 2.7L or 3.2L

15182 Oxygen Sensor - OE Type Before Catalyst

15183 Oxygen Sensor - OE Type After Catalyst

15738 Oxygen Sensor - Universal Type Before Catalyst

15738 Oxygen Sensor - Universal Type After Catalyst

2004 986 Boxster 2.7L or 3.2 L

16506 Oxygen Sensor - OE Type Before Catalyst

15506 Oxygen Sensor - OE Type After Catalyst

15733 Oxygen Sensor - Universal Type Before Catalyst

15733 Oxygen Sensor - Universal Type After Catalyst

See my comment above on the Porsche part numbers for model years 2003 and 2004. I would remove the old part temporarily and look at the part number on the old part before ordering.

The difference is that the Universal parts do not contain the already fitted wiring harness and you must use the one from your existing and crimp attach (DO NOT SOLDER) it to the new sensor and make the connection watertight. The difference between the before and the after is generally the length of the wiring harness. Better to get the OEM (Original Equipment Manufacturer...the company that makes the part originally and sells it to Porsche who then put their part number on it and stock it as a Porsche part) type made specifically for Porsche. But there is no need to pay Porsche prices. Buy the Bosch ones online. I would stay away from Walker or other generic sensors marketed as working with the Porsche. You can verify the part # at Bosch's site.

Since I wanted the exact part Porsche used but without the Porsche markup, I bought the Bosch part from AuohausAZ for $143.16 delivered, when the list on the part was $261.

Even Cheaper But More Work/Risk

On the 986boxster forum, there is talk of using a USOS-4000 part from Speedy Auto Parts and reusing the existing pigtail by crimping it (DO NOT SOLDER) it to the sensor and then using heat-shrink tubing to cover the splice and make it waterproof. This part goes for less than $40 each! Or from Parts Train who has better shipping prices. Or at Auto-Parts-Warehouse for $26.

Seems like too much work and risk to me. IMHO, YMMV

Change 2 or 4?

When you get a “Tired Sensor” code on the OBD2 reader, it is telling you that the sensor is not reacting as fast as it should. Therefore the engine computer isn’t benefiting from the feedback from the sensor and is probably making bad decisions based on outdated information. The result is a waste of gas (poor mileage) and/or a dumping of the wrong mixture into the catalytic converters which can cause them to go bad (big $$$).

Some mechanics will try and tell you to replace the 2 matching or even all 4. You don’t need to do that. They are supposed to last 100K miles...some do, some don't. The computer will tell you via codes when they are starting to fail. But that means you have to occasionally read the codes as the "Tired Sensor" code doesn't cause a check engine light. Perhaps the thing to do is be sensitive to the way the engine runs at startup and, at any abnormality (stumbling. rough running) check the codes with a code reader. That way you can get the sensor replaced before it goes totally bad and causes $$$ damage to the cats. My pre-cat went "tired" at about 52k, I replaced the other pre-cat sensor a few thousand miles later just for the sake of balance in the signals...both newish. I'm still running on the original post-cat sensors.

Covered by Warranty?

Read your warranty as it varies by state. For example, in California the answer might be yes while in Virginia the answer to the same set of facts as to in-service date and mileage might be no.

Change them yourself?

If you are young and supple or have access to a lift, absolutely. This is a trivial mechanical job.

If not, see your Porsche mechanic. He should charge no more than 2 hours (book rate times whatever his hourly rate is plus whatever fees he tacks on for waste disposal, etc) to diagnose, replace the O2 sensor (most of that waiting for the cat to cool), reset and test drive.

Or take the part you saved on by buying the Bosch part by mail order to your local muffler shop and negotiate a half hour labor charge. They have the lift, and the tools, there is no special knowledge needed. I know there book says more but you've done the diagnosis and have the part in hand so it is a simple put the car on the lift, remove, replace, lower the car process.

The actual replacement takes perhaps 5 minutes. There should be some time spent rechecking after a short drive to make sure the code is now still clear and the new sensor isn't defective. But the codes that indicate sensor failure are so explicit that replacement usually solves the problem. I've gotten the replacement only done for as little as a half hour labor charge when I didn't have access to a lift to do it myself.

Here is where they are located on some aftermarket headers/catalysts and on some stock ones.

They are the holes in the top (aftermarket) set, the sensors are the things with cords attached on the bottom (stock) set. This picture is from a later Boxster, one with 2 sets of sensors.

The connection looks like this. To find it, trace the blue wire back from the sensor. To disconnect the plug from the socket, you press down and in on the center section and then pull.

These pictures courtesy of shumtoby.

The sensors are installed with a torque of 55Nm/41ftlbs using a short handled torque wrench. I installed mine tightly without worrying about the torque. Purists would object.

Which Side is Bank 1? 2?

The code will tell you which side and before or after. "Before" means the sensor closer to the engine; "after" means the sensor nearest the muffler.

This picture shows the engine as you would see it with the access cover behind the seats removed. Left on this picture is the passenger side on a US car. Cylinders 1,2,3 (Bank 1) are on the passenger side of a US Spec car. Drivers side in a UK or AU car.

The error codes involved with the O2 sensors (for a 1997-1999 DME 5.2 car) look like this. There are separate lists of codes for '97-'99, '00-'02 and '03-'04 cars due to the differing Drive Management Electronics (DME). Rather than list them all here, I will provide this link to www.renntech.com's Boxster forum. Once there, select 'Quick Menu", then select "OBD P-Codes", accept the disclaimer that you won't post the whole list, then select the DME codes for your year Boxster (986) '97-'99 DME 5.2, '00-'02 DME 7.2 or '03-'04 DME 7.8. Basically, they look like this.

Tools Needed?

Do not remove the O2 sensor with an adjustable wrench. The darn things have heat cycled so many times they are a tough removal and you need a wrench that makes contact on all six sides or you risk rounding the nut which makes removal painful and expensive. There are also special sockets and wrenches sold that have a cut-out that allows the wires to fit through a slot while the socket turns and which allow the use of a torque wrench. You need a 22 mm metric. The special tools cost from $15 to $30.

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