How many O2 sensors should you replace?

Must be a "no brainer" with no questions on this subject, so I'll break the ice with a ~ dumb one. Always replace all four?

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This post was auto-generated based upon a question asked on our tech article page here: Pelican Technical Article: Boxster Oxygen Sensor Replacement - 986 / 987

[Wayne 962]

It depends. The first two are the most crucial to proper operation of the car. The rear two are basically just there to check the operation of the catalytic converters. So, when asked this question, I typically tell people to be sure to replace the two front ones, and then the rear ones after that if you want to. But the front two are what measure the mixture of the engine and are most vital to the proper operation of the car. That said, they do wear out over time, and if you're taking the time to replace one or two, I think you might want to replace all of them "while you're in there."

- Wayne

[mikefocke]

I replaced one and drove that way for over a year before deciding to replace the other one but there was no real need for me to do it. Never did replace the 2 rearmost ones.

I replaced just the one that threw the code (typically the front passenger side seems to be the one to go first...threw code saying "aging" sensor).

[randy_k]

I as well just replaced the front ones. Pretty easy DIY. About an hour or so.
Randy

[pbkirby]

What is the recommendation on how many miles/years to replace the front o2 sensors ?

[JFP in PA]

First of all, you should not be changing any O2 sensors until they have been properly tested with a multi meter. If the sensors pass the electrical tests sequence outlined in the service manual, there is nothing wrong with them and they should not be changed. If one fails the tests, replace that one, period. We see far too many cars that have had O2 sensors and/or MAF's changed for no apparent reason, and quite often without solving any issues.

There is no rational reason I am aware of for changing sensors that are fine, you are wasting your money............

[The Glademister]

Originally Posted by JFP in PA

First of all, you should not be changing any O2 sensors until they have been properly tested with a multi meter. If the sensors pass the electrical tests sequence outlined in the service manual, there is nothing wrong with them and they should not be changed.

Care to share those pins and values with us?

[JFP in PA]

Fully covered in the OEM service manuals; pin configurations and values differ somewhat according to year and location (e.g.:first or second sensor, heated or not).

Do you have a specific model year and O2 sensor location in mind?

[The Glademister]

Originally Posted by JFP in PA

Do you have a specific model year and O2 sensor location in mind?

Yes, sir. 1999 Boxster (not S) front O2 sensors. We may be deleting the rear sensors or at least ignoring them. It's an autocross car and will be getting performance exhaust. Punching out the cats will certainly trigger a CEL won't it

[JFP in PA]

Originally Posted by The Glademister

Yes, sir. 1999 Boxster (not S) front O2 sensors. We may be deleting the rear sensors or at least ignoring them. It's an autocross car and will be getting performance exhaust. Punching out the cats will certainly trigger a CEL won't it

OK, test values to determine if a sensor and its harness are working:

For the wiring harness-
* Ignition switch on, connect a voltmeter between pins 3 and 4, you should see ~450mV
* Connect voltmeter to pin 3 and ground, you should see ~720 mV
* Start and run the engine for 3 min, connect the voltmeter between pins 1 and 2, you should see battery voltage.

If the harness passes these tests, it is fine.

Checking sensor function-
* Warm the engine, connect a PST II/PIWIS or Durametric unit (or an OBD II scan tool capable of reading sensor voltages), if the sensor shows voltage fluctuations between 100 and 800 mV, it is fine.

Checking the sensor heater-
* Unplug the sensor, with the meter set on ohms, connect sensor pins 1 and 2, you should see 1.8-2.5 Ohms at 68 degrees F.
* Connect pin 1 and the sensor body, you should see infinite resistance.
* Connect pin 2 and the sensor body, you should again see infinite resistance.

If the harness and sensor pass these tests, there is nothing wrong with it......

[turboflyer]

Originally Posted by Wayne at Pelican Parts

It depends. The first two are the most crucial to proper operation of the car. The rear two are basically just there to check the operation of the catalytic converters. So, when asked this question, I typically tell people to be sure to replace the two front ones, and then the rear ones after that if you want to. But the front two are what measure the mixture of the engine and are most vital to the proper operation of the car. That said, they do wear out over time, and if you're taking the time to replace one or two, I think you might want to replace all of them "while you're in there."

- Wayne

Wayne just a quick question. not directly related but i thought i read where you can space out the post cat o2 sensors to get a cleaner reading and no CEL. Are you up on that. I already replaced the front two thanks.

[qstoria]

When changing the primary o2 sensors, do I need to disconnect the battery before proceeding?

If not, will the ECU fault codes for failed or aging o2 sensors disappear once the new ones are in there and operating correctly or do I need to have the codes cleared via the Durametric or PST2 software?

I have my radio codes so if that is the only issue regarding disconnecting the battery to clear fault codes then that is fine by me.

Any input would be much appreciated as I am trying to save some $$ by doing the sensors myself as I just spend big cash on the LN IMS upgrade!!

[mikefocke]

Any code reader will clear the aging sensor CEL. Both times I needed/wanted to change one of mine, I had em changed ($49 each, I supply sensor) at a muffler shop for convenience me not having a lift and being cheap not wanting to pay for Porsche-specialized experience. Nothing special about the car being a Porsche associated with the changing of the sensors. I'm sure they didn't disconnect the battery, they wouldn't even have known where to find it. 15 minutes work but about a half hour elapsed because you have to let the cats cool down after driving onto the lift. A fan helps.

Get the Bosch ones (see my web page for part numbers) with pigtails of the correct length. You can't solder em and you can't twist em on as well as the factory.