At 09:18 AM 10/21/02, Patrick McKiernan wrote:

>It seems like they all fail, eventually, but _why_ do O2 sensors fail?
>I've never done any type of research on how the O2 sensor actually works; beyond layman's terms. [My understanding is that the o2 sensor is a variable resistor that regulates a voltage within the 0 to 1 volt range. The resistance varies in accordance to the o2 levels in the exhaust. The newer style sensor is manufactured with a heating element that gets the sensor up to the ~600 degree exhaust temperature more quickly during engine warm-up. No need to explain how the voltage is used by the "brain".] Right?

Close - but the sensor is not a variable resistor. It actually generates a voltage between zero and one volt dc in response to the amount of free oxygen in the exhaust stream.

>Over time/use, what happens to these sensors? Do they fail due to heat cycling?
>Is the element permeated with carbon and/or other exhaust deposits?

Most common problem is apparently "poisoning", or the contamination of the sensor with trace material that prevents it from generating a voltage. Lead, boron, bismuth, and a host of other stuff (mostly metallics) will poison the sensor.

>When they fail, what are the "usual" symptoms?
>(one such symptom may be that gas mileage suffers...tough to tell when your WORM GEAR IS FRAG! :-)

The ECU runs the engine in either "open loop" or "closed loop" mode. When in open loop (upon cold start, full throttle, etc.) the ECU relies upon pre-programmed "maps", which are complex three-dimensional look-up tables relating fuel flow and/or ignition timing to sensor inputs, such as the signal from the mass air flow (MAF) sensor, the rpm sensor, etc.

In closed loop, the input from the oxygen sensor is used to "trim" or slightly adjust the values from the maps so as to control the air/fuel mix close to stoichiometric, or perfect combustion.

If there is no input form the oxygen sensor, the ECU stays in open loop mode. Fuel economy goes down, emissions go up. Drivability may or may not change much, depending upon how closely the maps match reality.

You can test the oxygen sensor by measuring the voltage output with a good digital voltmeter. In closed loop, the voltage should usually fluctuate between 0.4 and 0.7 vdc.

If your problem is a strong smell from the exhaust, suspect a bad plug wire or plug.

Wally Plumley
928 Specialists



A failed O2 sensor is a common cause of failing Smog. It is easy enough to check. Get a VOM (Volt meter). Pull the bottom board on the fuse panel - there are two Phillips screws holding it in place. Look towards the left side of the panel. There should be a green wire attached to a funny looking plug. It is distinctive because of size.

Disconnect the plug and insert the positive lead of the VOM into the portion of the wire coming up from the muffler - female end. Connect the negative lead from the VOM to ground. Set the meter to read 2 Volts DC. Start the car and let it run for a minute or two. You should read voltage varying between 0.8 volts to 1 volt. It is possible that the reading are low or you just never get a reading. If you do not read the correction voltages the O2 sensor is probably bad.

There is one other connection to check. The O2 sensor has three wires - the main sensor wire and two wires to power an internal heater. The heater is powered by 12 volts. Before replacing the O2 sensor check to be sure it is getting power. Beside the big sensor connector is a double connector for the power. Pull this one apart and measure the voltage coming from the fuse panel. It should be 12 volts. If it is not then you need to trace down the source or substitute a new ignition switched source.

The best bet for replacement is a universal O2 sensor from 928 International. I found 928 had a better price than the local supply houses.
Do not remove the wiring for the old O2 sensor. Instead cut them off at the O2 sensor in the muffler. The O2 sensor is screwed into the muffler. Replace the old sensor with the new universal one. The wires are Teflon coated to protect against the heat so they are difficult to strip. I used crimp connectors to make the connection, covered the crimps with shrink tubing and then tucked the connection point up under the heat shield.

Good luck and I hope this helps. With a bad O2 sensor the HC and Co would be up as well since the car will be running rich.

Dan the Pod Guy
Portia's Parts



I did this on my 944. Here's info for that:

Bosch doesn't make too many sensors, it's the wiring (length/plugs) that makes them different. I'll wager that the Bosch 3-wire generic or Ford part will work fine. At $40 worth a try.

The part numbers can all be cross-referenced through the Bosch site for that. (I don't have that link available.)



The Lambda sensor can be replaced by a universal model (3-wire)
PORSCHE 928 S4 GTS / 5.4
BJ: 86 -
Thread: M18 x 1,5
Operating voltage 0-1Volt
Resistance of the heater element 4-5 Ohm