The oxygen sensor (also called an O2 sensor) is one of the most important elements of the modern fuel injection systems. A finely tuned fuel injection system with an oxygen sensor can maintain an air/fuel ratio within a close tolerance of .02 percent. Keeping the engine at the stoichiometric level (14.6:1 air/fuel ratio) helps the engine generate the most power with the least amount of emissions.
The oxygen sensors are located in the exhaust system of the engine, and they sense the oxygen content of the exhaust gases. There are a total of four on the Carrera--two for each catalytic converter on the car. The sensor located just in front of the catalytic converter measures the mixture of the exhaust gasses exiting the engine. The sensor located after the catalytic converter is used to measure the performance of the converter by comparing the O2 levels before and after. The amount of oxygen in the exhaust varies according to the air/fuel ratio of the fuel injection system. The oxygen sensor produces a small voltage signal that is interpreted by the electronic control unit (ECU) of the fuel injection system. The ECU makes constant adjustments in fuel delivery according to the signal generated by the oxygen sensor in order to maintain the optimum air/fuel ratio.
There are a few signs that your oxygen sensor may be failing. In general, it is difficult to diagnose problems with the sensor, unless all of the other components in the fuel injection system have been checked and determined to be operating correctly. Some of the symptoms of a failed oxygen sensor system are:
- Irregular idle during warm-up
- Irregular idle with warm engine
- Engine will not accelerate and backfires
- Poor engine performance
- Fuel consumption is high
- Driving performance is weak
- CO concentration at idle is too high or too low
- Check engine light is illuminated
In general, if the oxygen sensor is not working, the car will be running very poorly and will also be outputting a lot of harmful emissions. If the signal received by the computer is out of its normal range, the 911 Carrera's computer will almost always give a warning signal that lights up the check engine lamp. Sometimes the computer may output an error code stating that the oxygen sensor is reading out of range, when in reality the values are out of range because there is something else wrong with the fuel injection system. Prior to replacing the oxygen sensors, make sure there are no other codes being recorded that may affect the O2 sensor readings. For more info on reading these fuel injection codes, see Pelican Technical Article: Reading Porsche 911 Carrera Fuel Injection Fault Codes.
If you disconnect the oxygen sensor and ground it to the chassis, the ECU will think that the car is running lean (not enough fuel), and will try to richen the mixture. At the other extreme, if you disconnect the oxygen sensor, and replace it with a small AA battery that supplies 1.5 volts, the ECU will think that the car is running really rich and attempt to adjust the mixture to be leaner.
Needless to say, troubleshooting the complete fuel injection system is beyond this project's scope. If you think that the oxygen sensors may be causing some of your fuel injection problems, they should be replaced. In general, I recommend that you do this every 30,000 miles. You have to jack up the car to gain access to the sensor (see Pelican Technical Article: Jacking Up Your Porsche 911 Carrera).
Using a 22mm wrench, simply remove the sensor from the exhaust pipe. On the Carrera, the sensors are very easy to reach. On many other cars, you would need a special deep socket with a slit cut in the side to remove it. The electrical plug for the O2 sensor simply unplugs from the chassis plug. New O2 sensors should have the same exact plug--ready to attach to your car. When you remove the O2 sensor, you will probably find that it is coated with black soot. This is normal for an old, worn-out O2 sensor. On our project car here, the O2 sensor was covered in motor oil and coolant. This is a bad sign that corresponded with the seized engine in my project car (I bought it that way). See Project 13 for more details on the problems sometimes found with these late-model Porsche engines.
Install your new sensor snug-tight, or if you have the proper slit-tool and a handy torque wrench, then tighten it to 40 ft-lbs (55 N-m). It's also a smart idea to add some anti-seize compound to the threads of the plug before you install it, but make sure the anti-seize doesn't get into any of the slits on the head of the sensor. Check the sensor first though, as new ones sometimes come with anti-seize already on the threads.
There are two different types of sensors you can purchase--generic ones that allow you to snip the connector off of the old sensor and put it on the new one, and original OEM sensors with the correct connector. On older cars, I used to use the generic sensors, but I've had problems with using them on these newer cars. Researching further, I discovered that the wires and connectors are very important on these O2 sensors. The Porsche factory workshop manuals state the following:
"911 Carrera 4 has a new, water-proof oxygen sensor. Water-proof means that the upper sensor section and housing are connected leak-proof with a later welded seam and previous reference air openings are omitted. Reference air is now taken via the connecting lead and plug connection. For this reason it is important to keep contact solutions, lubricants, liquids or similar products out of the 3-pin plug, since they would lead to sensor failure"
Soldering wires together can interrupt the reference air signal and lead to problems with the sensor. I've also had problems with the Bosch factory Posi-Lock connectors that are supposed to work with these newer sensors. Because the relationship between the connector and the O2 sensor is so vital to the proper reference signal, I recommend that you only use the correct sensors with the proper plug.
Shown here are the two Oxygen sensors for the passengers side of the car (red arrow shows the pre-catalytic, yellow shows the post catalytic converter). On the 997 Carreras one of the sensors is located on the catalytic converter, but the removal and replacement procedure is the same. The green arrows, show the wires that go to the engine shelf, where the connectors are located. I have often found that the new O2 sensors come with the correct plug, but sometimes the cable is way too long. If this is the case, then secure the cable with a nylon zip tie. Make sure that the cable is not located anywhere near any exhaust components--you don't want the heat melting the cable to the O2 sensor. The photo inset shows the special tool that is sometimes required to remove O2 sensors in hard-to-reach places (available from PelicanParts.com).