Almost all Porsches from about 1984 on use a sophisticated Bosch engine management system called Motronic. The Motronic system (also called the Digital Motor Electronics or DME) is hands down the best overall fuel injection system that you can use when you consider price and performance. Ignition timing and fuel delivery are all controlled by a digital map stored within the main fuel injection (DME) computer. The computer takes input from a variety of sensors that are located on the engine: engine coolant temperature, crank angle, throttle position, exhaust gas oxygen (mixture), ambient air temperature and mass air flow. The DME is programmed from the factory with certain performance characteristics so that the engine will react well under a host of varying conditions.
As with any electronic device, components can fail, triggering problems with the system. The system is designed to react to these failures and indicate them to the driver, so that they can be fixed. If one of the computer's sensors is not working properly, then the computer may not be able to successfully identify the current state of the engine and choose the appropriate fuel mixture or timing advance level. When this happens, the fuel mileage drops, engine performance suffers, emissions increase and the car typically illuminates the 'check engine' light.
Pre-1995 Porsches were equipped with what is known as OBD I (On Board Diagnostics Level I). Starting in 1996, they were equipped with a more advanced version called OBD II, which was mandated by the US government, in order to standardize automotive repair and diagnostics. The OBD system is responsible for monitoring and checking all of the fuel injection sensors and systems in the vehicle and sending a visual error message to the instrument panel if it finds a problem or irregularity with one of them. If there is a problem with a sensor or component, the computer logs a Diagnostic Trouble Code (DTC) in the main computer until it is read and reset.
In order to accurately find the sensor and fix the problem, you will need to find out which error code is being triggered by the computer. There is no method to pull these codes out of your Porsche without the use of a computer. The factory has produced a version of this tool for use by Porsche dealers, called the Porsche System Tester 2. Unfortunately, finding one of these is next to impossible, and they cost about $4000 used anyways. There's a newer version of the PST2 called the PIWIS, but getting your hands on one of those costs about $20,000 and a $1,000 maintenance contract from Porsche. You can indeed use a standard off-the-shelf OBD II reader available at any auto parts store, but it will only give you the standard read-out codes for the fuel injection system: you will not be able to do any extra diagnostics on any other area of the car.
Thankfully though, there is aftermarket software available produced by Durametric that performs almost all of the important reading functions that the PST2 does. It runs on a Windows laptop computer and the cost is about $300 for the home-based version that will allow you to read codes on up to three cars. If you're planning on working on your Cayenne yourself, I suggest that you pick up this essential tool.
With a standard OBD II code reader, you can access and reset the codes on the car that are related only to the fuel injection system. The PST2 and Durametric software allow you to dig deep into the various systems of the car and read values from the various systems of the car (airbag, ABS, Tiptronic transmission, alarm, seat memory, heating & A/C). The Durametric tool does not program the computer though: you are still at the mercy of the dealer when it comes to making changes to any of the non-fuel injection settings on your car.
When you obtain the trouble codes output by the PST2, you can look them up in the Porsche ODB II factory diagnostic book (expensive), or look up the code online. The factory book will have extensive sections describing the various faults and what is needed to fix them. Typically a web search will provide you with the information you need as well.
Tip: Wire harnesses are a major cause of fault code problems. As the cars get older, the wiring harnesses have a habit of becoming what is commonly known as 'work hardened.' This causes the wiring to become brittle and often break inside of its plastic sheath. Only by testing the continuity of the wires end-for-end will you be able to determine whether the wire is broken or not.
Here it is, the holy grail of Porsche code reading. The Porsche System Tester 2 (PST2) is a rare and expensive tool that can be used to diagnose multiple car systems on a variety of Porsches. Every dealer has one of these, or the later-model PIWIS tester so that they can quickly diagnose problems and change settings on the various computer systems within the car.
Both the PST2 and the Durametric software interface with the car through the OBD II port located beneath the steering wheel on the lower left hand side of the car. Make sure that the plug is firmly seated as it has a tendency to occasionally fall out.
Here is a screenshot of the Durametric software available for diagnosing various system problems with your Cayenne. The software is nearly as powerful as the reading functions on the original Porsche PST2 or PIWIS and is a required diagnostic tool for the do-it-yourself enthusiast.