Cambridge, England

Porsche  Electronics specialist

Diagnostic Tester Frequently Asked Questions

JDSPorsche now offers a Porsche  Diagnostic Tester at a competitive price to assist your fault finding. The  following FAQ has been compiled for your information, and we hope you find  it interesting and useful.

E-mail us at JDS Porsche for our current charges.  If you need advice or help regarding our diagnostic tester feel free to  e-mail us with no obligation.


These notes relate to the first generation of Porsche cars  fitted with on board diagnostics. These officially started  with the 1988 Model Year, and these FAQs end with the 1994  models.

In 1988, Porsche  introduced a fault diagnostic system for their models 928S4  and 944S. This system uses the fault diagnosis facility  introduced by Bosch into the “LH Jetronic” and “Motronic”  engine fuel and ignition control computers used on those  vehicles. The 1987 MY 928 S4’s were also wired for  diagnostics, and it is possible to easily upgrade the ECUs to  allow  diagnosis.

Q: How do  the on board diagnostics work ?
A: The various ECUs used  in the cars can detect implausible signals from the various  actuators and sensors in the car. When it encounters such a  situation, a fault code is stored in a memory chip in the ECU.  The fault codes can be retrieved by the diagnostic tester.  These systems were introduced in order to monitor emissions  critical systems, in response to increasing environmental  regulations. The diagnostic facility was also a great benefit  to workshop personnel, face with increasingly complex  electronic systems. So the “intelligence” lies in the ECUs in  the vehicle, the diagnostic tester just retrieves that  information.

These stored  fault codes are held in volatile memory in the ECUs are the  information stored there is lost if the vehicle battery is  disconnected for a few  seconds.

Q: Anything else ?  It sounds as though the ECUs in these cars are rather  clever
A: The ECUs can also be programmed with a  suitable tester. to test the various actuators and switches on  the vehicle, under operator control. It is also possible to  clear stored fault codes, reset the idle speed on some cars,  and count knocks (detonation “pinking” knocks) while driving  the car.

Q: What would I  need to read these codes ?
A: Here is a little  history on Porsche Diagnostic testers, starting with the 9268.

The  Porsche 9628 diagnostic tester

This  was the first Porsche Diagnostic tester, introduced for the  1988 model year. At that time, the only Porsche models  equipped with on board diagnostics were the new 928S4 and the  944S. The tester was capable of reading stored fault codes,  and also testing various actuators and switches on the  vehicle, under operator control. It could also clear stored  fault codes, reset the idle speed of the 928, and count knock  (detonation “pinking” knocks) while driving the car.

The operator  interface was a four digit numerical display and two buttons.  When a fault code was displayed, it was necessary to look up  the code in the manual. The factory workshop manuals for these  cars give excellent fault finding checks for each of the fault  code numbers.

Q: This  tester would do what I want, can I buy one  ?
A: No, not a new one, they  have long been superseded by more capable testers such as the  KTS300 “Hammer”, and newer units. They rarely come onto the  second hand market.

Q: I didn’t even  know my car had a diagnostic socket, where is it  ?
A: For 1988 MY 928 cars, it is a 12 way  rectangular connector located on the mounting plate that hold  the LH and EZK ECUs, in the passenger side footwell. In the  1988 MY 944, it is mounted above the DME engine management  ECU. 1987 928S4 was also fitted with this connector, but their  LH and EZK ECUs weren’t fitted with the software required for  diagnosis. However, it is possible to easily upgrade the ECUs  to allow diagnosis. Contact JDSPorsche for further  details.

For the 1989 and  later 928 cars, the connector changed to a special (unique to  Porsche) 19 way connector, which is located under the cover at  the side of the passenger seat, between the seat and the  door.

Q: What about the  later cars, didn’t they also have on board diagnosis  ?
A: Yes, all the models introduced since 1988  have had diagnosis systems. So the 944S2, 944 Turbo, 968, and  the 964 Carrera 2 and Carrera 4 (911s) also have diagnosis.,  using the same 19 way circular connector. More details on  these cars later.

Q:…..and the later  cars ?
A: When vehicle manufacturers started  fitting on board diagnosis systems, they all had their own  proprietary versions. There were some attempts at industry  standards in the early 1990’s , and by about 1995 most cars  were then fitted with the OBDII system (On Board Diagnostic  second version, which has become a mandatory industry  standard). This document does not cover these later vehicles,  for which generic fault code readers are readily  available.

Q: What about the  “Check Engine” lamp on the dash of some cars, can’t that be  used to read fault codes ?
A: Yes, on some cars it  is enabled, and can be made to flash out some of the fault  codes that relate to emissions problems. Although common on US  market cars after about 1992, this option was not always  activated on other market cars. Refer to the Workshop manual  for your car for more details.

Q: You mentioned  the early 9268 tester was superseded by the KTS 300 “Hammer”  ?
A: Yes, just a year later in 1989, Bosch  produced for Porsche a version of their KTS 300 tester, which  was known as the KTS301, with special diagnostic software for  the Porsche ECUs.. Porsche refer to this tester as the 9288.  The Bosch reference number is the KTS  301.

Q: So  what is the difference between the KTS300 and the KTS 301  ?
A: For the KTS 300 Bosch produced software for  the KTS tester for many makes of cars. This software is in the  form of plug in modules. They also produced the various types  of diagnostic leads for the various vehicles, because at that  time each manufacturer had their own version of connector.  Each software module contains test software for many makes of  vehicle.

The KTS 301 was  Porsche specific. It had a Porsche only software module, and  the special 19 way circular connector, which was also  specially made for Porsche, and not generally  available.

Q: So can I just  plug a different software module in to any KTS 300 or 301, and  with a suitable lead diagnose any car ?
A: Yes.  The modules and leads are just plug  in.

Q: What are  typical fault codes ?
A: Here is a list of  possible fault codes from the 928 LH (fuel injection) ECU

1500 No faults stored
1111 Supply voltage too high  or low
1112 Idle throttle switch faulty
1113 Full load  throttle contact fault
1114 Engine water temperature sensor  fault
1121 Airflow sensor fault
1122 Idle stabiliser  circuit fault
1123 Lambda sensor signal too rich
1124  lambda sensor signal too lean
1125 lambda sensor  fault
1131 Ignition monitor relay closed
1132 Ignition  monitor relay ground Terminal AL.
1141 LH ECU  faulty
1000 LH ECU output end

For the EZK (ignition) ECU, the following fault codes are  reported

2500 No faults  stored
2112 Idle contact fault
2113 Full load contact  fault
2114 Engine temperature sensor (second section)  fault
2115 Idle/full load contact fault
2121 load signal  from LH fault
2126 Auto transmission safeguard switch  fault
2131 Knock sensor 1(front) fault
2132 Knock sensor  2 (rear) fault
2133 Knock control in EZK faulty
2134  Hall sensor fault
2141 EZK control unit faulty
2000 EZK  output end

Q: you  mentioned actuator tests ?
A: yes, the  tester will allow you to individually test various  actuators and switches, to help with fault diagnosis.  For example, you can switch the fuel injectors  repeatedly on and off, so you can check they are all  clicking. If not, then you can meausre the voltages on  the connectors, while they are being activated. This is  very useful for fault finding.

The fuel  injector are pulsed while fault finding takes  place.
The idle actuator is switch across its range  to enable fault finding
The inlet plenum resonance  flap is actuated repeatedly to enable fault finding in  the vacuum feed, or the actuating solenoid.
The vent  valve for the petrol tank carbon canister filter is  repeatedly activated.
The function of the throttle  closed switch is verified.
The function of the wide  open throttle switch (WOT) is verified.
The function  of the idle speed correction when the aircon is  activated is verified.

Q: There are a  number of other ECUs that can be diagnosed as well, aren’t  there ?
A: Yes, on the 928, there is the PDS  electronically controlled limited slip differential ECU that  can be diagnosed, and also the Airbag ECU and the RDK tyre  pressure monitoring system

The 964 Carrera  models have a heater/aircon ECU, and the PDAS s ECU which  controls the 4 wheel drive system of the Carrera 4. The 964  also has a ECU for the Tiptronic option, where fitted.

Q: I would like a  diagnostic tester for my Porsche, but I can’t find one I can  afford...
A: Check out the JDSPorsche diagnostic  tester. This is affordable for the home mechanic or  Independent Porsche Specialist garage

Copyright JDSPorsche 2003 Issue  1

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