DIY OBD Adapter: Build Your Own Diagnostic Tool (Articles)

Translated on 10-08-2026, 13:14.

KKL Diagnose Adapter - OBD DIY

Bei allen Fahrzeugen des Volkswagen Konzerns (Audi, Bentley, Seat, Skoda und VW) ab etwa Baujahr 1993 lassen sich über die Diagnoseschnittstelle mehr oder weniger wichtige Informationen der Motormanagements abfragen. Bei aktuellen Fahrzeugen ist eine Diagnosesoftware zur Servicerückstellung sogar zwingend notwendig. Dies kann mit einem Adapter (Diagnosesystem) für die USB/serielle Schnittstelle des PC bewerkstelligt werden, dann braucht man nur noch einen preisgünstigen Laptop/Notebook und die entsprechende Software dazu. Dies ist vor allem für kleinere Werkstätten und Bastler interessant, ohne die Möglichkeit des Zugangs zur Onboard Diagnose artet die Fehlersuche an der heutigen komplexen Systemen der Motorelektronik leicht zum Ratespiel aus, obwohl das Auslesen des Fehlerspeichers schnell Klarheit verschaffen könnte. Für ältere Fahrzeuge reicht ein Diagnoseadapter mit KKL (K-Line), hier ist die Bauanleitung dazu. DiThe attached list, under the section "HEX-USB Diagnostics," mentions specific vehicle models for which professionally manufactured diagnostic systems are available.

There are several other programs that allow individuals to gain insight into modern engine electronics at affordable prices. One of them is VCDS from Ross-Tech, for OBD2 via CAN-Bus (vehicles from 2008 onwards, across manufacturers), and another is the dieselschrauber KOBD2Check.

The last VCDS software version that works with self-built interfaces (e.g., our B1 or U1 kit) is the manually unlockable VCDS-Lite from Ross-Tech, which is still being maintained (label files, Autoscan, plugins where possible).

In order for VCDS-Lite to work with a self-built adapter, the adapter must meet certain criteria in its transmission characteristics. Starting with version 504.1, VAG-COM/VCDS always queries the license data within the diagnostic system; manual activation is no longer required (dongle function).

The same applies to the diesel mechanic OBD2 software KOBD2Check (dongle function): https://shop.dieselschrauber.org/en/obd2-diagnose-kit-p-321.php

Repair manuals on how to use the diagnostic tools can be purchased directly from VW (Erwin), among other places. The most common procedures can be found on the VCDS, information about the procedure, repair manuals overview.

Now, regarding the adapter issue - what should be considered when building it yourself?

Since the vehicle's diagnostic signals are not compatible with those on the computer's COM port/USB port, both VCDS-Lite and an adapter are required for communication.
For DIY soldering projects, schematics designed with a minimalist approach are often suggested. These work flawlessly in many cases, but sometimes they don't. In some instances, increasing the value of resistor R7 to approximately 10 kOhm proves to be necessary.
effective remedy.

Other details of the standard circuit can also lead to problems:

The high-low switching threshold of T2 is in the range of 0.6 volts due to its control, which is only about 5% of the total voltage swing of nominally 12 volts. This is especially true in conjunction with the relatively small original value.
For the Pullup R 7, the on-board electronics may occasionally experience problems because the K-line may not pull down far enough for each bit.
When the 0.6 volts are just reached, individual characteristics of optocouplers 3 and T2, operating temperatures, the current onboard voltage, impedance ratios of the COM port, and the limited slew rate on the K-line can lead to timing shifts in the high-low transitions detected by the PC, or bits may be completely lost, which manifests as synchronization problems, various error messages, etc.
In particular, laptops with low-power outputs on the COM port can cause problems with the adapter-side circuitry of the RTS and TXD lines.
received: Due to the antiparallel diodes at the inputs of the optocouplers, high currents flow during low levels, just like during the activation of the optocouplers. Since the RTS output, which is switched to low when receiving data, is loaded again via resistor R3 and the output of optocoupler 3, this "unnecessarily high" current can potentially overload the COM port when the K and L lines are not activated, thus causing communication errors.
Depending on the optocoupler used and the input current, saturation-related delays in signal edges can occur, with rising and falling edges being delayed for different lengths of time: a typical characteristic of optocouplers that can lead to communication problems due to bit length errors.

Furthermore, an optical functional check is not possible during operation, and ultimately, the selection of optocouplers can be problematic: in some cases, types that are difficult to obtain are specified.

Instead of adapters with optocouplers, types without galvanic isolation between the circuits of the PC and the car can generally be used. With a well-designed circuit configuration, these are generally superior to adapters with optocouplers in terms of their transmission characteristics.
Only in cases of problematic combinations of ST generators and diagnostic PCs can adapters without optocouplers sometimes enable more stable communication.
However, such adapters can cause problems if they are not the only connection between the car and the PC, for example, if a diagnostic laptop is powered from the vehicle's electrical system. This can happen when errors or electrical incompatibilities occur.
In the worst-case scenario, the laptop and/or the vehicle's electronics could be damaged.

Anyone who wants to have the greatest possible security in such cases should still use an adapter with galvanic isolation between the circuits of the PC and the car.

To reduce or minimize the problems associated with optocouplers, the...
Standard circuit redesigned and expanded.
The new design includes:

a fixed voltage divider that sets the high-low detection threshold for the K-line to approximately 6 volts
D1 and D2, which reduce the load on the COM port when the K and L lines are not active
VR1 to VR3, to adjust the control currents for the optocouplers so that the shortest signal durations with equal delays of rising and falling signal edges are achieved. The optimum depends individually on the optocouplers used and the impedances of the PC interface. For VR1 to VR3, dust-insensitive, encapsulated versions should be selected; spindle trimmers are not required.
LEDs 1 - 3 as visual indicators for the signals on the L-line (LED 1) and K-line (LED 2 lights up during data flow towards the car).
D3 in the data flow direction towards the PC)
includes a transistor output stage for the L-line as well, to make the selection of optocouplers (coupling factor!) less critical

. Furthermore, the following were provided:

R 13, C 1, and C 2 to better suppress interference from the vehicle's electrical system
R 16 and R 17, which will discharge any static charges via the PC chassis instead of through the signal pins (for this, the diagnostic connector must first be connected to the vehicle)
a transistor output stage for the L-line as well, to make the selection of optocouplers (coupling factor!) less critical and to allow synchronization with the K-line
an optional voltage monitoring circuit using R 14, R 15, and LED 4 (which turns off below 8 volts).

. Note:
At high voltages and low impedance of the RXD pin on the COM port, LED 3 may constantly glow faintly, even if the OBD connector is not plugged in or no data is being received from the vehicle.This is not a fault in the adapter and does not affect its function; it is simply caused by the relatively simple circuit of LED 3 and can be ignored. Nevertheless, data reception from the vehicle can still be detected by observing a brighter flashing pattern of LED 3.

If the diagnosis is only to be performed on vehicles without an active L-line, then components R 1 through 4, VR 1, OK 1, LED 1, and T 1 can be omitted during the adapter assembly.

Adjustment of the trimmers

Connect the adapter to the diagnostic PC and the OBD socket.
(Note: If the diagnostic PC is replaced with a different model, readjustment may be necessary for reliable communication.)
Set all trimmers to their maximum values. In the VCDS-Lite LED test, LEDs 1 to 3 must blink in sequence - sometimes individually.
partially together. Using VR 1 and 2, the brightness of LED 1 and 2 should be adjustable.
In the repeated VAG-COM adapter test, find the position of VR 2 where the adapter is no longer recognized. Then, adjust VR 2 halfway towards the "recognized" position. Proceed in the same way with VR 3.
Adjust VR1 optically to the same position as VR2.
In dialog mode, determine the STG (Signal Transduction Group) with the highest measurement block sampling rate. Determine the adjustment ranges for VR2 and VR3 where communication is still stable. Adjust both trimmers to the middle of their respective green range. If necessary, adjust VR1 optically again like VR2.

If the adapter behaves differently, the functions on the test bench can be checked as follows.
A voltage source of approximately 12 volts is required, along with a voltmeter and a set of measurement cables with alligator clips – or alternatively, some pieces of wire and a soldering iron to create the connections. All measured voltages refer to [something - the context is missing here].
Vehicle-side mass.

Preparation:

Connect pin 5 of the PC connector (GND) to the vehicle's ground connection.
Connect pin 4 of the PC connector (DTR) to the vehicle's 12-volt power line.
Set all potentiometers to their maximum position.

Tests (measure voltages against ground; the voltages to be applied are also understood as being relative to ground):

- Connect pins 3 and 7 of the PC connector (TXD and RTS) to the vehicle's ground connection and apply 12 volts from the vehicle side. In this case, both the K-line and L-line must carry 12 volts. The collector of T3 should not have any voltage. LEDs 1 through 3 should not light up.
- Instead of connecting pin 7 of the PC connector to ground, apply 12 volts. LEDs 1 and 3 must light up; this corresponds to a switching state of the LED test in the VAG-COM menu.
Then connect pin 7 to ground again.
- Apply 12 volts to pin 3 of the PC connector (TXD). LED 2 must light up. The collector of transistor T3 must be at 12 volts. This corresponds to the data flow from the PC to the car.
- Connect pin 3 of the PC connector (TXD) and the K-line to ground. LED 3 must light up as long as the K-line is connected to ground. This corresponds to the data flow from the car to the PC.

Any other logical lighting behavior of the adapter will lead to malfunctions.

Christian designed a PCB layout for the above circuit diagram.
"Thank you very much!"

Adapter test software from Andreas (AST) and a user manual from Ulf, available for download.

For vehicles with control units split across two K-lines, the following

designapplies.

Johannes has created files for

Eagle: schematic, board.

Based on previous experience, a properly configured 2.4 adapter is usually sufficient for a stable connection in most cases.
Due to its relatively simple basic concept, it has some weaknesses that can make the communication unstable under extreme conditions (e.g., bit errors caused by significant fluctuations in the on-board voltage when starting the engine, extremely long cable lengths with high capacitance between the PC and the adapter, very unfavorable data from the COM port).

Furthermore, it can only communicate with a K-line, and the data lines on the vehicle side are not protected against voltage spikes from the on-board network.

The "Worst-Case Design" 4.0 offers advantages compared to...
The following improvements are available from version 2.4:

- High slew rate and low output impedance at pin RXD for problematic conditions related to the COM port.
- Constant bit lengths starting from a 9-volt operating voltage.
- Option to switch to 2 K-lines with separate receive LEDs, indicating which control unit is connected to each K-line (not yet practically tested).
- Protection against voltage spikes on all diagnostic lines.
- More evenly distributed current load on the COM port.

The principle of the protection diodes D11 to D16 can also be applied to other adapters.
If dual K-line diagnosis is not required, then D4, 12 and 16, LED 4 and R10, as well as the switch can be omitted; cable 15 is connected directly to the L-line.
For vehicles without an L-line, D1, 13 and 16, R1 - 4, VR1, OK1 and T1 can be omitted.

If a simple plug-and-play adapter is needed to facilitate communication, for example, by minimizing signal delays and distortions in critical combinations of vehicle, diagnostic PC, and software, then this one can be used.
Circuit for a diagnostic adapter without galvanic isolation and without MAX232.
They are used and operate stably at up to approximately 500 KB, depending on the cable capacities and characteristics of the COM port.
However, as with all adapters without galvanic isolation for the PC and vehicle sides, there is an increased risk of damage to the PC and vehicle electronics under unfavorable conditions (e.g., faulty adapter construction, powering the PC from the vehicle's electrical system using unsuitable power supply circuits or transition resistors, loose connections, etc.).

LED1 displays the data sent to the vehicle, LED2 displays the received data, LED3 indicates the control of the L-line, LED4 shows whether the operating voltage is sufficient (it goes out below 8 volts), and LEDs 5 and 6 indicate interference voltages between the vehicle and the PC, which can affect communication.

For vehicles without the L-Line, parts D 8, LED 3, T 4, and R 10 to 12 can be omitted.
will be.

For owners of older optocoupler adapter versions from Ulf...

Common problems
(Author: Ulf, Rainer)

Unfortunately, the data exchange between the PC and the vehicle is so complex due to the involved hardware and software that the initial attempts using VCDS-Lite are often described as "plug and pray" (i.e., simply plug it in and hope for the best).

"I can't connect to any STG at all!"

The most common causes are:

1. The adapter, including the connecting cable, is incorrectly designed/constructed/defective.

2. The electrical specifications of the adapter and the PC interface are not compatible. This can happen relatively easily due to the wide variation in characteristics of COM ports, especially when using adapters with optocouplers, which are often not very tolerant regarding the data transmitted through the COM port.
Adapters with corresponding adjustment options can provide a solution, such as the versions 1.1, 2.4, and 4.0 shown here.

3. On-board errors (ignition not switched on, cable breaks in the area of the diagnostic connector, etc.).

4. störende Software-Konfiguration des PC (vgl. Sure, here is the translation of the text from German to English:

"No.

"

5. Computer malfunction / VCDS-Lite installation error.

"I cannot access all control units (STG), only some functions work, and VCDS-Lite constantly displays error messages."

This can mainly be caused by the following reasons:

6. Significant bit-length errors in the adapter impede communication, especially with STGen devices using higher baud rates.
Design or adjustment errors in the adapter are most likely to occur in circuits using optocouplers.

7. The adapter's onboard logic-low detection threshold is lower than the logic-low output level of the "silent" STGs, which often (and still compliant with ISO standards!) provide 1 to 1.5 volts instead of the ideal value of 0 volts.
Design or calibration errors in the adapter; ideally, a logical "0" should be recognized as soon as and while the K-line voltage is less than half of the vehicle's on-board network voltage.
Attempt to fix the issue (if the actual adapter circuit cannot be corrected): Connecting 1 to approximately 4 diodes, such as 1N4002 or similar, in series and in forward bias within the ground wire leading to the OBD connector (pin 4) might solve the problem with some luck.

8. The overall software configuration of the PC is interfering with the VCDS-Lite dialog.
Solution: Close or uninstall as many background programs as possible, or verschandeln up the startup files.
If necessary, try a different PC with a different software configuration; using identical hardware makes it possible to more reliably pinpoint the cause of the problem within the software.
More information can be found on Uwe Ross's homepage.

9. The data format of older STGe devices (especially those from 1Z engines) is not fully compatible with the version of VCDS-Lite being used.
Joergs has come up with the following solution:
It can be helpful to gradually decrease the value for 'Blk Int' in the program options.
Regarding the general settings for 'Char Int' and 'KW2 Delay': decreasing these two values can improve performance on slower computers, while increasing them may help with faster systems.
Thank you, Jörg!

"I'm getting inconsistent text when I read the error memory of the STGe device!"

Take some time to reflect and make sure you have the proper license for your VAG-COM/VCDS-Lite version!

When purchasing adapter kits without an included wiring diagram, you are essentially buying a "black box" regarding potential (not necessarily guaranteed!) problems related to items 1, 2, 6, and 7.
This also applies to (www-)auctions, flea markets, etc., where design flaws from private/unknown manufacturers are apparently sold off even more readily than in regular retail .

If an adapter is recognized by VCDS-Lite or if a check using the provided adapter test software yields a positive result, then only adapter problems listed under numbers 1 (excluding the cable and connector on the vehicle side!), 2, and 6 can be largely ruled out.
Error number 7 can still occur, especially when using the widely used optocoupler minimal circuit and related designs, which may also be included in various commercially available adapters.

In addition to everything else, items 6 and 7 in particular can also vary depending on the onboard voltage, ambient temperatures, etc. (which makes them quite unpredictable).

For these reasons, the purchase of pre-made adapters from unknown manufacturers or suppliers, or with unclear designs, should be carefully considered.
If high reliability is desired from the outset, circuits like those described or linked here should be preferred (with the exception of the minimal designs). Furthermore, support is often available in this forum, and various problems have already been addressed previously, which can be found using the search function.

But:
Manufacturing defects and flaws can occur in any individual component, therefore creating an almost incomprehensible variety of error patterns

. If measuring instruments are also missing, and/or the person seeking help lacks basic electronic knowledge, then providing meaningful support via remote diagnosis is practically impossible!

https://shop.dieselschrauber.org/en/rks-can-can-bus-interface-c-24.phpFor training purposes, teaching, or professional work on the powertrain, the OBD2 diagnostic software KOBD2Check is a good option and is available in the shop:

Here is an example of boost pressure, measured with VAG-COM} (Excel file).