FAQ
TL;DR: In a 2003 Sharan, after 8h34m of tracing, "all K lines are gray with a white stripe," and the additional‑heater controller was the K‑line blocker. Isolate the K‑line and test with that controller unplugged. [Elektroda, zuberek7, post #17324448]
Why it matters: This FAQ helps VW Sharan 7M owners and techs fix no‑communication OBD issues by tracing K‑line faults fast.
Quick Facts
- OBD-II pinout: 7 = K-line (ISO 9141/14230), 16 = +12 V, 4/5 = ground. [“OBD-II Connector Pinout”]
- Sharan K-lines are gray/white; the immobilizer’s K-line is white. [Elektroda, zuberek7, post #17324448]
- K-line splice cluster sits in the thick loom behind the left fuse/relay box. [Elektroda, zuberek7, post #17323786]
- Auxiliary heater controller can short the K-line and block diagnostics to all modules. [Elektroda, zuberek7, post #17324448]
- To locate a K-line short, measure the line and unplug the splice connectors one by one. [Elektroda, ociz, post #17047605]
Why won’t my 2003 Sharan connect to CDiFA/VCDS?
A module is likely pulling the shared K-line down. In this documented case, the additional heater controller blocked communication across modules. Unplug the heater controller and retest diagnostics. If communication returns, repair or replace that controller or its harness. [Elektroda, zuberek7, post #17324448]
Where is the K-line junction I should check first?
Follow the gray/white K-line from the OBD socket to a splice cluster. It is inside a thick loom behind the fuse and relay box on the left of the steering wheel. Access the loom, open the wrap carefully, and expose the splice connectors for testing. [Elektroda, zuberek7, post #17323786]
How do I isolate a shorted K-line quickly?
Use this 3-step method:
- Measure K-line voltage at OBD pin 7 with ignition on.
- Unplug K-line splice connectors under the dash one by one.
- After each unplug, retry the scan until communication returns; the last removed branch is faulty. [Elektroda, ociz, post #17047605]
Does the engine ECU’s K-line route through the immobilizer?
Yes. The engine ECU K-line passes through the immobilizer box. If that path fails, you may still talk to the immobilizer itself. As one expert put it: "Short-circuit K and W to immo forest and diagnosis should return." Verify the gray/white and white wires at the immo plug. [Elektroda, grala1, post #17045602]
What color are the Sharan’s K-line wires, including immobilizer?
All K-lines are gray with a white stripe. The immobilizer K-line is white. Trace and confirm before cutting, because multiple gray/white lines join in the loom, and identification requires testing continuity. "All K lines are gray with a white stripe. only from the immobilizer is white." [Elektroda, zuberek7, post #17324448]
Why did a direct jump to OBD pin 7 only talk to the engine ECU?
A direct wire connected your scan tool only to the engine ECU’s K-line, bypassing the shared splice. That isolated the engine module from the rest, so only the engine answered while other controllers remained unreachable on the faulty K-line branch. This behavior matches the reported test outcome. [Elektroda, zuberek7, post #17044950]
Can the additional heater (Webasto) block diagnostics on the K-line?
Yes. The additional heater controller can hold the K-line bus down and kill communication to all modules. Disconnect that controller and check if scanning resumes. If it does, repair the heater controller or its corroded harness before reconnecting it to the K-line. [Elektroda, zuberek7, post #17324448]
How many joined K-line wires might I find in the under-dash loom?
Expect multiple splices. In the documented teardown, the gray/white K-line from the OBD socket was welded to four other K-line wires in the loom. One of those branches blocked diagnostics until isolated from the bus. This shows how a single bad branch can affect all modules. [Elektroda, zuberek7, post #17312249]
How do I verify OBD-II power before chasing K-line faults?
Check for battery voltage at OBD pin 16 and good grounds on pins 4 and 5. Without stable +12 V and ground, no scan tool will connect, regardless of K-line integrity. Confirm power first to avoid misdiagnosing a communications fault. [“OBD-II Connector Pinout”]
Could a failed ECU cause total no-communication on the K-line?
It can. A malfunctioning ECU may disrupt bus communication and appear as a global no-comm condition. Rule it out by isolating ECU’s K-line branch at the splice and testing tool communication with other modules. Replace or repair the ECU only after confirming isolation results. [Elektroda, sagem173, post #17045494]
After adding missing fuses, my battery died overnight. Related to no-comm?
It can be related. Reinstalling missing fuses can re-energize a faulty retrofit or corroded circuit, such as the auxiliary heater harness. That can both drain the battery and load the K-line. Pull the suspect fuse, unplug the heater controller, and recheck current draw and diagnostics. [Elektroda, zuberek7, post #17047468]
If the immobilizer path is faulty, can I still run diagnostics?
Often, yes. You may retain communication with the immobilizer even if the engine ECU path via immo is broken. Bypassing or shorting the K and W lines at the immobilizer plug for testing can restore scan access, confirming a path fault. Proceed carefully and document wiring. [Elektroda, grala1, post #17045602]
