FAQ
TL;DR: In this Audi A4 B5 case, 1 switch restored ECU diagnostics; “I did it on the switch.” [Elektroda, golabek008, post #16950238]
Why it matters: If your OBD reads HVAC/ABS but not the engine ECU, a simple K/W-line fix can save hours of guesswork for 1.8 LPG owners.
Quick Facts
- Symptom pattern: OBD connects to air conditioning and ABS, but not to the engine ECU. [Elektroda, golabek008, post #16935039]
- Likely cause: immobilizer emulator or a missing/incorrect K–W diagnostic line connection. [Elektroda, spinacz, post #16934869]
- Practical fix: tie K and W lines or add a toggle switch to restore ECU comms. [Elektroda, spinacz, post #16937911]
- Confirmation tip: “wrap the key in aluminum foil” test indicates immo-off behavior. [Elektroda, adamski123, post #16935194]
- Tools used and verified: VAG/Viaken interfaces worked on other cars; the A4 still started and ran. [Elektroda, golabek008, post #16934611]
Why does my Audi A4 B5 read HVAC/ABS but not the engine ECU over OBD?
Because the engine ECU’s diagnostic line is interrupted or rerouted, often by an immobilizer emulator or miswired K/W lines. Other modules (HVAC/ABS) sit on separate diagnostic lines, so they still reply. Restoring the K–W path brings the ECU back online. [Elektroda, spinacz, post #16934869]
What are the K-line and W-line in this context?
They are single-wire diagnostic lines used on older VAG cars. The ECU expects K and sometimes W lines present to communicate. If these lines are cut, not bridged, or hijacked by an emulator, the scanner cannot talk to the ECU. [Elektroda, adamski123, post #16937398]
Could an immobilizer emulator block ECU diagnostics?
Yes. If someone installed an emulator and didn’t connect the diagnostic line correctly, ECU diagnostics are unavailable while other modules still work. Reconnecting K and W restores access. [Elektroda, spinacz, post #16935043]
How can I quickly test if my car is immo-off?
Try the foil-key test. Wrap the key in aluminum foil so only the metal tip is exposed, then start. If it starts, the immobilizer is disabled. [Elektroda, adamski123, post #16935194]
What wiring colors did owners see when fixing this?
One owner reported green/black and green/red wires tied with a red lead from the immo can. He used those to sort the K/W connection and diagnose. [Elektroda, golabek008, post #16939771]
Will joining K and W lines stop the car from starting?
No. A helper advised joining K and W, then testing start and diagnostics. The car should start normally once the lines are correctly connected. [Elektroda, spinacz, post #16937911]
What’s the simplest working fix reported in the thread?
A toggle switch between the diagnostic lines. The owner confirmed “I did it on the switch” and could read errors even with the engine running. [Elektroda, golabek008, post #16950238]
Do VAG/Viaken interfaces fail here, or is it the car?
The interfaces were fine. They worked on other cars. The A4 still started and ran, indicating a vehicle-side diagnostic line issue, not tool failure. [Elektroda, golabek008, post #16934611]
My climate control also stopped connecting later—what does that imply?
It suggests broader diagnostic wiring issues, not just the ECU path. Re-check the splices and any emulator connections affecting shared signal routes. [Elektroda, golabek008, post #16939759]
How do I implement the switch-based workaround?
- Identify the green/black and green/red diagnostic wires near the immo box.
- Insert a simple toggle switch to bridge the K and W lines when needed.
- With the switch on, connect your scanner; switch off after. [Elektroda, golabek008, post #16950238]
Is there an edge case where K/W reconnection still fails?
Yes. Some emulators block diagnostics regardless of K/W bridging. In that case, add a switch or re-route around the emulator to regain access. [Elektroda, spinacz, post #16937911]
What if my immo box shows only three wires—does that change things?
It can indicate a modified setup or emulator presence. Trace those three leads and look for a nearby broken plug or spliced greens to locate K/W. [Elektroda, golabek008, post #16936837]
Can LPG conversion be the reason for no ECU comms?
The thread points to immobilizer/emulator wiring rather than LPG. The car with 1.8 LPG ran fine; fixing K/W restored ECU diagnostics. [Elektroda, golabek008, post #16950238]
How do I know the fix actually worked?
You should establish ECU comms and read fault codes. The owner confirmed successful live diagnostics even on a running engine after adding the switch. [Elektroda, golabek008, post #16950238]
What’s a quick expert tip from the thread?
“Connect these K and W, then check.” Start there before complex steps; it often restores ECU communication immediately. [Elektroda, spinacz, post #16937911]
Any concrete numbers I can look for when inspecting?
Look for three wires at the immo module in modified setups and a single added switch bridging two diagnostic leads to re-enable OBD ECU access. [Elektroda, golabek008, post #16936837]
