Transmission Clutch Interlock Safety Switch Open Circuit
Your transmission's safety switch—which prevents the engine from starting unless the clutch is fully disengaged—has an open electrical circuit, like a broken wire in a lamp cord. The ECU can't detect whether the clutch pedal is actually pressed, so it won't allow normal operation.
The ECU monitors the clutch interlock switch voltage signal to verify the clutch pedal is depressed before allowing starter engagement. When the circuit opens, the signal voltage stays high or disappears entirely, triggering a fault. The ECM expects a clean low signal when clutch is pressed and high signal when released.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Switch signal voltage | 0.5V when pressed (closed), 4.5–5.0V when released (open) | No signal or constant high voltage (open circuit) |
| Circuit continuity | <5 ohms closed, >1M ohms open | Infinite resistance (broken wire/connector) |
Code P1806 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P1806 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.