Cylinder #1 High To Low Side Short
Cylinder #1's fuel injector circuit has an electrical short between the high and low voltage sides, like a wire touching where it shouldn't. This prevents proper fuel injection and causes misfires.
The ECU monitors voltage and current through the cylinder #1 injector driver circuit. It detects abnormal voltage drops or shorts when commanding the injector on and off. A short between high and low side indicates a wiring or component failure.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Injector voltage drop | 0.5-2V during activation | >2.5V or <0.1V (short detected) |
| Injector coil resistance | 12-14 ohms | <5 ohms (short) or open circuit |
Code P1266 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, P1266 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.