P1261: Cylinder #1 High To Low Side Short

P1261 flags a short circuit between the high side and low side of the fuel injector circuit for cylinder 1. Modern fuel injector driver circuits use a peak-and-hold or saturated-switch design where the PCM precisely controls current through the injector solenoid. When the high side and low side of that circuit are unintentionally connected -- either inside the injector itself or in the external wiring -- the PCM detects the abnormal current signature and disables the injector to protect the internal driver circuit from damage.

The most common cause is a failed injector with an internal winding short. Fuel injectors contain a solenoid coil, and like any coil, the winding insulation can break down over time -- particularly in engines with heat-soak issues or contaminated fuel. External wiring faults are the second most common cause: the injector harness for cylinder 1 passes close to exhaust components on many engines, and heat can degrade the insulation until the high and low side wires contact each other or a ground point.

Test the injector first by measuring its solenoid resistance -- a specification between 12 and 16 ohms is typical for high-impedance injectors. A near-zero or shorted reading confirms the injector itself is the issue. If the injector measures correctly, unplug the injector and check the harness resistance between the high and low side pins at the PCM connector. A replacement injector is a straightforward repair once isolated; a wiring fix requires finding and insulating the damaged section.