Cylinder #3 High To Low Side Open
Cylinder #3's fuel injector circuit has an open connection between the high and low sides, meaning the injector won't fire properly. It's like a broken wire in a light switch—the electrical path is interrupted so no fuel sprays into that cylinder.
The ECU monitors the fuel injector circuit resistance and voltage drop across cylinder #3's injector during firing commands. It detects when current cannot flow from the high side to the low side (ground), indicating an open circuit or wiring break.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Injector Circuit Resistance | 12-16 ohms | Open circuit (infinite/very high resistance) |
| Injector Voltage Drop | 10-12V during pulse | No voltage drop detected (open line) |
Code P1278 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, P1278 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.