While
- (NFET, injector, ignition IGBT, ...) low-side output switches use common cathode flyback to +12V
- high-side (PFET) output switches use common anode flyback to GND
Tests before ECU powerup, retest after ANY change around the injectors or supply/GND:
With ECM connected, but output load (solenoid) NOT, measure the flyback integrity with DVM at the solenoid connector.
Low voltage flyback (almost always used, eg. for +4 PFETs) measures appr. 500..1500mV in DVM diode mode (with DVM+ red probe at GND). Very quick test, touch each PFET high-side switch output pin to verify, take note.
Test flyback path integrity for every high-side and low-side output that will (or might ever) be connected.
See also: