Electrical/Electronic Controls and Onboard Diagnostics
What you must know
CNG buses use an ECM/PCM with dedicated fuel-system strategies, plus a fuel-type switch on bi-fuel units. Know how to use a scan tool: read and clear DTCs, view freeze-frame, monitor live data (fuel pressure, fuel temperature, fuel trim, O2, MAP), and run bidirectional tests (injector balance, solenoid actuation). Know the wiring basics: power and ground integrity, 5-volt reference circuits, and how a shorted sensor reference can take down multiple sensors. Know the fuel shutoff solenoid circuit and the pressure/temperature sensor signal.
Traps
A common exam trap is clearing codes before capturing freeze-frame - you lose the evidence. Another is assuming a no-start is a fuel problem when a failed crank sensor or a blown fuse for the ECM power circuit is the cause. Also know that a shorted 5V reference can disable several sensors at once, producing multiple codes that look unrelated. Finally, verify the fuel gauge and the actual fuel pressure separately - a faulty sender doesn't mean an empty tank.
Rule of thumb
Before replacing parts, check powers, grounds, and the 5V reference. Multiple unrelated sensor codes often mean one shared reference or ground fault, not multiple failed sensors.
Worked example
A bus sets codes for MAP, TPS, and fuel pressure simultaneously. Instead of replacing three sensors, check the 5V reference at each; find it pulled to 0V. Disconnect sensors one at a time until the reference returns - the shorted sensor is the culprit.