General Powertrain Diagnosis
Start with the three fundamentals
Every no-start, misfire, or driveability complaint reduces to spark, fuel, compression, and timing. The L1 exam expects you to prove the mechanical side before condemning a control system. A P0301 does not mean replace the coil; it means cylinder 1 is contributing less power. Confirm with compression, cylinder leakdown, relative compression on a scope, or a power balance test.
Reading the instruments
A vacuum gauge at sea level should read a steady 17-21 in-Hg at idle. A low steady reading points to late valve timing, a large vacuum leak, or worn rings. A needle that flicks regularly suggests a leaking valve; a reading that drops off at higher RPM suggests a restricted exhaust. Compression should be within roughly 10-15% cylinder to cylinder, and a wet test that raises a low cylinder points to rings rather than valves.
Wiring and voltage drop
Load-test circuits with a voltage drop, not an ohmmeter. More than about 0.5 V on a ground or power feed is excessive. Wiggle-test harnesses while watching live data.
Traps
Assuming the DTC names the failed part; skipping a visual inspection and TSB or calibration check; taking a vacuum reading on a cold engine; and condemning a PCM when the real fault is a corroded ground or chafed harness.
Rule of thumb
Swap-and-move: if a misfire follows an injector or coil when you move it to another cylinder, the component is at fault. If the misfire stays put, look at that cylinder's compression, valvetrain, or harness.