General Engine Diagnosis
Symptom first, system second
A general engine diagnosis begins with verifying the customer complaint and duplicating the condition. Record the symptom - crank/no-start, stall, misfire, lack of power, noise, vibration, fluid loss, or abnormal exhaust - and check for DTCs and freeze-frame data before touching hardware. Then confirm the three fundamentals: adequate compression, a correct air/fuel mixture, and spark at the right time. A vacuum gauge, compression gauge, cylinder leak-down tester, and exhaust back-pressure gauge answer most mechanical questions quickly.
Separate mechanical from control
A base-engine fault (worn rings, burned valve, jumped timing chain, leaking head gasket, restricted exhaust) will mimic sensor or fuel faults. Use cylinder contribution/balance tests, a running compression test, and a cooling-system hydrocarbon test to confirm. A vacuum gauge that reads low and steady indicates late timing or a vacuum leak; a needle that drops rhythmically points to a valve or ignition problem.
Traps
Do not replace parts because of a single DTC - a P0300 is a symptom, not a diagnosis. Do not overlook the air intake or exhaust. A restricted converter causes low power and can set lean or misfire codes. Verify with a back-pressure test rather than guessing.
Worked example
Cranks, no start. Spark is strong at all plugs, fuel pressure is within spec, injector pulse is present, and compression is even. Rule out ignition and fuel delivery, then check exhaust back-pressure and base timing; a plugged converter or jumped chain is the likely cause.