Symptom Diagnosis

How do I diagnose a crank but no start?

Confirm the basics of fuel, spark, compression and timing in order, using scan data for crank sensor rpm, security status and fuel pressure before replacing parts.

How do I diagnose an engine overheating complaint?

Short answer: Check coolant level and leaks, fan operation, thermostat function and coolant flow, then test for combustion gases in the coolant if the basics check out.

Start with the simple things. With the engine cool, check coolant level and condition, and pressure test the system for leaks. Low coolant is the most common cause of overheating.

Next, check cooling fan operation using the scan tool's active tests and by watching the fans as temperature rises. Compare the scan tool temperature reading with an infrared or thermal reading to make sure the sensor is accurate. A thermostat stuck closed causes rapid overheating; feel or measure upper and lower hoses to confirm flow.

If the basics are fine, look for restricted radiators, failed water pumps, air pockets, or a combustion leak. A block tester detects combustion gases in the coolant, a sign of a head gasket or cracked head. Never open a hot cooling system, and keep hands away from fans, which may start without warning.

How do I diagnose a rough idle with no codes?

Short answer: Look at fuel trims, misfire counters and mode 6 data, check for vacuum leaks with smoke, inspect ignition parts, and compare cylinder contribution to find the cause.

Rough idle without a code often means the problem is below the threshold that sets a code. Start with scan data. Fuel trims show whether the engine is running lean or rich. Misfire counters and mode 6 data show which cylinder misfires even when no code has set.

Vacuum leaks are a frequent cause, especially on engines with large intake manifolds and PCV systems. A smoke test finds them quickly. Check spark plugs, coils and injectors on the cylinder that shows misfires. A power balance or cylinder contribution test using the scan tool's bi-directional controls can confirm a weak cylinder.

Also consider carbon buildup on throttle bodies and intake valves, idle relearn procedures after cleaning, and engine mounts that make a normal idle feel rough. Keep hands and tools clear of belts and fans while the engine runs.

How do I diagnose hesitation on acceleration?

Short answer: Check for codes and fuel trims, then look at airflow, fuel delivery, ignition under load, and throttle data during a road test, using graphing to see what happens at the moment of hesitation.

Hesitation means the engine does not respond smoothly when the driver presses the pedal. Begin with codes, including pending ones, and freeze frame data. Check fuel trims for lean conditions that show up under load.

During a road test, graph throttle position, pedal position, airflow, fuel trims, oxygen sensors and ignition data. Look at what changes right when the hesitation happens. Common causes include airflow sensor problems, vacuum leaks, weak fuel delivery, ignition misfire under load, and throttle body or transmission issues.

Test suspect components with a scope or fuel pressure gauge. Have a helper drive while you watch data. Fuel system testing involves pressurized fuel: wear safety glasses, relieve pressure as the maker describes, and keep ignition sources away.

How do I diagnose a no-crank condition?

Short answer: Confirm battery state, then follow the starting circuit: security or neutral safety inputs, starter relay, starter command and voltage drop across the cables and connections while trying to crank.

A no-crank means the starter does not turn the engine. Start by testing the battery, since a weak battery is the most common cause. Then check for immobilizer or security messages, and confirm the transmission range input sees park or neutral.

Use a scan tool to see whether the module commands the starter. Check the starter relay and fuses. Measure voltage at the starter control terminal while cranking. Voltage drop tests on the positive and ground cables find poor connections that look fine visually.

If the starter receives full voltage and does not turn, the starter or engine may be at fault; check that the engine turns by hand. Disconnect the battery before removing the starter, keep clear of belts and fans, and wear safety glasses.