First: does the engine actually crank?
“Won’t start” describes two very different problems. Listen while a helper turns the key or presses Start—without standing in front of the vehicle.
A no-start complaint splits first by whether the engine turns. Keep that branch and the missing combustion requirement separate before testing parts.
- 01ClassifyCrank normally, crank slowly, or no crank
- 02PreserveCodes, security clues, recent work, and conditions
- 03SeparateElectrical, spark, fuel, air, timing, or compression
- 04TestUse the safest targeted check available
- 05VerifyConsistent starting and no returning evidence
Follow the sequence, stop when evidence changes the path, and verify the exact vehicle information before acting.
What happens when you try to start it?
Before you begin
Keep sparks and flames away. Do not bypass safety interlocks. Keep hands, clothing, and test leads clear of belts and fans. Many tests require vehicle-specific procedures.
Most likely cause groups
Battery voltage and cranking speed
The engine may turn yet still crank too slowly or pull voltage below what modules and ignition systems need.
Ignition or fuel delivery
Loss of spark, injector control, fuel pressure, or usable fuel can produce similar symptoms.
Air, sensor, or security input
A major air restriction, implausible sensor signal, or immobilizer intervention can prevent starting.
Compression or valve timing
Mechanical faults need compression, leak-down, relative compression, vacuum, or timing evidence.
Diagnostic steps
Preserve clues and scan every accessible module
Record codes, freeze-frame data, warning lights, and recent work. Security, communication, crank/cam, and power-supply codes can redirect the entire diagnosis.
DO NOT clear codes yet.Confirm electrical basics during cranking
Observe cranking speed and test battery/cable condition using the correct procedure. A simple resting-voltage reading does not prove battery health.
Compare measured results with exact service information.Establish whether spark and injector control exist
Use approved testers and methods. Do not hold a plug wire or improvise around ignition voltage or fuel vapor.
Evaluate fuel delivery with appropriate equipment
Listen and observe first. Pressure, volume, and command tests vary greatly by system; direct-injection systems can retain hazardous pressure.
Test mechanical condition when evidence points there
Use compression, relative-compression, leak-down, vacuum, or timing checks appropriate to the engine. Verify all specifications from exact service information.
Repair, clear, and verify under the original conditions
Confirm the engine starts consistently and that the original codes, data patterns, and symptoms do not return.
Use the reusable evidence test
The IAR test page turns this workflow into a recordable sequence: preserve the failed-start pattern, save scan context, confirm cranking power, separate the missing combustion requirement, and only then choose deeper mechanical testing.
Common mistakes
- Buying a fuel pump because it is quiet.Some pumps are difficult to hear; verify command and delivery correctly.
- Replacing a crank sensor from one code.The code can reflect wiring, timing, power, ground, or signal issues.
- Using starting fluid indiscriminately.It can be unsafe and unsuitable for some engines and fault conditions.
- Ignoring cranking speed.“It turns” is not the same as “it turns fast enough with stable voltage.”
When professional help is the right next step
Stop when diagnosis requires high-pressure fuel service, high-voltage hybrid/EV access, immobilizer programming, internal-engine work, advanced waveform analysis, or equipment and training you do not have.
Sources and verification
This general workflow follows a professional engine-performance diagnostic framework. Use the exact vehicle’s service information for procedures, hazards, expected values, and specifications.