The Fundamental Starting Requirements
For an internal combustion engine to start and run, it needs three things simultaneously in the right quantities and at the right time:
- Fuel — the right amount of petrol or diesel delivered to the cylinders
- Spark — a strong spark at the right moment (petrol engines; diesel uses heat from compression)
- Compression — sufficient mechanical compression in the cylinders to ignite the mixture
When the engine cranks but won't fire, one or more of these is absent or inadequate. A systematic diagnosis works through each in order.
Diagnosing No-Start With a Good Crank
Step 1 — Check for fault codes. Connect an OBD-II scanner (or have a workshop do it). Fault codes often point directly to the cause: P0335 (crank sensor), P0230 (fuel pump circuit), P0300 (misfire), etc.
Step 2 — Check for spark. Remove a spark plug lead or coil-on-plug unit, insert an old spark plug, hold it against the engine block (earthing it), and have a helper crank the engine. You should see a bright blue spark. A weak orange spark or no spark indicates an ignition system problem — coil pack, crank sensor, distributor, or spark plugs.
Step 3 — Check for fuel. Listen for the fuel pump priming hum when you turn the ignition to "on" before cranking. Use a fuel pressure gauge to check rail pressure. Alternatively, a small squirt of starting fluid (or even carby cleaner) into the intake — if the engine fires briefly then dies, fuel delivery is the issue.
Step 4 — Check compression. A compression test uses a gauge threaded into the spark plug holes. Low or zero compression in one or more cylinders indicates a serious mechanical problem: burnt valve, broken piston ring, head gasket failure, or in severe cases a seized engine.
Common Causes of Crank No-Start
- Fuel pump failure or blown fuel pump fuse
- Crank position sensor failure
- Immobiliser active
- Flooded engine (excess fuel)
- Failed ignition coil or coil pack
- Timing belt or chain failure (no compression)
- Head gasket failure (low compression)
When to Stop DIY Diagnosis
If you've confirmed spark and fuel pressure are present and the engine still won't start, the problem is mechanical (compression, timing, internal fault). This requires a compression test, and if that shows a problem, professional diagnosis from a mechanic. Continuing to crank a hydrolocked or severely damaged engine will cause more harm.
Cost of Diagnosis
Most mechanics charge $80–$150 NZD for a no-start diagnostic session. This typically includes fault code reading, basic system checks (spark, fuel pressure), and a recommendation. Repairs then depend on the identified fault.