Diagnose intermittent crank-no-start
The short answer
A crank-no-start means the starter turns the engine over but the engine does not fire and run. An engine needs four things to start: adequate cranking speed, fuel delivered at the right pressure, spark (on gasoline engines), and an engine computer that knows where the crankshaft is and has authorized the start. When the fault is intermittent, one of these is dropping out only some of the time. The most reliable way to find which one is to test while the failure is happening, because a car that starts normally in the workshop will usually pass every test.
This guide applies broadly to modern fuel-injected passenger vehicles. The specific test points, specified values, relay locations and security-system behavior depend on the make, model year, engine and market. This article does not supply manufacturer specifications. Use the service information for your exact vehicle for any measured values.
Diesel engines differ in important ways. They have no spark ignition and they rely on high injection pressure and, in many cases, glow plugs or intake heaters. Diesel-specific causes are noted where relevant, but the spark checks below apply only to gasoline engines.
Stop first: conditions that call for professional diagnosis
Raw fuel smell, visible fuel leaks or wet fuel lines. Stop cranking and do not attempt to start the engine. Cranking that sounds unusually fast, light or even. This can indicate a broken timing belt or chain, or lost compression. On interference engines, continued cranking can cause further internal damage. Grinding, clunking or a starter that engages and then stops abruptly. Repeated long cranking attempts. These can overheat the starter. On gasoline engines, unburned fuel can wash cylinders and load the catalytic converter. Smoke, a burning smell or hot wiring near the battery, starter or fuse boxes. Any work on a high-pressure fuel system, such as gasoline direct injection or common-rail diesel. These systems hold dangerous pressure and need proper tools and procedures.
Record the pattern before testing
An intermittent fault leaves clues in when it happens. Before replacing anything, note the circumstances of each failure. Patterns point to specific systems far more efficiently than guessing at parts.
Engine temperature: only when hot (for example, a restart after a short stop), only when cold, or both. Time parked: only after sitting for hours or overnight, which points toward fuel pressure bleed-down. Weather and moisture: rain, washing or humidity, which suggests wiring or connector moisture. Fuel level: only when the tank is low. Key or fob behavior: whether the security or immobilizer light behaves differently on failed attempts, or whether a spare key works. What eventually fixes it: waiting, tapping something, wiggling a connector, cycling the key, or holding the accelerator pedal.
Likely causes and what separates them
Pattern or observation | Plausible area | What to check next; Fails mainly when hot, starts after cooling | Crankshaft or camshaft position sensor, ignition components, fuel pump | Check for crank signal and spark during a hot failure; look for stored sensor codes; Long crank after sitting, then starts normally | Fuel pressure bleeding off overnight (check valve, leaking injector or regulator) | Check whether fuel pressure holds after shutdown, using the manufacturer's test method; No fuel pump hum at key-on during a failure | Fuel pump relay, fuse, wiring, inertia or crash cutoff switch, pump | Check power and ground at the pump during the failure; Security light flashes or stays on abnormally | Immobilizer, key transponder or fob battery | Try the spare key; read body or security codes; Slow or uneven cranking | Battery, terminals, grounds, starter, cables | Clean and inspect connections; have the battery and starting system load-tested; Starts after holding the accelerator, or smells of fuel | Flooding | Confirm the vehicle's clear-flood procedure in the owner's manual; Fails in wet weather | Moisture in connectors, ignition components or wiring | Inspect connectors and harness routing for damage; Diesel: worse when cold | Glow plugs or intake heater, fuel gelling, air in fuel, low injection pressure | Professional diagnosis with diesel scan data
Checks to do while the engine won't start
Because the fault comes and goes, plan your checks so you can carry them out the moment it reappears. Several need only your eyes and ears.
1) Listen for the fuel pump at key-on
Many vehicles run the fuel pump briefly when the ignition is switched on, producing a faint hum from the tank area. If you hear it on good starts but not on failed ones, focus on pump power: the relay, fuse, wiring and the pump itself. Some vehicles only run the pump while cranking or after receiving a crank signal, so the absence of a prime is meaningful only if your vehicle normally primes.
2) Watch the tachometer while cranking
On many vehicles the tachometer needle moves slightly during cranking when the engine computer sees a crankshaft position signal. If the needle moves on good starts but stays flat during a failure, a crank sensor or its wiring becomes a strong suspect. This is not universal; some instrument clusters do not display cranking speed, so confirm what is normal for your car on a good start.
3) Note the security indicator
An immobilizer that fails to recognize the key typically allows cranking but blocks fuel or spark. Watch how the security light behaves on good and bad attempts. If a spare key starts the car reliably, the original key or its transponder is a likely cause. Behavior varies widely by manufacturer, so compare against the owner's manual.
4) Scan for trouble codes
Read stored and pending codes with a scan tool, ideally right after a failure, along with any freeze-frame data (a snapshot of sensor readings recorded when the code was set). Treat a code as a lead, not a verdict. For example, a crankshaft sensor code can come from the sensor, its connector, damaged wiring, or a problem with the reluctor ring it reads. A crank-no-start can also leave no code at all, especially in fuel delivery and immobilizer faults on some vehicles. A scan tool that can show live data such as engine speed during cranking, and that can read security or body modules, gives far more insight than a basic code reader.
5) Test for spark (gasoline engines)
Use a proper inline spark tester rather than holding a plug lead near metal, which can damage ignition electronics and give an unreliable result. Spark present during a failure points toward fuel, timing or compression. No spark on any cylinder points back toward the crank signal, ignition power, or the engine computer and its power supply.
6) Confirm fuel pressure and injector pulse
On port-injected engines with a test port, a fuel pressure gauge shows whether pressure reaches specification during the failure and whether it holds after shutdown. A test light designed for injector circuits can show whether the computer is pulsing the injectors. If pressure is fine but injectors are not being pulsed, look at the crank signal, security system and computer power. If pulse is present but pressure is low, look at the pump and its supply circuit. Direct-injection and diesel systems should be checked through scan data or by a professional because of their high operating pressures.
7) Check power and ground connections
Loose battery terminals, corroded grounds and a weak battery can let voltage dip during cranking enough to reset or starve modules, even when the starter still turns. Inspect and clean terminals and accessible ground straps. A load test at a parts store or workshop gives a more meaningful result than a voltmeter reading at rest. Also inspect the main relay or engine computer power relay circuit if your vehicle's service information identifies one.
Catching an intermittent fault
If the car starts every time you try to test it, a few methods help. Gently wiggle connectors and harness sections at the crank sensor, fuel pump relay and ignition switch while the engine idles. A stall or stumble when you move one section narrows the search. Monitor engine speed and fuel-related data with a scan tool on a good start so you have a baseline to compare against. Note any fix that works, such as waiting ten minutes or tapping the fuel tank, because that observation is itself diagnostic evidence.
Avoid replacing parts on suspicion alone. Crank sensors, fuel pumps and relays are common causes, but each produces a recognizable pattern when tested during the failure. Fitting parts without that confirmation often leaves the real fault, frequently a wiring or connection problem, in place.
When to hand it over
Take the vehicle to a qualified technician if any stop-first condition applies, if the basic checks do not separate fuel, spark, crank signal and security, or if the vehicle uses high-pressure fuel injection. Bring your record of when the failures happen and what you observed. Many workshops struggle to diagnose a fault they cannot reproduce, so a detailed pattern log, along with any codes and freeze-frame data you captured, can save diagnostic time.
Gaps in this guide
This article does not include manufacturer-specified fuel pressures, sensor resistance values, battery or cranking voltage thresholds, relay locations, or model-specific technical service bulletins. Those depend on the exact vehicle and should come from the vehicle's service information or a dealer.