Diagnosing why your car won’t start can be frustrating, but with a systematic approach, you can pinpoint the issue and potentially fix it yourself. This comprehensive Car Not Start Diagnosis Chart will guide you through the troubleshooting process, helping you identify whether the problem lies within the starter system, ignition, or fuel delivery. Remember, working on cars can be dangerous, so always proceed with caution and at your own risk.
Does the starter crank and turn over the engine when you turn the key?
If the starter doesn’t crank, first consider the basics. Ensure your automatic transmission is in Park (P) or Neutral (N). A faulty neutral safety switch might prevent the starter from engaging if the car isn’t properly in Park or Neutral. In manual transmission vehicles, a similar safety switch is often linked to the clutch pedal.
Does the starter spin at high speed or make a clashing or grinding sound without turning the engine?
This often indicates a problem with the starter motor’s pinion gear not properly engaging with the engine’s flywheel. The starter solenoid is responsible for pushing the pinion gear to mesh with the flywheel when you turn the key. If the solenoid is failing, corroded, or sticking, the starter might spin without cranking the engine. While some suggest tapping the starter with a hammer, this is rarely a permanent fix.
Another possibility is damaged teeth on the flywheel’s ring gear. Over time, teeth can break or wear down, especially in specific spots where the engine usually stops. Try manually rotating the engine slightly using a socket wrench on the crankshaft pulley to engage a different section of the flywheel. Safety first: Ensure the ignition is off and remove the wrench before attempting to start the car again.
Does the battery read above 12 Volts?
A digital voltmeter is essential for accurate battery diagnosis. A test light is insufficient to determine precise voltage. A healthy battery at rest should read around 12.6 to 12.8 volts. Turning on the headlights shouldn’t cause the voltage to drop below 12V significantly. A low reading suggests an undercharged battery or a battery unable to hold a charge.
If the voltage is low, attempt a jump start. If the car starts with a jump, the issue likely lies with the battery or the charging system. For non-sealed batteries, check the electrolyte levels and top up with distilled water if needed. A specific gravity tester can further assess battery health. Batteries have a limited lifespan, typically rated in months on the battery case. If your battery is old, shows signs of swelling, or gets excessively hot during charging, it may have a dead cell and needs replacement.
With the engine running (and the car in Park or Neutral with the parking brake engaged), check the battery voltage again. It should now read above 14V, indicating the charging system is working. If it remains below the initial battery voltage, suspect issues with the alternator or voltage regulator. The voltage regulator is often less expensive and easier to test. You’ll need a wiring diagram for your specific vehicle to identify the terminals for testing the alternator and regulator. Remember, the alternator needs power from the voltage regulator to generate an electric field and charge the battery. A common alternator failure is diode rectifier malfunction, which can lead to AC output instead of DC, causing battery charge and discharge cycles.
Have you cleaned the battery terminals and connections?
Often overlooked, corroded or loose battery terminals are a frequent cause of starting problems. If you hear only clicking from the starter relay or the engine cranks very slowly, check the battery terminals first. Loose connections can sometimes be temporarily improved by twisting the connector. However, the proper fix involves disconnecting the terminals, cleaning both the terminals and the inside of the connectors with a wire brush or sandpaper, and ensuring a tight, clean connection upon reassembly. Inspect and clean the ground connection point where the negative battery cable attaches to the vehicle’s chassis or engine block as well.
Cars utilize a starter relay to handle the high current flow to the starter motor, protecting the ignition switch. When you turn the key to “start,” the starter relay solenoid closes, sending power to the starter. A faulty starter relay or a blown fuse in the starter circuit can prevent power from reaching the starter. Caution: While risky and potentially damaging, in emergency situations, you can attempt to bypass the starter relay to directly power the starter for diagnostic purposes only. Similarly, you can bypass the ignition switch to activate the starter relay solenoid. Extreme caution is necessary when bypassing safety circuits like the neutral safety switch; ensure the vehicle is securely in Park or Neutral to prevent accidental movement.
If issues persist, removing and testing the starter motor outside the car is the next step. Always disconnect the negative battery cable before removing the starter to prevent electrical shorts and potential injury. Inspect the starter terminals for corrosion. To test the starter, use jumper cables to connect the starter case to the battery’s negative terminal and the starter’s main terminal to the battery’s positive terminal. The starter motor should spin and the pinion gear should extend.
Does the engine fire and try to catch, or does it just crank over endlessly without ever attempting to start?
If the engine cranks without any sign of firing, it indicates a lack of either spark or fuel. Prolonged cranking without any ignition attempts is not effective and may point to a more significant issue. If the engine initially tries to start but then just cranks without catching, suspect a fuel delivery problem. This could be due to fuel starvation or engine flooding. Some vehicles, especially older ones, might experience fuel evaporation in the carburetor after being parked, requiring extended cranking. Backfiring often suggests a timing issue, while an engine fire is a serious emergency requiring immediate attention with a fire extinguisher and indicates both a timing problem and a potential fuel leak.
Is a spark getting to the spark plugs?
Checking for spark at the spark plugs is crucial. Avoid directly touching spark plug wires while the engine is cranking to prevent electric shock. A safer method is to disconnect a spark plug wire, insert a screwdriver or spark tester into the boot, and hold the metal shaft near a grounded metal part of the engine while someone cranks the engine. A strong spark will be visible and audible as a bright, snapping spark. Checking one spark plug wire is usually sufficient initially, although issues with individual distributor contacts or spark plug wires are possible. Even with spark at only some plugs, the engine should still attempt to fire.
Alt Text: Car not starting diagnosis flowchart diagram, visually guiding users through troubleshooting steps for common car starting problems related to starter, battery, ignition, and fuel systems.
Is there a spark from the coil secondary?
If there’s no spark at the spark plugs or the spark is weak, the ignition coil is a likely culprit. Faulty spark plug wires with high resistance can also impede spark delivery, although this is less common for sudden no-start issues. To test the coil, use the screwdriver method on the coil’s high-voltage output terminal while grounding the coil’s negative terminal intermittently (touch and release) with a jumper wire. This simulates the points or electronic ignition signal and should produce a spark from the coil if it’s functional. Remember, coils are transformers and require a changing current to generate high voltage.
Do you have a mechanical distributor?
Mechanical distributors are recognizable by a round distributor cap with spark plug wires connected around the perimeter and a central coil wire. Underneath the cap, a rotor spins on a shaft driven by the engine’s camshaft, synchronizing spark delivery with cylinder timing. Spark timing is adjustable by rotating the distributor cap. Older systems use points, which mechanically interrupt the ignition circuit, while newer systems use Hall effect or magnetic pickups to signal the ignition control module.
Distributor rotor contacts and cap terminals can corrode or erode over time due to high voltage. Inspect and clean these contacts. For distributor caps with wire contacts, rotating the wires can sometimes extend their life. Magnetic pickups can fail, causing intermittent stalling or hard starting. Testing them requires a shop manual and is often more complex than replacing the relatively inexpensive part. For electronic distributors, consult the vehicle’s service manual and check for OBD II codes.
Reading 12V+ at coil primary?
With the ignition key in the “run” position, there should be battery voltage at the positive terminal of the ignition coil (primary winding). No voltage here means the coil isn’t receiving power, and you need to trace the ignition circuit back to the ignition switch and battery. Avoid directly jumping battery voltage to the coil for testing, as this can bypass safety circuits and potentially cause damage if a short circuit exists.
Obtaining a wiring diagram for your vehicle is essential for effective electrical troubleshooting. Modern cars have complex wiring harnesses, and wire colors can change within the harness. A wiring diagram allows you to systematically check voltages and continuity at connectors, pinpointing open circuits or shorts without unwrapping wiring bundles. Wiring diagrams are also invaluable for locating short circuits that cause fuse failures.
Is fuel getting to the last fuel filter in line?
Most cars have multiple fuel filters, including one in the fuel tank and a final filter just before the carburetor or fuel injection system. Professionals use fuel pressure gauges to verify correct fuel pressure. A simpler method is to remove the final fuel filter, check if it’s full of fuel, and attempt to blow air through it in the direction of fuel flow (indicated by an arrow on the filter). A clogged filter will be difficult or impossible to blow through. If the filter is empty, fuel isn’t reaching it, indicating a potential issue like an empty fuel tank, fuel pump failure, vapor lock, or a blockage in the fuel lines or earlier filters. Fuel leaks are also possible, often accompanied by a gasoline odor.
Caution: Testing fuel pump operation by redirecting fuel into a container and cranking the engine is highly flammable and dangerous. Modern cars have vapor recovery systems and vented gas caps. A blocked vapor recovery system or fuel tank vent can create a vacuum in the fuel tank, hindering fuel delivery. Try loosening or removing the gas cap to see if you hear a whooshing sound, which might indicate a vacuum issue. If the car starts or runs better afterward, a blocked vent is likely the problem.
Does the vehicle have electronic fuel injection or a carburetor?
Fuel-injected cars can suffer from injector failures or blockages, leading to starting and running problems. Fuel injection systems can be multi-point (individual injectors for each cylinder) or single-point (throttle body injection). Diagnosing fuel injection issues requires a vehicle-specific manual and knowledge of computer-controlled fuel delivery systems, including checking injector pulse signals, voltage supply, and related sensors.
For older cars with carburetors, using engine starting fluid sprayed into the carburetor throat (while holding the throttle open) can help diagnose fuel delivery problems. If the engine starts and runs briefly on starting fluid, it confirms a fuel or carburetor issue. Carburetors can become gummed up, and carburetor cleaner can be helpful, but a thorough cleaning often requires carburetor disassembly and rebuilding. However, carburetors can last for many years without needing a rebuild.
Does the car start, run a few seconds (or longer), and then stall?
If the engine starts briefly but stalls, it could be related to the spark control system, timing, or insufficient cranking speed. Slow cranking can be caused by a weak battery, corroded battery cables, or a failing starter motor. Try jump-starting the car, even if the battery voltage seems okay. The added power might increase cranking speed, and if the car starts and runs normally, a battery or starter issue is likely.
Check the OBD (On-Board Diagnostic) or pre-OBD/OBD II codes.
Your owner’s manual or a repair manual will provide instructions on how to retrieve diagnostic codes. Many cars, even pre-OBD II, can display fault codes by blinking the “check engine” light in a specific pattern. OBD II systems have a diagnostic connector that can be accessed with a code reader. Some auto parts stores offer free code reading services. Code readers are also available for purchase, ranging from basic code readers to more advanced scanners with code descriptions.
Does the engine fire right up but stall as soon as you release the key back to the “run” position?
This symptom strongly suggests a faulty ignition switch. However, ignition switches can be expensive, so thorough diagnosis with a wiring diagram is recommended before replacement. In an extreme emergency situation where immediate mobility is crucial, you could “hotwire” the car by bypassing the ignition switch’s “run” circuit. This involves connecting the ignition system’s “run” circuit directly to a power source, effectively keeping it powered even when the key is released from the “start” position. Caution: This is a temporary emergency measure and may cause damage or battery drain if the “run” circuit remains active when the engine is off.
If the car stalls in the rain or damp weather, it’s very likely related to the high voltage ignition components: coil, wires, or distributor cap.
Moisture significantly reduces insulation resistance, making it easier for high voltage spark to arc to ground. Stalling or poor running in wet conditions is a common indicator of a cracked coil, cracked distributor cap, or failing spark plug wires or boots. At night, while the engine is running in Park or Neutral with the parking brake engaged, open the hood and inspect for visible sparks arcing from ignition components to ground. Any arcing is abnormal and indicates a problem.
If the car stalls or runs poorly after driving through puddles, water splash may have reached the distributor or coil, causing excessive arcing until it dries. Distributors mounted low at the front of the engine are particularly vulnerable and should have a protective shield. If drying off ignition components resolves the issue, replace or shield the affected parts or avoid driving through deep puddles.
If the car stalls when warm, possible causes range from a simple incorrect idle setting (on older cars) to a failing ignition computer (on newer cars).
Investigate standard fuel delivery problems, as fuel issues can always cause stalling. Vacuum leaks in the intake manifold or vacuum lines can also lead to stalling and poor performance, as can various sensor failures. Modern cars utilize numerous sensors, including O2 sensors and MAP sensors. Sensor failures can prevent starting or cause poor running, hard starting, and reduced fuel economy. A sudden drop in fuel mileage without changes in driving habits is a strong indicator of a potential sensor or engine management issue.
If the car stalls when cold, a stuck choke (on carbureted engines) causing an overly lean mixture should be checked first. The EGR (Exhaust Gas Recirculation) valve can also contribute to cold stalling, as can vacuum leaks, especially in older vehicles. A method to check for vacuum leaks is to carefully spray engine starting fluid around vacuum lines and the intake manifold while the engine is running. If the engine speed increases (RPM surge), it indicates that the starting fluid is being drawn into the engine through a vacuum leak. Caution: This method carries a fire risk, even with a cold engine. Vacuum gauges are a safer method for detecting vacuum leaks.
In some cases, intermittent cold starting problems can be caused by temperature-sensitive electronic components like the spark control computer. If warming up the computer module resolves the issue, component replacement is usually necessary.
