How to Diagnose and Fix a Car That Won't Start
Systematically diagnose and revive a non-running budget car by testing the battery, starter, ignition spark, fuel delivery, and engine air intake to get it idling.
- Read the tools, materials, safety notes, and steps below for free.
- Tap Start Camera Coach when you want hands-free voice help.
- Allow camera and microphone. Point your phone at the work; the coach watches your progress and tells you the next step.
🧰 Required Tools & Equipment
- ✓ Digital multimeter
- ✓ Spark plug socket and ratchet set
- ✓ Inline spark tester
- ✓ OBD-II code scanner
- ✓ Wire brush
- ✓ Jump starter pack or jumper cables
📦 Materials & Supplies
- ✓ Aerosol starting fluid
- ✓ Electrical contact cleaner
- ✓ Fresh unleaded gasoline (2 gallons)
- ✓ Dielectric grease
⚠️ Safety Checkpoints & Warnings
- • Keep a Class B dry-chemical fire extinguisher within arm's reach when testing fuel lines and ignition components.
- • Ensure transmission is set to Park (or Neutral on manual) with wheels firmly chocked before attempting to crank.
- • Work outdoors or in a garage with wide-open bay doors to prevent toxic carbon monoxide buildup.
📋 Step-by-Step Guide (6 Steps)
Test Battery Health and Clean Corroded Terminals
Set your multimeter to 20V DC and measure across the battery posts; a healthy resting battery must read at least 12.4V to 12.6V. If voltage drops below 10.0V while attempting to crank, jump or charge the battery. Unbolt oxidized terminals, scrub lead posts to a bright metallic shine using a wire brush, and tighten clamps firmly.
- • Multimeter displays a steady 12.4V+ resting voltage.
- • Battery terminal posts and clamps show bare, shiny metal without powdery white/green buildup.
- • Testing voltage on the clamp instead of directly on the lead post, hiding poor terminal contact.
- • Over-tightening battery terminal bolts until the soft lead stretches or snaps.
Pull Diagnostic Trouble Codes (DTCs)
Locate the 16-pin OBD-II diagnostic port beneath the driver's side dashboard and plug in your code scanner. Turn the ignition key to the ON position without cranking the engine to allow the computer modules to power up. Read all stored, pending, and permanent codes, noting issues related to the crankshaft position sensor, camshaft sensor, or security immobilizer.
- • Scanner successfully establishes communication with the ECU.
- • Fault codes or a 'No Codes Found' status appear on the scanner screen.
- • Scanning with the key in the accessory position instead of the full ON position, preventing ECU communication.
- • Immediately clearing codes before writing them down for reference.
Verify Ignition System Spark
Disconnect an ignition coil or spark plug boot and install an inline spark tester between the boot and the spark plug. Have an assistant crank the engine for 3 to 5 seconds while you observe the tester's transparent window. Look for a rhythmic, bright blue-white spark; a dim yellow spark or total absence indicates bad coils, plug wires, or a faulty crank sensor.
- • Visible bright flashing light pulsing inside the spark tester window while the engine cranks.
- • Holding the spark wire directly by hand, resulting in a painful 20,000+ volt shock.
- • Assuming all cylinders have spark after only testing a single cylinder on multi-coil setups.
Perform Starting Fluid Fuel Delivery Test
Turn the ignition to ON and listen near the rear seat or gas tank for a 2-second hum from the electric fuel pump priming. Loosen the intake boot clamp, pull the rubber duct back slightly from the throttle body, and spray a 2-second burst of starting fluid directly past the throttle plate. Immediately crank the engine: if it starts, stumbles, and dies, your ignition and compression are good, pinpointing a fuel delivery failure.
- • Engine briefly fires up and revs for 1 to 3 seconds before stalling.
- • Spraying excessive amounts of starting fluid, which washes oil off cylinder walls and risks an intake backfire.
- • Spraying before the mass airflow sensor instead of directly into the throttle body bore.
Inspect Air Intake Path and Throttle Valve
Unlatch the engine air cleaner box, pull out the pleated air filter, and inspect the lower housing for rodent nests, acorns, or debris restricting airflow. Look into the throttle body barrel to ensure the throttle butterfly valve rotates smoothly without sticking against oily carbon sludge. Wipe away heavy carbon deposits along the perimeter using a shop towel lightly dampened with intake cleaner.
- • Air cleaner housing and intake ducts are completely clear of physical obstructions.
- • Throttle plate moves smoothly through its full travel and returns cleanly to its resting stop.
- • Forcing drive-by-wire electronic throttle plates open manually with screwdrivers, stripping internal plastic gears.
- • Spraying harsh solvent directly into electronic sensor openings.
Add Fresh Fuel and Execute Controlled Start
Pour 2 gallons of fresh unleaded gasoline into the tank to dilute any varnish or old fuel that sat during storage. Cycle the ignition key between OFF and ON three times for 3 seconds each to fully prime fuel lines and build rail pressure. Turn the key to crank in 5-second intervals, allowing the starter motor to rest 15 seconds between tries until the engine catches and settles into an idle.
- • Tachometer needle rises steadily to 800-1200 RPM without dying.
- • Exhaust note remains consistent and engine runs smoothly on its own fuel supply.
- • Cranking the starter continuously for over 15 seconds, which overheats and burns out the starter motor windings.
- • Running completely varnished fuel through injectors, causing immediate injector clogs.
Credit: inspired by an original tutorial by ChrisFix — watch the original on YouTube.
Don’t have everything?
- No Inline spark tester? Spare known-good spark plug with threaded body held firmly against unpainted engine ground
- No Battery terminal wire brush? Coarse steel wool or 120-grit sandpaper wrapped around a screwdriver shaft
- No Starting fluid (ether)? Non-chlorinated flammable aerosol brake cleaner