Car Won't Start
A practical guide to the most common reasons a car will not start, from battery issues to fuel and ignition faults.
Automotive troubleshooting
Understand car symptoms before you replace parts.
Plain-English diagnostic guides that start with safety, explain likely causes, and help you talk to a mechanic with better notes.
High-intent symptom guides
Car Cranks But Won't Start
How to narrow down a normal-cranking no-start into fuel, spark, air, compression, or sensor problems.
Clicking Noise When Starting
What a single click or rapid clicking usually means when you try to start a car.
Check Engine Light Flashing
Why a flashing check engine light is urgent and what to do next.
Check Engine Light On But Car Runs Fine
Why a steady check engine light can still matter even when the car feels normal.
Car Shakes When Accelerating
Common causes of vibration under acceleration, from misfires to axle and mount problems.
Common OBD-II codes
P0300: Random or Multiple Cylinder Misfire
P0300 means the engine computer detected misfires that are not limited to one cylinder.
P0171: System Too Lean Bank 1
P0171 means the engine computer is adding fuel because it sees too much air or not enough fuel on bank 1.
P0420: Catalyst System Efficiency Below Threshold
P0420 means the catalytic converter is not storing oxygen as expected, or the sensor data makes it appear that way.
P0455: Large EVAP System Leak Detected
P0455 means the evaporative emissions system cannot hold pressure or vacuum during a leak test.
P0128: Coolant Temperature Below Thermostat Regulating Temperature
P0128 usually means the engine is warming up too slowly or not reaching the expected operating temperature.
P0135: Oxygen Sensor Heater Circuit Bank 1 Sensor 1
P0135 means the upstream oxygen sensor heater circuit is not working as expected.
P0301: Cylinder 1 Misfire Detected
P0301 means the engine computer detected a misfire on cylinder 1.
P0302: Cylinder 2 Misfire Detected
P0302 means cylinder 2 is misfiring often enough for the computer to flag a fault.
P0303: Cylinder 3 Misfire Detected
P0303 indicates a misfire isolated to cylinder 3.
P0304: Cylinder 4 Misfire Detected
P0304 means the engine computer has detected misfire activity on cylinder 4.
P0174: System Too Lean Bank 2
P0174 means the engine computer is adding extra fuel because bank 2 is running lean.
P0430: Catalyst System Efficiency Below Threshold Bank 2
P0430 means the catalytic converter monitor for bank 2 is below the expected efficiency threshold.
P0440: EVAP System Malfunction
P0440 means the evaporative emissions system failed a general leak or flow test.
P0442: Small EVAP Leak Detected
P0442 means the EVAP system has a small leak, often too small to see without testing.
P0456: Very Small EVAP Leak Detected
P0456 points to a very small leak in the fuel vapor system.
P0101: Mass Airflow Sensor Range or Performance
P0101 means the MAF sensor reading is outside the range the computer expects.
P0113: Intake Air Temperature Sensor Circuit High
P0113 means the intake air temperature signal is higher than expected electrically, often indicating an open circuit.
P0118: Engine Coolant Temperature Sensor Circuit High
P0118 means the coolant temperature sensor circuit is reading out of range, often as extremely cold.
P0507: Idle Control System RPM Higher Than Expected
P0507 means idle speed is higher than the computer expects.
P0700: Transmission Control System Malfunction
P0700 means the transmission control module has requested the engine computer to turn on the warning light.
P0740: Torque Converter Clutch Circuit Malfunction
P0740 means the torque converter clutch control circuit or operation is not behaving as expected.
P0172: System Too Rich Bank 1
P0172 means the engine computer is removing fuel because bank 1 appears to be running too rich.
P0102: Mass Airflow Circuit Low Input
P0102 means the mass airflow sensor signal is lower than expected.
P0441: EVAP Incorrect Purge Flow
P0441 means purge flow through the EVAP system is not what the computer expected.
P0443: EVAP Purge Control Valve Circuit
P0443 means the electrical circuit for the purge control valve has a fault.
P0496: EVAP Flow During Non-Purge
P0496 means fuel vapor is flowing when the EVAP purge system should be closed.
Parts and systems to understand
Battery
The battery stores electrical energy for starting and stabilizes voltage for the vehicle's electronics.
Starter Motor
The starter motor turns the engine fast enough for combustion to begin.
Alternator
The alternator recharges the battery and powers the electrical system once the engine is running.
Spark Plugs
Spark plugs ignite the air-fuel mixture inside gasoline engine cylinders.
Ignition Coils
Ignition coils turn low battery voltage into the high voltage needed to fire spark plugs.
Oxygen Sensors
Oxygen sensors monitor exhaust oxygen so the engine computer can adjust fuel mixture and catalyst checks.