Bad alternator: dimming lights, battery warning, stalling. Bad fuel pump: no start, stalling, loss of power under load. Bad catalytic converter: rattling, sulfur smell, failed emissions. Bad wheel bearing: humming or grinding that increases with speed. Bad ignition coil: misfire, rough running, flashing check engine light.
Recognizing symptoms of a failing part before it fails completely prevents breakdowns, expensive secondary damage, and unsafe driving conditions. Use the AI diagnosis tool with your specific symptoms for a ranked cause list.
Worn spark plugs cause rough idle, hesitation under acceleration, reduced fuel economy, hard starting, and a flashing or steady check engine light with misfire codes P0300 through P0308. Spark plugs fire thousands of times per minute — over tens of thousands of miles, the electrode erodes and the gap widens, weakening the spark. Symptoms appear gradually, so many drivers do not notice until fuel economy drops or a misfire code appears. Replacement cost: $20 to $60 in parts for a 4-cylinder engine DIY, $100 to $250 at a shop. Replace at the manufacturer interval: 30,000 miles for copper, 60,000 to 100,000 miles for iridium or platinum.
A stuck-open thermostat causes the engine to run cold, the temperature gauge to stay low, poor fuel economy, and the heater to blow lukewarm air. A stuck-closed thermostat causes rapid overheating, the temperature gauge to spike into the red, and steam from under the hood. The thermostat is a simple valve that opens at a specific temperature to allow coolant flow — when it fails, it fails in one direction. P0128 is the common code for a thermostat stuck open. Replacement cost: $10 to $30 in parts DIY, $150 to $300 at a shop. A $15 part that prevents a $2,500 head gasket failure.
Worn brake pads cause squealing when braking (wear indicators touching the rotor), grinding when braking (metal backing plate contacting the rotor), longer stopping distances, a soft or spongy pedal, and vibration through the pedal. Some pads have wear indicators that squeal at light braking pressure to warn you before the metal contacts the rotor. Once you hear grinding, the pads are gone and the rotor is being damaged. Replacement cost: $30 to $100 in parts per axle DIY, $150 to $350 at a shop. Replace pads when friction material reaches 3mm thickness — do not wait for grinding.
Dim headlights, a battery warning light on the dashboard, slow or clicking starts, electrical accessories working slowly or not at all, and eventually a dead battery and stalling. A failing alternator cannot keep the battery charged — the car runs on battery power alone until it dies, usually within 30 to 60 minutes of driving.
Symptoms of a failing fuel pump include difficulty starting, sputtering at highway speeds, loss of power under load, stalling, and a whining noise from the fuel tank. The car may start fine when cold but fail to restart when warm. A fuel pressure test confirms the diagnosis — healthy pumps maintain 30 to 80 PSI depending on the vehicle.
A bad wheel bearing produces a humming, growling, or rumbling noise that gets louder with speed and changes when turning. The noise typically starts around 30 to 40 MPH and sounds like driving over rumble strips. To identify which wheel, swerve left and right at speed — the noise gets louder when you load the bad bearing.
Only for a short distance. A bad alternator means the car is running entirely on battery power. Depending on the battery condition, you have roughly 15 to 60 minutes of driving time before the battery dies and the engine stalls. Drive directly to a repair shop — do not use accessories like AC, radio, or headlights to maximize remaining battery life.
Reviewed by Vukašin Herbez — Automotive Editor · Last updated 2026-10-14