Start with what the car is doing
If you have not read the codes yet, start here. Each page describes what the symptom feels like, which codes usually produce it, and the free checks to run before you buy anything.
- Car Shakes When Accelerating A shake under acceleration is almost always a misfire or an ignition part that is breaking down under load. It is not normal, and it gets worse.
- Car Cranks But Won't Start The starter turns the engine normally but it never fires. That separates it from a battery problem and points at spark, fuel, or the crank signal.
- Rough Idle The car idles unevenly but smooths out once you drive. That pattern usually means a vacuum leak, a dirty airflow sensor, or one weak cylinder.
- Engine Misfire Symptoms A misfire is a cylinder failing to burn its fuel. The car shakes, loses power, and dumps raw fuel into the converter, which is why a flashing light means stop.
- Car Stalls At Idle The engine dies when you stop but runs fine once you are moving. That points at an idle air problem, a dirty throttle body, or a vacuum leak.
- White Smoke From Exhaust Thin white vapor on a cold morning is normal. Thick white smoke that keeps coming once the engine is warm usually means coolant is entering the exhaust.
- Blue Smoke From Exhaust Blue smoke means the engine is burning oil. It is usually valve guides or worn rings, and it gets worse the longer you leave it.
- Car Smells Like Gas A raw fuel smell is a safety issue, not a nuisance. It can be an EVAP vapor leak or an actual liquid leak, and the second one is a fire risk.
- Car Smells Like Rotten Eggs That sulfur smell is hydrogen sulfide from the catalytic converter. A little is normal; a strong smell that keeps up usually means the converter is working overtime.
- Engine Ticking Noise A tick that follows engine speed is usually valvetrain or an exhaust leak. A tick on a cold start that fades can mean low oil pressure at the top of the engine.
- Grinding Noise When Braking Grinding under braking usually means the pads are worn past the friction material and the backing plate is cutting into the rotor. Stop driving and look.
- Car Overheating Overheating warps heads and blows gaskets in minutes, not weeks. Pull over and let it cool before you open anything.
- Heater Not Blowing Hot Air If the engine reaches normal temperature but the vents blow cool, the problem is in the heater core or the blend door, not the engine.
- Battery Dies Overnight A battery that goes flat overnight has a parasitic drain, not a bad battery, unless the battery is already old. Test the drain before buying anything.
- Check Engine Light Flashing A flashing check engine light means an active misfire is dumping raw fuel into the converter. Stop driving. This is the one warning you do not ignore.
- Check Engine Light After Refueling A light that comes on right after a fill-up is almost always an EVAP code from a loose or bad gas cap. Tighten it, clear the code, drive a full tank.
- Reduced Engine Power Message That message means the computer has cut throttle authority on purpose to protect the engine. The car will only creep. Read the codes before driving far.
- Car Jerks When Shifting A jerk at the shift point is the transmission, not the engine, unless it also misfires. Read the transmission module, not only the engine module.
- Clicking Noise When Turning A rhythmic click that speeds up with wheel rotation and gets worse in a tight turn is a worn CV joint. It will not fix itself.
- Car Vibrates At High Speed A vibration that appears above 50 mph and fades when you slow down is almost always wheel balance or a tire problem, not the engine.