Diagnose your car by the symptom you're seeing.

Common car symptoms

Pick what your car is doing. Each guide explains the symptom, how urgent it is, what to check first, and the OBD2 codes most likely behind it.

• Engine overheating Overheating means the cooling system can't shed heat fast enough — low coolant, a stuck thermostat, a failing water pump, a bad fan, or airflow blockage. Some related faults also set a coolant-temperature OBD2 code. 4 likely codes
• Engine misfire A misfire is when a cylinder fails to fire properly — you feel a stumble, shake, or loss of power, and the check-engine light may flash. It comes down to a missing ingredient in that cylinder: spark, fuel, or compression. 3 likely codes
• Reduced engine power / limp mode Reduced engine power — often shown as a dashboard warning or “limp mode” — is the computer deliberately capping power to protect the engine or transmission after it detects a fault. It's a protective response, not a random glitch. 10 likely codes
• Engine stalling Stalling is when the engine quits running on its own — at idle, coming to a stop, or while driving. It means the engine briefly lost the air, fuel, or spark it needs to keep running. 9 likely codes
• Transmission problems Transmission trouble shows up as harsh or delayed shifts, slipping, or a warning light, and often sets a transmission-related OBD2 code (P07xx and related). Causes range from low/old fluid and a faulty solenoid to a sensor feeding bad data. 5 likely codes
• Rough or unstable idle A rough idle means the engine runs unevenly when you're stopped — it shakes, the RPM needle bounces, or it feels like it might stall. It usually points to something disrupting the air, fuel, or spark balance at low RPM. 10 likely codes
• Hard starting / long crank Hard starting means the engine cranks longer than normal before it catches — often worst on a cold morning. It usually traces to fuel delivery, the air/fuel mixture, or a sensor feeding the engine computer bad startup data. 9 likely codes
• Hesitation when accelerating Hesitation is a momentary stumble, bog, or flat spot when you accelerate — the engine doesn't respond cleanly to the throttle. It usually means fuel, air, or spark isn't keeping up with demand. 10 likely codes
• Engine vibration or shaking Shaking has two big buckets: engine-related (a misfire or rough running, often with a code) and chassis-related (tires, wheel balance, motor mounts, or brakes, usually with no code). When the engine is the cause, an OBD2 code is the fastest clue. 5 likely codes
• Poor fuel economy A sudden drop in fuel economy usually means the engine is running too rich (too much fuel), working harder than it should, or a sensor is feeding the computer bad data so it miscalculates the mixture. 10 likely codes
• Engine knocking or pinging A metallic knock or “ping” under acceleration is often pre-ignition or detonation — fuel burning at the wrong moment in the cylinder. Carbon buildup, wrong fuel octane, overheating, or a bad knock sensor can all contribute. 6 likely codes
• Vacuum leak (hissing / whistling) A vacuum leak lets unmetered air into the engine, leaning out the mixture. The classic signs are a hiss or whistle under the hood, a rough or high idle, and lean OBD2 codes. It's one of the most common driveability faults. 3 likely codes
• Failed emissions / smog test Most emissions failures come down to a stored OBD2 code (often emissions-system related) or readiness monitors that haven't completed. The check-engine light being on is an automatic fail in most programs, regardless of how the car drives. 10 likely codes
• Exhaust smoke (black, white, or blue) Exhaust smoke color tells a story: black = running too rich (excess fuel), blue = burning oil, white = burning coolant or just startup steam. Black smoke often pairs with rich-mixture OBD2 codes, which are the easiest to chase first. 3 likely codes
• Grinding noise when braking A metal-on-metal grinding sound when you press the brakes almost always means the brake pads have worn down to the backing plate and are cutting into the rotor. Every mile you drive on them makes the repair more expensive. 0 likely codes
• Squealing or squeaking brakes A high-pitched squeal when braking is often the built-in wear indicator telling you the pads are near their limit. It can also be harmless surface rust or brake dust — the timing and conditions tell you which. 0 likely codes
• Soft or spongy brake pedal A brake pedal that sinks toward the floor or feels spongy usually means there's air or moisture in the hydraulic system, a fluid leak, or a failing master cylinder. Firm braking depends on an air-free, sealed system. 0 likely codes
• Car pulls to one side A steady pull to one side while driving straight usually points to alignment or uneven tire pressure; a pull only under braking points to a sticking caliper or uneven brakes. Each has a very different fix and cost. 0 likely codes
• Steering wheel shakes When the shake depends on speed, it's usually a tire balance or bent-wheel issue; when it happens only under braking, warped or uneven brake rotors are the usual cause. The trigger tells you where to look. 0 likely codes
• Whining noise when turning A whine or groan that gets louder as you turn — especially at full lock — usually points to low power-steering fluid or a failing power-steering pump. Air or a leak in the system is a common root cause. 0 likely codes
• Clicking noise when turning A rhythmic clicking or popping that shows up when turning at low speed — sharpest at full lock — is the classic sign of a worn outer CV joint, usually after its protective boot has torn and let grease out and grit in. 0 likely codes
• Car won't start, just clicks Rapid clicking when you turn the key usually means the battery is too weak to spin the starter; a single loud click with no crank more often points to the starter or its connections. A quick battery test separates the two. 0 likely codes
• Battery keeps dying A battery that repeatedly goes flat usually comes down to one of three things: an old battery that no longer holds charge, an alternator not recharging it while you drive, or a parasitic drain pulling power while the car is off. 0 likely codes
• AC not blowing cold When the AC blows warm, the most common cause is low refrigerant from a slow leak, followed by a compressor that won't engage or an electrical/control fault. Air conditioning is a sealed system, so it should never need topping up unless it's leaking. 0 likely codes
• Heater blowing cold air A heater that blows cold usually comes down to the engine not reaching temperature (a stuck-open thermostat), low coolant, air trapped in the system, or a clogged heater core. The heater borrows heat from the engine's coolant, so anything that starves it of hot coolant kills the heat. 0 likely codes
• Burning smell while driving A burning smell is your car flagging something hot that shouldn't be. The smell itself is the biggest clue: burnt oil, sweet coolant, hot brakes, or acrid electrical each point to a different system — and different urgency. 0 likely codes
• Clunking noise over bumps A clunk or knock when you hit bumps points to play in the suspension — most often worn sway-bar links, strut mounts, ball joints, or control-arm bushings. The looseness that makes the noise also affects handling. 0 likely codes
• Car vibrates at highway speed A vibration that appears in a specific speed band (often 55–70 mph) and smooths out above or below it is the signature of a tire balance or wheel problem. It can also come from a worn driveline or suspension part. 0 likely codes