Diagnose Your Car by Symptom
No trouble code? Start with what you're actually experiencing — a warning light, a noise, smoke, a smell, or how the car drives. Each page explains the likely causes, the OBD-II codes behind it, how serious it is, and what to do next.
Most car problems announce themselves as a symptom before they ever throw a code — a dashboard light, a new noise, a puddle, or a change in how it drives. Browse the 38 symptoms below, or snap a photo and let Au7o diagnose it for your exact vehicle at au7o.io.
Brake Warning Light On
Red brake light on? It can mean low brake fluid or a hydraulic leak, a real safety risk. Learn the causes, codes, and how urgent it is to get checked.
Engine Temperature Light On
Red temperature light on or gauge in the red? Your engine is overheating. Learn the causes, codes, and exactly what to do to avoid costly engine damage.
Flashing Check Engine Light
A flashing check engine light means an active misfire that can wreck your catalytic converter. Learn the causes, the codes, and exactly what to do now.
Gas Smell in Your Car: Causes, Dangers & What to Do
A gas smell in your car can be a loose gas cap or a dangerous fuel leak. Learn the common causes, how serious it is, what to check, and when to stop driving.
Grinding Noise When Braking: Causes & How Urgent It Is
Grinding when you brake usually means metal-on-metal contact from worn pads. Learn the causes, how dangerous it is, and why you shouldn't keep driving on it.
Oil Pressure Light On
Red oil pressure light on? It can mean dangerously low oil pressure. Learn the causes, codes, and why you should stop the engine to avoid major damage.
ABS & Traction Control Light On
ABS and traction control lights on together? Learn the most common causes, the OBD-II codes behind them, how serious it is, and what to do next.
Airbag (SRS) Light On
Airbag light staying on? Your SRS may not deploy in a crash. Learn the common causes, the codes behind it, and why you should get it checked soon.
Battery / Charging Light On
Battery light on while driving? It usually means a charging problem, often the alternator or belt. Learn the causes, codes, and how far you can drive.
Black Smoke From Exhaust: Causes of a Rich Fuel Mixture
Black exhaust smoke means your engine is burning too much fuel. Learn the common causes, the OBD-II codes involved, how serious it is, and what to do.
Blue Smoke From Exhaust: Why Your Car Is Burning Oil
Blue or gray exhaust smoke means your engine is burning oil. Learn the common causes, how serious it is, what you can check, and when to see a mechanic.
Burning Oil Smell in Your Car: Causes and What to Do
A burning oil smell usually means oil is leaking onto hot engine parts. Learn the common causes, how serious it is, what to check, and when to see a mechanic.
Car Cranks But Won't Start: Top Causes & Fixes
Engine cranks but won't fire up? Learn the common causes (fuel, spark, sensors), the likely OBD-II codes, and what an owner can check before calling a mechanic.
Car Pulls to One Side: Causes & How to Fix It
Car drifting or pulling left or right? Learn the common causes (alignment, tire pressure, brakes), how serious it is, and the quick checks to do before a shop.
Car Shakes at Highway Speed: Causes & How to Fix
Steering wheel or whole car shaking at highway speed? Learn the common causes (tires, bearings, rotors), how serious it is, and what to check before a shop visit.
Car Won't Start, Just Clicks
Car clicks but won't start? Learn what rapid vs single clicking means, the most likely causes (battery, terminals, starter), and exactly what to check first.
Check Engine Light On
Steady check engine light on? Learn the most common causes from a loose gas cap to O2 sensors, the OBD-II codes behind it, and how urgent it really is.
Clicking Noise When Turning: Causes & How Serious It Is
A clicking or popping noise when you turn usually points to a worn CV axle joint. Learn the causes, how urgent it is, and when to replace the axle.
Clunking Noise Over Bumps: Suspension Causes & Fixes
A clunk over bumps usually points to worn sway bar links, strut mounts, or control arm bushings. Learn the causes, how serious it is, and what to check.
Coolant Leak Under Car: Causes & Is It Safe to Drive
Green, orange, or pink puddle under your car? Learn what's leaking coolant, why overheating is the real danger, and whether it's safe to keep driving.
Engine Hesitation or Stalling: Causes & What to Do
Engine hesitating, bogging down, or stalling? Learn the common causes, the likely OBD-II codes, how serious it is, and what to check before you see a mechanic.
Engine Knocking or Ticking Noise: Causes & How Serious
Engine knocking or ticking ranges from a harmless lifter tick to serious rod knock. Learn the causes, real OBD-II codes, and how urgent each one is.
Hard or Harsh Shifting: Causes & How to Fix It
Transmission shifting hard, jerking, or clunking between gears? Learn the common causes, likely OBD-II codes, how serious it is, and what to check first.
Humming or Roaring Wheel Noise: Wheel Bearing Causes & Fixes
A humming or roaring noise that grows with speed usually means a worn wheel bearing or uneven tire wear. Learn the causes, ABS codes, and how serious it is.
Loss of Power While Driving: Causes & What to Do
Car suddenly losing power or stuck in limp mode? Learn the common causes, the likely OBD-II codes, how urgent it is, and what to check before you see a mechanic.
Oil Leak Under Car: Causes, How Bad It Is & Fixes
Found dark brown oil under your car? Learn the most common leak sources, how dangerous it is, what you can check yourself, and when to see a mechanic.
Rattling on Cold Start: Causes & How Serious It Is
A rattle that appears for a few seconds on cold start can be a harmless heat shield or a worn timing chain tensioner. Learn the causes and how urgent each is.
Rotten Egg Smell From Car: Sulfur Smell Causes & Fixes
A rotten egg or sulfur smell usually points to a failing catalytic converter or a rich-running engine. Learn the causes, how serious it is, and what to do.
Rough Idle: Why Your Engine Shakes at a Stop & Fixes
Rough idle? Learn the most common causes (vacuum leaks, misfires, dirty sensors), whether it's safe to drive, the likely OBD-II codes, and what to do next.
Squealing or Squeaking Brakes: Causes & When to Worry
Squeaky brakes can be a harmless rust film or a wear-indicator warning you pads are low. Learn the causes, how serious each is, and when to get them checked.
Sweet Coolant Smell in Car: Causes and What to Do
A sweet, maple-syrup smell means your car is leaking coolant. Learn the common causes, how serious it is, what to check, and when to see a mechanic.
Tire Pressure (TPMS) Light On
Tire pressure light on? Usually one tire is low, often from cold weather. Learn the causes, what a flashing TPMS light means, and how to fix it fast.
Transmission Fluid Leak: Red Fluid Causes & Fixes
Reddish fluid under your car and rough shifts? Learn what causes transmission leaks, how serious low fluid is, and what to check before damage sets in.
Transmission Slipping: Causes, Danger & Fixes
Transmission slipping or RPMs flaring without speed? Learn the common causes, likely OBD-II codes, how serious it is, and what to check before major repairs.
Transmission Whining Noise: Causes & How Serious It Is
A whining transmission often just means low or old fluid, but can signal worn bearings or a failing torque converter. Learn the causes and what to do first.
White Smoke From Exhaust: Steam or Blown Head Gasket
White exhaust smoke can be harmless steam or a blown head gasket. Learn how to tell the difference, what causes it, how serious it is, and when to stop driving.
Car AC Not Blowing Cold Air
AC blowing warm air? Learn the most common reasons your car's A/C stopped cooling - from low refrigerant to a bad compressor - and what to do next.
Puddle of Water Under Car: Normal or Should You Worry
Clear water under your car after running the AC? Learn when a water puddle is totally normal A/C condensation - and the few times it actually signals a problem.