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 98 symptoms below, or snap a photo and let Au7o diagnose it for your exact vehicle at au7o.io.
Brake Pedal Goes to the Floor: Causes & Urgent Fixes
A brake pedal that sinks to the floor is a safety emergency. Learn the four main causes, why it's dangerous, and exactly what to do right now.
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.
Burning Plastic or Electrical Smell in Car: Causes & Fixes
A burning plastic or electrical smell often means melting wire insulation or a short. Learn the likely causes, fire risk, and what to check before driving.
Car Dies While Driving Then Restarts: Causes & Fixes
A car that dies while driving then restarts usually means a failing crank sensor, fuel pump, or ignition switch. Here's how to pinpoint it safely.
Car Overheats on Highway but Not in Town: Causes & Fixes
Overheating at highway speed but fine in town usually means a clogged radiator, lazy thermostat, or worn water pump. Here's how to pin it down safely.
Car Won't Go Into Gear: Causes, Fixes & What to Check
A car that won't go into gear often means low transmission fluid, a clutch hydraulic issue, or a shift solenoid fault. See causes, codes, and fixes.
Engine Overheats in Stop-and-Go Traffic: Causes & Fixes
Overheating in traffic but not on the highway usually points to a cooling fan, thermostat, or low coolant. Learn the causes, codes, and what to do.
Engine Revs but Car Doesn't Move: Causes & Fixes
Engine revs but the car won't move? Top causes: low transmission fluid, a worn clutch, torque converter failure, or a broken axle — and what to do next.
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.
Exhaust Fumes Smell Inside the Cabin: Causes & Dangers
Smelling exhaust inside the car can mean an exhaust leak or fuel vapor leak. Learn the causes, the carbon monoxide risk, and what to do right away.
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.
Milky Oil: Coolant Mixing With Engine Oil Causes & Fixes
Milky, creamy oil usually means coolant is leaking into your engine. Learn the common causes, how serious it is, and what to check before you drive.
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.
AdBlue / DEF Warning: Causes, No-Restart Countdown & Fixes
AdBlue or DEF warning with a no-restart countdown? Learn what's wrong with the SCR system, the real OBD-II codes, and how to avoid a no-start lockout.
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.
Battery Light Comes On and Off: Causes & What to Do
Battery light flickering on and off? It usually points to worn alternator brushes, a failing regulator, or a slipping belt. Here's how to pin it down.
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.
Brake Pedal Pulsates When Braking: Causes & Fixes
A pulsating brake pedal usually means uneven rotor wear, pad deposits, hub runout, or a sticking caliper. Here are the likely causes and what to do next.
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 Battery Keeps Dying Overnight: Causes & How to Fix
Battery dead every morning? Learn the top causes from parasitic drain to a bad alternator or battery, and how to track down what's killing it.
Car Bounces After Bumps: Causes, Fixes & What It Costs
Car keeps bouncing after bumps? Worn shocks or struts are the top cause, followed by weak springs and bad strut mounts. Here's how to test it and fix it.
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 Heater Not Blowing Hot Air
Heater blowing cold air? The most common causes are low coolant, a stuck thermostat, or a bad blend door actuator - and how to pinpoint yours fast.
Car Hesitates or Bogs Down When Accelerating: Causes
Hesitation or bogging on acceleration usually means a fuel, air, or ignition problem. Learn the most likely causes, codes, and what to do next.
Car Jerks or Surges When Accelerating: Causes & Fixes
Car jerking or surging on acceleration usually points to a misfire, fuel delivery, or air-fuel issue. Learn the likely causes, codes, and what to do next.
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 Rolls Back in Drive on a Hill: Causes & What to Do
Car rolling back in Drive on hills? Low transmission fluid, a worn torque converter, or weak clutches are common causes — here's what to check first.
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 Shakes at Idle but Smooths Out Driving: Causes & Fixes
Car shaking or vibrating at idle? Misfires, worn motor mounts, vacuum leaks, and dirty throttle bodies are the usual causes. Here's how to narrow it down.
Car Stuck in Park: Causes & How to Release the Shifter
Shifter stuck in Park? Usually a bad brake light switch, interlock solenoid, or shifter cable bushing. Here's how to check it and get moving safely.
Car Vibrates When Accelerating: Causes & Fixes
Vibration that appears when you press the gas and fades when coasting usually means a worn inner CV joint, motor mount, or driveshaft. Here's how to tell.
Car Won't Accelerate in Limp Mode: Causes & How to Fix It
Car stuck in limp mode and won't accelerate? See the most likely causes — turbo boost faults, throttle body, pedal sensor — and the right next step.
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.
Coolant Reservoir Bubbling: Causes & What to Do
Bubbling in your coolant reservoir can mean trapped air, a weak radiator cap, or head gasket trouble. Here's how to tell them apart and what to do next.
Cruise Control Stopped Working: Causes & Fixes
Cruise control stopped working? A faulty brake light switch, blown fuse, or wheel speed sensor is usually to blame. Here's how to pinpoint and fix it.
Dashboard Lights Flickering: Causes & What It Means
Flickering dash lights usually mean a charging or grounding issue. Learn the common causes, how serious it is, and what to check first.
Delayed Shifting or Transmission Lag: Causes & Fixes
Delayed shifting or a lag before the transmission engages usually means low/old fluid or a solenoid issue. Learn the causes, codes, and what to do.
Door Locks Lock and Unlock by Themselves: Causes & Fixes
Door locks cycling on their own? A worn lock actuator switch, sticky key fob, or a glitching body control module is usually why — here's how to pin it down.
DPF Light On: Diesel Particulate Filter Causes & Fixes
DPF warning light on your diesel? Learn what's clogging the filter, the real OBD-II codes, how to trigger a regen, and when it needs a mechanic.
Droning Noise at Highway Speed: Causes & Fixes
A drone or hum that rises with speed usually means a wheel bearing or uneven tire wear. Learn how to tell them apart and how urgent the fix is.
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.
Engine Stalls at Idle: Common Causes and What to Do
An engine that dies at idle usually means a vacuum leak, dirty throttle body, or failing idle control. See the likely causes, codes, and fixes here.
EPC Light On: Causes, Codes & Is It Safe to Drive
EPC (Electronic Power Control) light on your VW or Audi? Learn the most common causes, real OBD-II codes, and whether it's safe to keep driving.
Exhaust Rattle at Idle: Causes & How to Fix It
A metallic rattle from under your car at idle usually means a loose heat shield, broken hanger, or failing catalytic converter. Here's how to pin it down.
Fuel Gauge Stuck or Jumping: Causes & Fixes
Fuel gauge stuck, jumping, or reading wrong? It's almost always a worn fuel-tank sending unit. See the common causes, what to check, and how to fix it.
Gear Shifter Loose or Not Engaging: Causes & Fixes
A loose, floppy gear shifter usually means a worn shift cable bushing or detached cable — often a cheap fix. Learn the likely causes and the next step.
Glow Plug Light Flashing: Diesel Causes & What to Do
Glow plug light flashing on your diesel? A flashing coil signals a fault, not just preheat. Learn the common causes, real codes, and what to do.
Grinding Noise When Shifting Gears: Causes & Fixes
Grinding when you shift usually means worn synchros, a dragging clutch, or low fluid. See the likely causes, how urgent it is, and what to do next.
Grinding Noise When Turning: Common Causes & Fixes
Grinding noise when turning? It usually means a worn wheel bearing, failing CV joint, or bent dust shield. Here's how to tell them apart and what to do.
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.
Headlights Flickering: Causes, Fixes & What to Check
Headlights flickering? It's usually a failing alternator, bad ground, or loose connector. Learn how to pinpoint the cause and what the repair costs.
Hissing Noise From the Engine Bay: Causes & Fixes
A hiss under the hood often means a vacuum leak or escaping coolant. Learn which is which, how dangerous it is, and what to check before you drive.
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.
No Crank, No Dashboard Lights: Causes & What to Check
Car totally dead with no crank and no dash lights? Learn the top causes from a dead battery to corroded terminals and grounds, plus what to check first.
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.
Popping Noise When Turning: Causes & Fixes
A pop or click that gets louder in tight turns usually means a worn CV joint. Learn the likely causes, the safety risk, and when to get it checked.
Power Steering Fluid Leak: Causes, Signs & What to Do
Spotting a power steering leak? Learn the common sources, whether it's safe to drive, and how to tell hydraulic from electric steering problems.
Power Steering Warning Light: Causes & Is It Safe
Power steering light on and steering feels heavy? Learn the common causes for EPS and hydraulic systems, real codes, and whether it's safe to drive.
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.
Red Fluid Leaking Under Your Car: What It Means
Red or pink fluid under your car is usually transmission or power steering fluid. Learn how to tell them apart and how serious the leak 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.
RPM Surging or Fluctuating at Idle: Causes & Fixes
A hunting, surging, or bouncing idle usually means a vacuum leak, dirty throttle body, or idle control fault. Learn the causes, codes, and fixes.
Service Engine Soon Light: Causes, Codes & What to Do
Service Engine Soon light on? Learn what it really means, the most common causes, real OBD-II codes, and whether you can keep driving safely.
Speedometer Not Working: Causes & How to Fix It
Speedometer not working? Most cases are a failed speed sensor, ABS tone ring, or cluster stepper motor. Learn how to find the cause and fix it.
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.
Steering Feels Loose or Car Wanders: Causes & Fixes
Steering feels loose or your car wanders? Worn tie rods, ball joints, or bad alignment are the usual causes. Here's how to pin it down and what it costs.
Steering Wheel Hard to Turn: Causes & Fixes
Steering wheel stiff or hard to turn? Low power steering fluid, EPS failure, a worn pump, or a slipping belt are the usual causes. Here's what to check.
Steering Wheel Shakes When Braking: Causes and Fixes
A steering wheel that shakes when braking usually means uneven (warped) front rotors. Learn the real causes, what to check, and when to see a mechanic.
Sunroof Leaking Water: Causes, Fixes & Drain Cleaning
Sunroof leaking water? Clogged drain tubes cause most leaks, followed by worn seals and misaligned glass. See how to find the source and what fixes cost.
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.
Ticking Noise When Accelerating: Causes & Fixes
Ticking that speeds up with RPM usually points to a lifter, low oil, or an exhaust leak. Learn the likely causes, how serious it is, and what to check.
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.
Tire Wearing on Inside Edge: Causes & How to Fix It
Tires wearing on the inside edge usually mean camber or toe is out of spec, or worn suspension parts. Here are the causes, costs, and what to ask the shop.
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.
Water Leaking Into Car Floor: Causes & Fixes
Water pooling on your car floor? The most likely causes—clogged AC and sunroof drains, heater core leaks—plus what to check first and when to see a shop.
Whining Noise When Accelerating: Causes & Fixes
A whine that rises with acceleration often points to the power steering pump, a pulley, the alternator, or the differential. Here's how to narrow it down.
Whistling Noise From the Engine: Causes & Fixes
An engine whistle usually means a vacuum, PCV, or boost leak letting air sneak in. Learn the likely causes, how serious it is, and what to check.
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.
Belt Squeal on Cold Start: Causes & Fixes
A loud squeal at startup that fades as the engine warms usually means a slipping or glazed serpentine belt. See the causes and how to fix it.
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.
Check Engine Light After Getting Gas: Causes & Fixes
A check engine light right after filling up is usually a loose gas cap or a stuck EVAP purge valve. Here's how to clear it yourself before paying a shop.
Key Fob Stopped Working: Causes & Easy Fixes
Key fob suddenly dead? Learn the most common causes from a dead coin battery to a blown fuse, plus quick fixes before you visit the dealer.
Musty or Moldy Smell From Car A/C Vents: Causes & Fixes
A musty, mildew smell when you turn on the A/C usually means mold on the evaporator or a dirty cabin filter. Here are the causes and how to clear it out.
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.