Diagnostics · Free tool
Free Car Symptom Diagnosis Tool
Pick what the car is doing. The tool lists the common causes, the sign that points to each one and how urgent the problem is.
- Free to use
- No signup
- Runs in your browser
What is the car doing?
Starting
Engine
Brakes
Running costs
Leaks and smells
Transmission
Comfort
Act soon
Listen to what it does when you turn the key
Common causes of car won’t start
- 01
Weak or dead battery
A rapid clicking, dim dashboard lights or no response at all.
- 02
Loose or corroded battery terminals
Power cuts in and out, or the car starts after the clamps are moved.
- 03
Failed starter motor
One solid click and the lights stay bright, but the engine does not turn.
- 04
No fuel reaching the engine
The engine cranks at normal speed but never fires.
- 05
Ignition or spark fault
Cranks normally, may cough, and a check engine code is often stored.
These are the usual causes, in the order most people check them. Only an inspection can confirm which one applies to your car.
Common causes, not a diagnosis of your car. Read the disclaimer. Updated .
02Your result
What your symptom diagnosis result means
A symptom has several possible causes. The result narrows the list and tells you which to rule out first.
- 01
The urgency
Whether you can keep driving, should have it seen soon or should stop now. Treat a stop ruling as final.
- 02
What to check first
The observation that splits the causes apart. It usually costs nothing and takes a minute.
- 03
The common causes
The usual explanations, each with the sign that points to it. They are possibilities, not a diagnosis.
What to do next
Read the stored codes
If a code is stored, it will narrow the list further than any symptom can.
OBD2 code lookupPrice the likely repair
Build an estimate from parts and labor before you call a shop.
Repair cost estimatorBrowse symptoms in the guide
What you can see, hear, feel or smell, and what each points to.
Diagnose by symptom
03How it works
How the symptom diagnosis tool works
Diagnosis by symptom is a process of elimination. The tool sets out that process for the most common complaints.
Symptom → first check → the sign that tells the causes apart → the most likely cause → confirm by inspection
Each step removes possibilities. The cause is whatever is left.
What each term means
- Symptom
- What the driver notices: a noise, a smell, a vibration, a warning light or a car that will not start.
- First check
- A simple observation that separates one group of causes from another.
- Telltale sign
- The detail that points to one cause over the others.
- Urgency
- How soon the car needs attention, from low risk to stop now.
What the tool assumes
- It covers 11 common symptoms on gasoline cars.
- Causes are the usual ones a mechanic checks first, not a ranking by probability for your car.
- One symptom can have more than one cause at once.
- Only an inspection, and often a code read, can confirm a fault.
04Worked example
Symptom diagnosis example: an overheating engine
The temperature gauge is climbing in slow traffic, then falls back once the car is moving again.
- 01
Pick the symptom
Engine overheating
Stop and check
- 02
Do the first check
Pull over, switch off and let the engine cool
Stop
- 03
Check the coolant level
When cold: at the mark, so low coolant is unlikely
Normal
- 04
Match the telltale sign
Overheats in traffic and at idle, cools at speed
Fan
- 05
Likely cause
A cooling fan that is not running
Failed fan
Most likely cause
Cooling fan
Air through the radiator at speed hides a failed fan. In traffic there is no airflow, so the temperature climbs. A shop can confirm it in minutes.
Try your own numbers05Background
How to diagnose a car problem by symptom
Mechanics start with the same questions every time: what does it do, when does it do it and what changed? You can ask them too. The answers do not replace an inspection, but they turn a vague worry into a short list.
| Symptom | How urgent | Check first |
|---|---|---|
| Car won’t start | Act soon | Note whether the engine cranks. A click with no cranking points one way, cranking without firing points another. |
| Rough idle or shaking | Act soon | Read the stored codes. A misfire code names the cylinder, which narrows the search at once. |
| Engine overheating | Stop and check | Pull over when it is safe and switch off. Do not open the cooling system while it is hot. |
| Check engine light is on | Act soon | Check whether the light is steady or flashing. Flashing means stop driving. |
| Brakes squeal | Act soon | Note when it happens. A squeal that stops when you press the pedal is different from one that starts when you do. |
| Shakes when braking | Act soon | Notice where you feel it. Through the pedal and steering wheel points to the front brakes. |
| Fuel economy has dropped | Low risk | Measure it properly over two or three tanks before you look for a fault. |
| Oil leak or burning smell | Act soon | Check the oil level on the dipstick now, and top it up if it is low. |
| White smoke from the exhaust | Act soon | Watch whether it clears once the engine is warm, and check the coolant level when the engine is cold. |
| Transmission slipping | Act soon | Check the transmission fluid level and condition if your car has a dipstick for it. |
| A/C not blowing cold | Low risk | Check that air is actually flowing from the vents. Weak airflow is a different fault from warm air. |
If the check engine light is on, read the code before anything else. A code names the system that reported the fault, which a symptom alone cannot do. Where there is no code, work from the symptom.
Some symptoms mean stop. An engine that is overheating, a red oil pressure warning or brakes that grind should end the journey. See diagnose a problem by symptom in the maintenance guide for the wider list.
06When to use it
When to use a car symptom diagnosis tool
When something has changed and there is no code to read, or you want to understand a code you have.
When something feels different
Put a name to a noise, smell or vibration before it gets worse.
Before calling a shop
Describe the problem in the terms a mechanic uses, and get a better answer.
When no code is stored
Many faults, such as brake noise or a leak, never set a code.
To decide what to try first
Rule out the simple, cheap causes before paying for a diagnosis.
Mistakes to avoid.
- Treating the first cause on the list as the answer without checking the sign that goes with it.
- Carrying on driving with a symptom the tool marks as stop.
- Replacing parts one at a time in the hope of hitting the fault.
- Describing the symptom to a shop as a diagnosis. Tell them what the car does, not what you think is broken.
07Related guides and tools
Keep going from here.
Guides that go with this tool
Related tools
- OBD2 Code LookupEnter any OBD2 code for its meaning, severity, likely causes and first steps.
- Check Engine Light CheckerSteady or flashing? Find out whether it is safe to keep driving and what to do next.
- Repair Cost EstimatorBuild a repair estimate from parts, labor hours and shop rate, and compare it with national averages.
08Questions
Car Symptom Diagnosis Tool: your questions.
How do I diagnose a car problem myself?
Start with what the car does and when. Read any stored codes, then use the first check for your symptom to separate the likely causes. Confirm the cause by inspection before you buy parts.
Why won’t my car start?
The usual causes are a weak battery, loose or corroded battery terminals, a failed starter or a lack of fuel or spark. A click with no cranking points to the battery or starter. Cranking without firing points to fuel or spark.
Why is my car overheating?
Common causes are low coolant, a stuck thermostat, a failed cooling fan and a leaking head gasket. Stop and let the engine cool, and never open the cooling system while it is hot.
Can a tool diagnose my car without a mechanic?
It can narrow the possibilities and tell you how urgent the problem is. It cannot confirm a fault. That takes an inspection, and often a code read or a test drive.
What should I tell a mechanic?
Describe the symptom, when it happens, when it started and anything that changed beforehand. That is more useful than naming the part you suspect.
09Estimates and sources
Read this before you rely on it.
Built and checked by CarTipsMentor Team. Last updated . How we check our work.
Sources
- [1]AAA Automotive, Time-Stamped Car Maintenance Checklist
- [2]U.S. Environmental Protection Agency, What To Do With the Check Engine Light (EPA420-F-00-041)
- [3]fueleconomy.gov (U.S. Department of Energy and EPA), Keeping Your Vehicle in Shape
- [4]CarMD, 2026 Vehicle Health Index (calendar-year 2025 repair data)