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When a dashboard warning light appears, most drivers immediately wonder how long does a diagnostic take on a car and whether it will cost time and money.
The truth is, a car diagnostic test can be quick, but in some cases, it requires patience. A proper diagnostic is not just about plugging in a computer — it’s about understanding what your car is trying to tell you.
In this guide, you’ll learn:
How long different diagnostics take
What happens during the process
Why rushing a diagnosis often leads to bigger repair bills
How Long Does a Diagnostic Take on a Car on Average?
On average, a car diagnostic takes between 30 and 60 minutes.
Here’s a realistic breakdown:
Computer diagnostic scan: 10–20 minutes
Fault code analysis: 10–15 minutes
Additional system checks: 15–30 minutes
So when asking how long does a diagnostic take on a car, remember that a fast scan does not always mean an accurate diagnosis.
To understand the technology behind this, read about on-board diagnostics (OBD-II).
How Long Does It Take to Get a Full Diagnostic on a Car?
A full diagnostic on a car usually takes 45 minutes to 2 hours.
Unlike a quick scan, a full diagnostic includes:
Reading and interpreting fault codes
Checking live sensor data
Inspecting wiring, connectors, and components
Performing manual tests where needed
If you’re wondering how long does it take to get a full diagnostic on a car, the answer depends on how complex the issue is — but accuracy is always worth the wait.
Learn more about full diagnostic testing from AutoTrader.
How Long Does a Car AC Diagnostic Take?
A car AC diagnostic typically takes 30 to 60 minutes.
During an AC diagnostic, a technician may:
Check refrigerant pressure
Inspect the AC compressor
Test electrical controls and sensors
Look for leaks using dye or pressure tests
Simple AC issues can be diagnosed quickly, but refrigerant leaks often require more time.
For deeper understanding, see how automotive air conditioning systems work.
How Long Does It Take to Diagnose an Electrical Problem in a Car?
Diagnosing an electrical problem in a car usually takes 1 to 2 hours, and sometimes longer.
Electrical faults are challenging because:
Problems can be intermittent
Multiple systems may be affected
Fault codes do not always appear
Common electrical issues include:
Battery drain
Sensor failures
Wiring or grounding faults
This is why electrical diagnostics are rarely rushed if done properly.
How Long Does a Computer Diagnostic Take?
A computer diagnostic alone usually takes 10 to 20 minutes.
This process:
Reads fault codes from the vehicle’s ECU
Identifies affected systems
Provides a starting point for diagnosis
However, a computer diagnostic does not confirm the repair. It must be followed by testing and inspection to avoid replacing the wrong parts.
You can learn more about vehicle computer diagnostics.
What Happens During a Car Diagnostic? Step-by-Step
A proper car diagnostic test follows a clear process:
Step 1: Scan the vehicle using a diagnostic tool
Step 2: Review and interpret fault codes
Step 3: Analyze live data from sensors
Step 4: Perform physical inspections
Step 5: Explain findings and repair options
This structured approach explains why diagnostic time can vary.
Dealer vs Independent Garage: Diagnostic Time Comparison
Dealership diagnostics
Time: 45–90 minutes
Brand-specific tools
Higher diagnostic fees
Independent garage diagnostics
Time: 30–60 minutes
Faster booking
More affordable
Mobile diagnostic services
Time: 30–45 minutes
Convenient
Limited deep testing
You can read more about choosing between garages at Which?.
Real-Life Example: Why Diagnostic Time Matters
A driver ignored a warning light because the car still drove fine. A later diagnostic revealed a failing oxygen sensor.
Without diagnosis:
Fuel economy dropped
Emissions increased
The catalytic converter risked damage
That 30-minute diagnostic prevented a repair costing hundreds more.
How long does a diagnostic take on a car if no codes show?
If no codes appear, technicians must manually test systems. This can take 1 to 2 hours.
Can a car diagnostic detect all problems?
No. Diagnostics mainly detect electronic faults. Mechanical problems still require physical inspection.
More details at AutoTrader.
Does insurance cover diagnostic tests?
Most insurance policies do not cover diagnostic costs unless linked to an approved repair claim.
When Should You Book a Car Diagnostic?
Book a diagnostic if:
The check engine light turns on
The car stalls or misfires
Fuel economy suddenly drops
Dashboard warnings appear
The vehicle fails an MOT emissions test
Early diagnostics prevent expensive breakdowns.
Final Verdict: How Long Does a Diagnostic Take on a Car?
So, how long does a diagnostic take on a car?
10–20 minutes for a computer scan
30–60 minutes for most diagnostics
1–2 hours or more for electrical or complex issues
Spending time on diagnostics saves money, prevents wrong repairs, and keeps your car reliable.
