car diagnostic test

What is Included In A Car Diagnostic Test In The USA


A car diagnostic test is an essential procedure for any vehicle owner. It provides a comprehensive assessment of a car’s overall health by analyzing the onboard computers and electronic systems. This test has become an indispensable tool for mechanics and car owners alike.

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How Does A Diagnostic Test Work?

A diagnostic test connects to the vehicle’s main computer, known as the ECU (Engine Control Unit) or PCM (PowerTrain Control Module), through a port, often located under the dashboard. The mechanic attaches a scan tool, which links to the various sensors and processors throughout the vehicle. It scans the ECU for any diagnostic trouble codes (DTCs) that indicate a malfunction or flaw in the system. The scan tool essentially talks to the different electronic control units, receives input signals from sensors, and generates a report.

Car Diagnostic Test
Car Diagnostic Test

Components Analyzed in A Diagnostic Test

A car diagnostic test checks the following major systems:

  • Engine – Scans components like ignition, fuel injection, emissions, and related sensors.
  • Transmission – Assesses transmission control module, clutch, gear ratio, and sensors.
  • Brakes – Checks ABS system, brake fluid levels, brake pads/linings.
  • Airbags – Monitors airbag control modules and crash sensors.
  • Battery – Tests charging and starting system, battery, alternator output.
  • Onboard Computers – Evaluates all electronic control units (ECUs) and their programming.
  • Other Components – Oxygen sensors, coolant temperature, mass airflow, throttle position, crankshaft position, etc.

The scan tool will scan through all the modules and analyze live data from various sensors and actuators while test-driving the vehicle. This enables the mechanic to pinpoint the root of the problem accurately.

Car Diagnostic Test
Car Diagnostic Test

The Diagnostic Testing Process Step-By-Step

Here is a step-by-step overview of the car diagnostic testing process:

Step#1. Connect The Scan Tool – The mechanic connects the scan tool to the vehicle’s computer port, usually under the dash.

Step#2. Enter Vehicle Information – Basic details like VIN, make, model, and mileage are entered into the scan tool.

Step#3. Retrieve Diagnostic Trouble Codes – The tool scans all systems and displays any diagnostic trouble codes pointing to issues.

Step#4. Assess Live Data – The mechanic can view live data from various vehicle sensors like coolant temperature, throttle position, air/fuel mixture, etc.

Step#5. Locate The Issues – Combining trouble codes and live data analysis enables accurate problem identification.

Step#6. Recommend Repairs – The mechanic recommends appropriate repairs and replacements based on the test findings.

Step#7. Clear Codes/Recheck – Once repairs are completed, codes are cleared and the system is rechecked to ensure proper functioning.

Car Diagnostic Test
Car Diagnostic Test

Benefits of Car Diagnostic Testing

There are several excellent reasons why regular diagnostic testing is critical:

  • Saves money – By detecting even minor issues early, major repairs are avoided, saving thousands in costs.
  • Avoids breakdowns – Consistent testing minimizes the chances of a vehicle breaking down unexpectedly on the road.
  • Maintains safety – Identifying and fixing brake, airbag, and other safety-related problems ensures continued safe operation.
  • Optimizes performance – Timely repairs improve fuel efficiency, acceleration, emissions, and overall performance.
  • Increases resale value – Meticulous maintenance and repair records based on diagnostics make the car more valuable at resale.
  • Provides peace of mind – Drivers experience greater comfort knowing their vehicle has been thoroughly evaluated and is in excellent condition.
Car Diagnostic Test
Car Diagnostic Test

When Should You Get A Diagnostic Test?

Here are some general guidelines on when to get a comprehensive vehicle diagnostic test:

  • Annually – A yearly health check is recommended for all vehicles.
  • Before long trips – Test vital components like engine, tires, brakes.
  • New & strange symptoms – Unusual noises, smells, dashboard warning lights.
  • Buying a used car – Essential to detect any hidden issues.
  • After an accident – To check airbags, chassis, sensors for proper functioning.
  • Significant mileage milestones – At 50,000, 100,000 and 150,000 miles.
  • Emissions or smog check – May be legally required in some states before registration.

Following scheduled diagnostic checks along with the manufacturer’s maintenance timeline can maximize a vehicle’s life span. Technological improvements now allow problems to be accurately identified even before they manifest symptoms. Taking advantage of this early detection is wise.

Car Diagnostic Test
Car Diagnostic Test

Diagnostic Testing Costs

The average cost of a comprehensive car diagnostic test in the USA is between $100-$150. However, prices can range from $50 for a basic scan at retail chains to $200+ at dealers and specialty shops.

Here are some typical diagnostic test costs:

LocationAverage Cost
Independent Repair Shops$100-$150
Retail Chains$50-$100
Mobile Mechanics$70-$110

The make, model, location, shop rates, and scan tool capabilities impact the final price. Luxury and hybrid cars with complex electronic systems tend to cost more for diagnostics. Overall, the price is reasonable considering how much money and trouble a test can save by identifying issues early.

Do-It-Yourself Diagnostics

There are some do-it-yourself diagnostic tools now available in the market, though they have limitations:

Pros: Convenience, lower cost, check basic codes

Cons: Limited capabilities, lack live data, cannot test all systems, difficulty deciphering codes

It is best to have a certified mechanic perform comprehensive testing to get an in-depth evaluation. However, DIY tools are handy for checking simple issues.

The Future of Diagnostic Testing

Vehicle diagnostics technology is advancing rapidly.

Here are some innovations shaping the future:

  • Faster testing through 5G internet speeds to vehicle data
  • Enhanced cybersecurity as vehicles become more connected
  • Detailed 3D mapping of components and faults
  • AI-assisted instant insights and recommendations
  • Over-the-air software updates and reprogramming of ECUs
  • Integration with digital service platforms, logistics, and parts databases

As vehicles get more complex, diagnostic testing will evolve to keep pace. The goal remains getting the most timely, actionable, and cost-effective assessment to keep cars running safely and smoothly.


In summary, a professional and thorough car diagnostic test can be one of the most valuable investments for any auto owner. The ability to evaluate the full range of integrated electronic systems allows problems to be identified early when they are cheaper to fix. This helps minimize breakdowns en route, and ensure optimal performance, safety, and reliability. As vehicle technology progresses, high-quality diagnostic testing becomes even more essential. Working closely with qualified mechanics and adhering to manufacturer service guidelines can keep a car running in peak condition for many years. So do not delay in booking your next diagnostic test.

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