Car Diagnostic Test: What It Is and Why You Need It

What is an Engine Diagnostic Test?

A car diagnostic test is the process in which a device (often referred to as ‘OBD scanner’ or ‘OBD reader’) is connected to the vehicle’s ECU (Electronic Control Unit), to look for error codes or unusual readings. The scanner works with sensors placed in different parts of the car, including the engine, transmission, exhaust system, airbag system and brakes. When a fault occurs, the sensors send signals that are saved as error codes. A diagnostic test helps read and understand these codes to decide the required maintenance procedure.

Purpose of Engine Diagnostic Tests

An engine diagnostic test saves both time and money by spotting issues at an early stage. Instead of waiting for a breakdown, the tests can reveal issues before they turn serious. It also helps find the exact cause of a warning light, such as the Check Engine light. Technicians can focus directly on the part that needs repair rather than replacing components (although from a commercial perspective, modern car components are generally engineered to be replaced rather than repaired).

Diagnostic tests also contribute to safety. For instance, if a fault occurs in the airbag system, the ECU will store an error code. This makes it possible to carry out repairs in a timely manner and prevent issues before they become too severe. Tests are also valuable in improving fuel efficiency and reducing emissions, as they can quickly identify problems such as a misfiring spark plug or a faulty Mass Air Flow (MAF) sensor.

How Diagnostic Tests Help in Car Engine Maintenance

A handheld device called the OBD scanner is connected to the Onboard Diagnostic port, usually located under the dashboard. The scanner communicates with the ECU and collects the stored information.

The scanner then displays codes (either on the device screen or a dedicated mobile app) that can be either standard ones used by all car manufacturers or special codes that belong to a particular brand. Each code corresponds to a specific fault. For example, a code might indicate that an ABS sensor is faulty, while another code indicates a misfiring cylinder.

Modern scanners do more than display codes. They also provide live readings from various sensors. These can range from fuel pressure, engine temperature, throttle position, to even the level of oxygen in the exhaust. Using this data, a technician can decide whether a specific part is damaged or if the fault can be localised to another component.

Types of Car Diagnostic Tools

Diagnostic tools are broadly divided into two types: a basic code reader that only displays error codes and clears them after the issue is resolved, and an advanced scan tool that provides detailed live data, graphs or even reprogrammes certain functions. Professional garages usually rely on advanced scan tools, while car owners often use simpler OBD scanners.

Why an OBD Scanner is an Essential Accessory

Having an OBD (or OBD-II) scanner in your car is quite handy. Most examples are reasonably priced and easy to operate. If the check engine light comes on, the owner can plug in the scanner to identify the potential issue. This helps avoid unnecessary visits to the service centre and reduces the risk of being misguided into expensive repairs. It also offers peace of mind on long journeys, as drivers can check for hidden problems before setting out. Modern OBD scanners offer smartphone connectivity as well.

Diagnostic tests can also prove beneficial while buying a used car. A quick scan can uncover past or current faults that might not be apparent during a test drive. It can also reveal if the owner has attempted to conceal issues by resetting warning lights without addressing the underlying problem. 

How OBD Scanners Detect Engine Remaps

It is not uncommon to tune or remap an engine to churn out more performance from its factory settings. Most tuning shops keep this bump in output within safe limits to keep things less obvious. It is worth knowing that manufacturers intentionally undertune engines for long-term reliability.

When an engine diagnostic test is carried out, the data and logs stored in the ECU can show if any unauthorised changes have been made. Data, such as flash counters, which record the number of times the software has been rewritten, are checked. If this number is higher than it should be, it may denote tampering. Checksum values, which confirm that the software matches the original factory version, are also strong indicators.

Even if someone restores the ECU to factory settings, there may still be hidden traces of tampering in specific memory locations. This abnormality helps manufacturers or authorised service centres detect ECU tampering and void the warranty.

Remember: every manufacturer is inclined to cancel a vehicle warranty on the grounds mentioned in the terms and conditions. Modifications, including but not limited to engine remaps, top the list of don’ts.

Limitations of Car Diagnostic Tests

While diagnostic tests are useful, they may not always tell the real story. For instance, an oxygen sensor code might appear, yet the real problem could be faulty wiring or a leaking exhaust. This is why basic car engine maintenance know-how and routine checks are essential. Moreover, some issues do not even create an error code. Mechanical faults, unless they directly affect an electronic actuator or sensor, are rarely detected.

An Important Car Engine Maintenance Tip

Experts advise regularly carrying out diagnostic tests, even if no warning light is displayed in the instrument cluster. Modern cars are complicated; minor problems can develop gradually. Finding these early helps avoid expensive repairs or parts replacement, while promising better reliability. Although they cannot fully replace a detailed inspection by a skilled technician, diagnostic tests remain a vital part of car maintenance.

FAQs

1. What does OBD stand for?

OBD stands for On-Board Diagnostics, a system that monitors a vehicle’s performance and reports issues through fault codes.

2. Where is the OBD port usually located?

In most cars, the OBD port is located under the dashboard near the steering wheel.

3. Can an OBD scanner tell me exactly what is wrong with my car?

Not always. The scanner provides error codes that pinpoint the area of the problem, but further inspection may be necessary to confirm the exact issue.

4. Do I need a professional mechanic to use an OBD scanner?

Basic scanners are user-friendly and can be used by car owners, but complex issues often require a technician to interpret the results better.

5. Can an OBD scanner clear warning lights?

Yes, many scanners allow you to clear codes and reset the check engine light; however, the problem may return if not addressed properly.