What Should You Do If Your Truck’s Diagnostic System Flags an Issue?

Modern trucks are equipped with advanced diagnostic systems that constantly monitor engine performance, emissions, transmission function, and other critical components. When an issue is detected, the system generates trouble codes that serve as warnings of potential malfunctions. These alerts should never be ignored—even a minor issue can escalate into a costly repair or unexpected breakdown.

At Southeast Fleet Services, we understand the importance of quickly diagnosing and addressing these warnings to keep your truck running efficiently. If your truck’s diagnostic system flags an issue, here’s what you need to do.

Why Your Truck’s Diagnostic System Matters

The onboard diagnostic system (OBD-II in light-duty vehicles, and ECM/ECU in heavy-duty trucks) continuously monitors:

• Engine performance and emissions
• Transmission operation
• Brake system and ABS functionality
• Fuel efficiency and injector performance
• DEF (Diesel Exhaust Fluid) and emissions compliance

When a problem is detected, the system triggers warning lights or fault codes, indicating that a critical system may be compromised. Ignoring these warnings can lead to:

• Increased repair costs
• Reduced fuel efficiency
• Unexpected roadside breakdowns
• DOT compliance violations

Steps to Take When Your Truck’s Diagnostic System Flags an Issue

When your check engine light, transmission warning, or other alerts appear, follow these steps to determine the severity of the problem and the best course of action.

Assess the Severity of the Warning

Not all diagnostic warnings require immediate action, but some indicate critical failures that could cause engine damage or safety risks.

Check Engine Light (CEL):

• Solid Light: Indicates a non-urgent but important issue (e.g., faulty sensor, emissions issue).
• Flashing Light: Signals an urgent problem (e.g., misfire, turbocharger failure) that needs immediate attention.

Transmission Warning Light: If the truck experiences slipping gears, hard shifts, or loss of power, stop immediately and inspect the issue.

Brake System and ABS Light: A warning light in your braking system could indicate low fluid levels, sensor failures, or worn components—all of which must be checked before continuing to drive.

Diesel Exhaust Fluid (DEF) and Emissions Warnings: Ignoring these can result in a forced engine derate, limiting power and speed until the issue is resolved.

Retrieve the Diagnostic Trouble Codes (DTCs)

Every truck with an Electronic Control Module (ECM) generates trouble codes (DTCs) when an issue arises. These codes provide insight into which system is malfunctioning and can help pinpoint the exact cause. Once you retrieve the code, consult your truck’s service manual or a professional technician to determine the next steps.

You receive DTC with an OBD-II scanner or truck diagnostic tool to read the codes. Many modern trucks display codes on the dashboard information screen. Some fleet management systems automatically send fault codes to maintenance teams.

Common DTC examples:

• P0200: Fuel Injector Circuit Malfunction
• P0300: Random/Multiple Cylinder Misfire
• P0401: Exhaust Gas Recirculation (EGR) Flow Insufficient
• P0700: Transmission Control System Malfunction
• P2463: Diesel Particulate Filter (DPF) Restriction

Perform a Basic Visual Inspection

Before heading to a repair shop, perform a quick visual inspection to check for obvious problems. If the problem seems simple, like a loose wire or low fluid, addressing it immediately can prevent further damage.

• Engine Bay: Look for loose hoses, disconnected wiring, or leaks.
• Fluid Levels: Check engine oil, coolant, transmission fluid, and DEF levels.
• Exhaust System: Look for excessive smoke, soot buildup, or strong fuel odors.
• Battery Terminals: Ensure the connections are clean and secure.

Seek Professional Diagnostic Testing

If the issue persists or cannot be resolved with basic troubleshooting, it’s time to schedule a full diagnostic scan with a certified technician. Schedule routine diagnostic testing every 30,000–50,000 miles, or whenever your truck’s warning system detects a problem.

Professional Diagnostic Testing
Ensures accurate diagnosis, preventing unnecessary part replacements. It helps fleet operators plan maintenance schedules efficiently. Maintenance will reduce downtime by identifying hidden issues before they cause breakdowns. 

Professional testing includes:

• Advanced ECM/ECU scanning to verify all active and historical fault codes.
• Live data monitoring to track engine and transmission performance in real-time.
• Component testing to check sensors, injectors, and actuators for faults.
• Exhaust system analysis for DPF, EGR, and DEF-related issues.

Immediate Actions to Take Based on the Diagnostic Codes

After identifying the fault codes, you need to determine whether to keep driving, stop for repairs, or seek emergency roadside assistance. Never ignore serious performance issues, as driving with a failing component can cause engine damage or system failure.

If the issue is minor (Non-urgent CEL or low-priority fault codes):

• Monitor the truck’s performance for changes in power, fuel efficiency, or shifting.
• Schedule maintenance at the next available opportunity.
• Keep an eye on temperature, oil pressure, and fuel consumption.

If the issue affects performance (Reduced power, rough shifting, or excessive smoke):

• Avoid long trips until the problem is diagnosed.
• Check fluid levels, hoses, and wiring for visible damage.
• Reduce load weight if possible to prevent further strain on the drivetrain.

If the issue is critical (Flashing warning lights, transmission failure, or overheating):

• Pull over immediately and turn off the engine.
• Avoid restarting the truck until a mechanic assesses the situation.
• Call for roadside assistance or mobile repair services.

Protect Your Truck with Timely Troubleshooting

A truck’s diagnostic system is designed to protect your vehicle, reduce downtime, and improve safety. Ignoring warning lights or fault codes can result in serious mechanical failures and compliance violations. By following these steps—assessing warning lights, retrieving DTCs, inspecting for damage, and seeking professional diagnostics—you can prevent costly repairs and keep your fleet running efficiently.

Southeast Fleet Services specializes in diagnostic testing, repairs, and fleet maintenance across North Carolina, South Carolina, and Georgia. If your truck has flagged an issue, don’t wait—schedule a diagnostic scan today to avoid unexpected breakdowns! For more information on the trucking industry, check out our DOT inspection checklist.