How to Identify and Address Your Fleet’s Top Fault Code

Fault codes are more than numbers, they show that your equipment isn’t working properly. When these issues go unresolved, it may result in problems, time wasting and expensive periods with nothing running. Since uptime matters greatly and time at the jobsite is limited, understanding and acting on fault codes is necessary for construction fleets.

We explain how to spot your fleet’s most common fault code, explain what it means and discuss actions you can take to avoid more problems. Handling any role in your company, you can make life easier for equipment by having a clear way to handle, interpret and complete fault code solutions.

We’ll go over various fault codes, display how to find your equipment’s main issues and explain how you can repair and avoid the same problems in the future.

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What is a Fault Code?

A fault code, also called a Diagnostic Trouble Code (DTC), is a standardized code generated by a vehicle or machine’s onboard diagnostics (OBD) system when it detects a malfunction. Should these problems affect major parts like the engine, transmission, emissions, hydraulics or sensors, the diagnosis system will be alerted via the codes.

Different Types of DTC Codes

Not all fault codes carry the same weight. Some demand immediate shutdown, while others are simply nudged to plan maintenance. The key for fleet managers is knowing which is which—and responding accordingly.

• Critical fault codes are high-severity alerts tied to issues like engine failure, low oil pressure, overheating, or brake system malfunctions. These can lead to catastrophic downtime, costly repairs, or even safety risks if ignored. The machines must be taken out immediately and the maintenance department should be notified straight away.

• Non-critical codes When the dashboard displays non-critical codes, it is likely due to a sensor issue, simple air conditioning or heating trouble or a reminder about needed servicing. Although these matters don’t need to be resolved quickly, they should be noted and resolved when the next maintenance window comes.

There are two main diagnostic standards used across vehicle classes:

• OBD-II: Used for both light and medium-duty trucks and vans that weigh between 6,000 and 26,000 lbs. Since 1996, every U.S. vehicle on the road must have one.
  
• J1939: Common in heavy-duty vehicles (26,001+ lbs), including construction equipment, cement trucks, and haul trucks. J1939 fault codes typically follow the SPN/FMI format.

Fault codes are used across both light-duty and heavy-duty equipment. In heavy construction and fleet vehicles, you’ll commonly see them formatted using identifiers like:

• SPN (Suspect Parameter Number)
  
• FMI (Failure Mode Identifier)
  
• MID (Module Identifier)
  
• CID (Component Identifier)
  
• EID (Element Identifier)

Each part of the code gives specific information:

• SPN 3364 might point to Diesel Exhaust Fluid (DEF) Quality
  
• FMI 2 could indicate data erratic or incorrect
  
• Together: SPN 3364 FMI 2 = DEF Quality Low (likely degraded or contaminated)

Here are the most common code combinations:

• SPN:MID
• MID:EID
• SPN:FMI
• FMI:MID:CID
• SPN:FMI:MID

Using diagnostic devices or fleet management systems, technicians and fleet managers look into and make sense of these codes. Being aware of these signs helps avoid failures, saves time and maintains the performance of machines.

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Key DTC (Diagnostic Trouble Code) Examples and What They Mean

Here are a few common examples across light, medium, and heavy-duty equipment:

SPN 157 FMI 18 – DEF Pressure Too Low

• System: Emissions (aftertreatment)
  
• Meaning: The system reported that the fluid line pressure for DEF (Diesel Exhaust Fluid) is not enough.
  
• Cause: Possibly, the DEF filter is blocked, air is present in the line or the DEF pump is failing.
  
• Action: Inspect DEF system for contamination or component failure. Clear the code only after confirming fluid quality and delivery.
  

SPN 789 FMI 7 – ABS Wheel Sensor Axle 1 Left Not Responding Properly

• System: ABS / braking
  
• Meaning: Left-side wheel speed sensor on Axle 1 is giving inconsistent or no signal.
  
• Cause: Loose connection, damaged sensor, or faulty wiring.
  
• Action: Inspect sensor alignment and wiring. Confirm with multimeter or scan tool before replacement.
  

SPN 168 FMI 1 – Low Voltage Detected

• System: Electrical system
  
• Meaning: Battery voltage has dropped below the required threshold.
  
• Cause: Weak battery, failing alternator, corroded terminals.
  
• Action: Test battery and alternator output. Clean terminals and check ground connections.

SPN 110 FMI 3 – Engine Coolant Temperature High

• System: Engine cooling
  
• Meaning: The coolant temperature is above normal operating limits.
  
• Cause: Low coolant, failed thermostat, blocked radiator, or fan malfunction.
  
• Action: Check coolant level, pressure-test the system, and inspect for leaks or flow restrictions.

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How to Identify and Address Your Fleet’s Fault Codes Step-by-step

Step 1: Know Your Fleet’s Fault Code Trends

Before you fix the problem, you need to understand what’s actually going wrong most often.

Pull Historical Fault Code Data

Use your telematics platform to look at fault code frequency across all machines. Filter by:

• Most common fault codes by count
• Most severe by downtime caused
• Most recurring by asset

Look for Patterns

Does one code keep popping up across a certain make or model? Is one site triggering more faults than others? Maybe your excavators are frequently throwing DEF-related codes when they work near steep inclines.

Step 2: Zero In on the #1 Fault Code

Once you’ve filtered the data, pick the most frequent or most costly code and dig in.

Let’s say: SPN: 789, FMI: 7 – Wheel Sensor ABS Axle 1 Left, Not Responding Properly

This code indicates a communication or performance issue with the ABS wheel speed sensor on the left side of axle 1. It may be caused by a damaged sensor, loose connection, or wiring fault, and could impact braking performance or disable ABS on that wheel.

Step 3: Identify Root Causes

Your top fault code is the symptom, not the disease. Time to dig deeper.

Investigate by Role

• Fuelers: Are they using the right DEF? Is the fill area clean?
• Operators: Do they ignore warnings? Are they topping off improperly?
• Mechanics: Are sensors inspected during PMs?

Check Environmental Factors

• Is the DEF stored properly (away from heat and sun)?
• Are dust, water, or fuel cross-contaminating it?

Use Inspections and Photos

Look at past equipment inspections. Are operators noting DEF issues? Any visual signs of contamination? Use your platform to flag this code automatically and notify your shop lead the moment it happens.

Step 4: Build a Fix Strategy

Now that you know what’s triggering it, here’s how to fix it—and prevent it.

Short-Term Response

• Drain and replace DEF if contaminated
• Clean DEF tank and replace filters
• Calibrate or replace sensors if faulty

Long-Term Prevention

• Train fuelers on DEF handling best practices
• Store DEF in shaded, sealed environments
• Add a checklist item for DEF quality in daily inspections
• Use Clue to auto-track when and where the code appears

Want to get ahead of it? Set up rule-based alerts in your platform to detect the first signs of DEF quality degradation before the fault code even hits.

Step 5: Close the Loop and Monitor

Once you've made the fix, monitor for results.

• Did the fault code frequency drop?
• Are specific machines or locations still triggering it?
• Are operators or fuelers following new procedures?

Use your maintenance data to prove that your fix worked. If it didn’t? Back to the data and the dirt until it does.

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How Clue Helps with Fault Code Detection and Interpretation

Clue simplifies fault code detection and interpretation by making complex diagnostic data accessible to everyone in the field, not just technicians.

Here’s how Clue supports your team

• Real-time Alerts: Get immediate fault code notifications tied to specific assets, operators, or locations.
• Human-Friendly Descriptions: Clue translates SPN/FMI, MID, and CID codes into plain language so teams can act fast without needing to decode the ECM.
• Automate fleet Maintenance: It streamlines the entire process by generating maintenance tasks automatically and keeping teams aligned through a single, connected platform.
• Historical Trends: View fault frequency over time to spot repeat offenders and root causes.
• Visualized Timelines: See exactly when and where a fault was triggered, and what happened right before and after it.
• Integrated Workflows: Automatically generate work orders and link inspections, photos, and comments to each code.

With Clue, you’re not just seeing the fault, you’re understanding it and building a repeatable process to resolve it, fast.

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Fault Code Management Best Practices for Fleet Managers 

Managing fault codes isn’t just about clearing warnings, it’s about building a proactive system that keeps your fleet reliable, safe, and ready to work.

• Centralize data: Use one system to view all fault codes across your fleet.
  
• Triage by severity: Flag critical (Red), maintenance-needed (Yellow), and info-only (Green) codes.
  
• Assign ownership: Make someone accountable for each fault’s resolution.
  
• Add context: Correlate faults with job conditions, operator habits, and environment.
  
• Track frequency: Spot patterns across machines or locations to catch recurring issues.
  
• Convert to action: Turn fault codes into work orders with clear tasks and timelines.
  
• Train operators: Ensure they understand and report codes—not ignore them.
  
• Inform PM: Use code history to refine and customize preventive maintenance intervals.
  
• Audit closures: Log fixes, causes, and response time for every resolved code.
  
• Use smart alerts: Get real-time notifications for urgent issues; batch non-critical ones into weekly reports.

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Final Thoughts

Your fleet is talking to you every day through fault codes. Most teams react. Great teams investigate, fix, and prevent.

By identifying your most frequent fault code and building a tactical response around it, you’re not just putting out fires, you’re building a system that gets smarter with every job. Over time, this discipline reduces downtime, protects your equipment investment, and strengthens communication between field and shop. Preventive maintenance is more important than just fixing what is broken. We have to know why a point of failure happens, notice it early and prevent it in the future.

Let the data lead. Let the field confirm. Let Clue help you close the loop and run a tighter, more reliable fleet every day.