Truck Fault Codes: Guide To Diagnostic Trouble Codes (DTCs)

Published on 09/23/2022

truck fault codes guide to diagnostic trouble codes dtcs

Modern trucks are increasingly complex and sophisticated, which makes servicing them harder than it used to be. The trucking industry has created a standardized truck fault code system so mechanics can accurately diagnose and fix problems.

Because knowing truck fault codes is crucial for your mobile fleet, the team at All Bay Diesel has put together this article discussing truck codes and why they are important.

Contact us today for excellent mobile diesel repair by All Bay Diesel!

What Are Truck Fault Codes?

In the past, mechanics would look under the truck hood to see what was wrong and call it a day. However, as more and more companies integrated complex computers and electronics into the truck design, diagnosing issues by eye became much more difficult.

The solution to this problem was truck fault codes. Truck fault codes, or diagnostic trouble codes (DTCs), were first introduced in 2009 and provide a standardized method of reporting and diagnosing truck problems. The point of truck codes was to establish a straightforward way of identifying problems with trucking systems for repair. 

How Do Truck Fault Codes Work?

Each DTC corresponds to a specific issue. Modern trucks have sensors that monitor truck components and track their performance. If a sensor detects a problem, it sends a signal to the engine control module, which activates the warning light on the dashboard.

All vehicles in the US since 1996 have reported diagnostic problems using either J1939 or OBD-II codes. The current standard for DTCs in heavy vehicles is J1939, while light vehicles use the OBD-II standard. Mechanics use a special tool to interface with a vehicle’s engine control module to identify DTCs and locate problems.

Modern truck companies can monitor a fleet’s condition by connecting to their engine control modules via telematics. This connection gives headquarters real-time updates about the status of their equipment. 

Types Of DTCs

Modern vehicles use two types of truck fault codes: critical and non-critical codes. 

Critical codes

These include codes for issues that can cause severe damage. Problems like coolant leaks or high engine temperatures are critical issues as they can permanently damage an engine. Fleet managers should handle critical codes as quickly as possible to prevent business interruptions.  

Non-critical codes

These include codes for issues that, while not immediately dangerous, require prompt attention. Non-critical DTCs include issues like high emissions, low tire pressure, or dirty air filters. 

Interpreting Truck Fault Codes

Mobile fleet managers must understand how to read truck fault codes to identify and fix issues. Each DTC has four numbers that identify where the problem is and what kind of problem it is. 

Suspect parameter number (SPN)

The first number is the suspect parameter number and tells which system has a problem. Current J1939 protocols reserve a 19-bit number for SPN identification that spans from 0 to 524287. This level of specificity ensures you know in which system the problem lies.  

Failure mode identifier (FMI)

The failure mode identifier indicates the specific issue at hand. For example, an FMI of 3 indicates that the voltage is higher than normal. Modern J1939 standards have 22 distinct FMIs that represent problems ranging from miscalibration and mechanical failure to high currents. 

Occurrence count (OC)

The occurrence count indicates how many times the issue has occurred. The more frequently the engine control module detects a defect from an active state, the more this number increases. 

Conversion method (CM)

The DTC CM is a short number that determines how to translate the SPN and FMI to identify a specific issue. Older heavy vehicles that use the J1939 standard have three conversion methods, while modern heavy vehicles use a fourth.  

Truck Fault Code Example

Now that you understand the major DTC components, let’s consider an example of DTC to illustrate how to read them. Say you diagnose your truck and get the code P0503.

In this case, the “P” indicates your powertrain as the main culprit, the “05” indicates the problem is with your speed sensor, and the “03” indicates the sensor voltage is too high. Please note that your truck may use additional codes, so consult the manufacturer’s guidelines for more information about manufacturer-specific codes. 

Considering how many things can go wrong with your truck, J1939 DTCs can get complex. Mechanics utilize special code readers that interface with the truck’s nine-pin J1939 connector to translate codes into an easily readable format.  

Common Truck Fault Codes

Below is a list of some of the most common DTCs and what they mean.

  •       P0101 indicates a flaw with the mass air flow sensor.
  •       P0442 indicates a malfunction with the emission control system evaporation.
  •       P0030 indicates a problem with the heater control circuit.
  •       P0606 indicates a malfunction with the powertrain control module.
  •       P0706 indicates a fault in a transmission range sensor.
  •       P0703 indicates a problem with the switch circuit voltage.
  •       P0110 indicates a problem with the air intake sensor.

 Truckers can use a diagnostic code reader to identify issues on the fly. Fleet managers can also monitor truck fault codes in their fleet to identify issues quickly.

The Importance Of Truck Fault Codes

It is difficult to overstate how important DTCs are for modern fleet operation. Fleet transports haul hundreds of thousands of dollars of goods daily, and a malfunction can significantly impact scheduling and productivity. 

Knowing DTCs can keep your fleet in top working condition. Having access to truck diagnostics allows fleet managers to diagnose issues accurately and stay on top of maintenance needs. 

Mobile Fleet Repair In The Bay Area

All Bay Diesel offers mobile truck fleet repairs and diagnostic services. Whether you need a simple tune-up or need to renovate your entire fleet, we can assist. When it comes to fleet maintenance, we should be your first choice.

Read our blog to know the differences between superchargers and turbochargers. If you have any further questions about truck fault codes or would like to schedule a maintenance appointment, contact us online or call today at (925) 522-1780!

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