Top 10 Critical Fault Codes That Require Immediate Action

Operating a fleet of (or above) 500 vehicles can yield an overwhelming amount of fault codes, commonly known as DTC or SPN error codes, on a daily basis. As a fleet manager, discerning which alerts need immediate attention and which can be deferred can prove to be a challenging task, especially when drivers are calling fault codes in or there are other fires to put out. But the importance of neglected critical alerts directly impact the safety and your company’s bottom line as it could potentially escalate into costly roadside breakdowns.

In this previous blog series, we explored the top 10 “noisy” codes that won’t disrupt a fleet manager’s day as well as the top 10 fault codes in truck maintenance. However, in this segment, we pivot our attention towards fault codes that demand immediate attention to prevent larger mechanical issues. We’ve analyzed over 18 million fault codes on our fleet maintenance platform in the last year, comprising both standard and Freightliner codes. Below are the top 10 critical fault codes in truck maintenance you should prioritize.

And don’t forget to try out our free fault code lookup tool for any alerts you require definitions for.

1. Fuel rail/system pressure – too low bank 1: P0087

Type: Fuel System

The ECM has identified a lower-than-acceptable fuel pressure within the fuel rail system. This could be due to a failing fuel pump, a blocked fuel filter, issues with the fuel pressure sensor, or fuel line leaks. Use a fuel pressure gauge to monitor the rail’s fuel pressure, inspect the fuel pump, filter, and lines for potential damage or clogs. Lastly, verify the condition and functionality of the fuel pressure sensor through electrical connections and a multimeter or scan tool.

Potential Solution: Depending on the issue, faulty parts may need repair or replacement, including the fuel pump, fuel filter, or fuel pressure sensor.

2. Fuel rail pressure sensor circuit range/performance bank 1: P0191

Type: Fuel System/Electrical System

This code suggests that the fuel rail pressure sensor for bank 1 is not operating within the expected parameters. This could be due to a malfunctioning fuel pressure sensor, a shorted or open circuit, or unreliable electrical connections. Start by examining the wiring and connections to the fuel rail pressure sensor visually. Use a scan tool to monitor the sensor’s operation. If it is functioning correctly, further investigate the wiring and ECM integrity.

Potential Solution: The faulty fuel pressure sensor or the associated wiring may require repair or replacement. If the ECM is causing the problem, it may necessitate reprogramming or replacement.

3. Fuel rail/system pressure – too high bank 1: P0088

Type: Fuel System

This code indicates that the ECM has identified a higher-than-usual pressure in the fuel rail system. Potential culprits could be a defective fuel pressure sensor, a failing fuel pump, or issues with the fuel pressure regulator. Initially, use a pressure gauge to check fuel pressure, followed by inspection of the fuel pressure sensor and regulator. The fuel pump’s performance should also be assessed.

Potential Solution: Resolve any fuel pressure irregularities by repairing or replacing faulty components such as the fuel pump, fuel pressure sensor, or fuel pressure regulator.

4. Glow plug/heater circuit “A”: P0380

Type: Electrical System

This code signifies a malfunction within the glow plug “A” circuit, possibly due to defective glow plugs, issues with the glow plug relay, or wiring complications. Check the glow plugs’ resistance with an ohmmeter and test the glow plug relay. Scrutinize the wiring for any visible damages like fraying or disconnection.

Potential Solution: Any defective glow plugs need replacement, any wiring issues need addressing, or the glow plug relay may need replacement.

5. Engine intake manifold #1 pressure: SPN102

Type: Engine System

This code denotes an issue with the pressure in the engine’s intake manifold. Potential causes could be a faulty intake manifold pressure sensor, intake air leaks, or a blocked intake manifold. Use a scan tool to check the intake manifold pressure and assess the sensor and its wiring if readings are abnormal. Inspect the intake manifold for any leaks or blockages.

Potential Solution: Leaks or blockages in the intake manifold need repair, a defective sensor needs replacement or damaged wiring requires repair.

6. Engine Protection Torque Derate: SPN1569

Type: Engine System

This code implies the engine is operating in a de-rated state to prevent further damage. Potential issues could include overheating, oil pressure problems, or other factors that might harm the engine. Use a scan tool to identify any additional fault codes that might reveal the issue’s source. Check the engine for signs of overheating or oil pressure issues.

Potential Solution: The solution may vary depending on the problem, but it could involve replacing the oil pump, addressing any overheating problems (such as a faulty thermostat or radiator), or repairing any other diagnosed issues.

7. Cylinder 4: injector circuit/open: P0204

Type: Fuel System/Electrical System

This code indicates a problem with the fuel injector circuit in cylinder 4, possibly due to a faulty injector, issues in the wiring harness, or poor electrical connection. Begin by examining the wiring and connections to the cylinder 4 fuel injector, and use a multimeter or scan tool to test the injector’s operation.

Potential Solution: Repair or replace the faulty fuel injector, address wiring issues or improve the electrical connection as required.

8. Transmission oil temperature above normal: SPN 5606

Type: Transmission System

This code suggests that the transmission oil temperature exceeds the standard range. This could be due to low transmission fluid, a blocked transmission oil cooler, or a defective transmission oil temperature sensor. Verify the transmission fluid level and condition, inspect the transmission oil cooler for blockages, and assess the transmission oil temperature sensor using a scan tool.

Potential Solution: Refill or replace low or degraded transmission fluid, clear any blockages in the transmission oil cooler or replace the faulty transmission oil temperature sensor.

9. Transmission shift modulator circuit failure: SPN 3359

Type: Transmission System/Electrical System

This code identifies an issue with the transmission shift modulator circuit. Potential problems could be a faulty shift modulator, complications in the wiring harness, or a poor electrical connection. Start by visually inspecting the wiring and connections to the shift modulator, then test the modulator’s operation using a multimeter or scan tool.

Potential Solution: Depending on the cause, repair or replace the faulty shift modulator, rectify the wiring issues, or improve the electrical connection as required.

10. Transmission oil life remaining: SPN4177

Type: Transmission System

This code may be activated by degraded transmission fluid or a faulty transmission oil life sensor. First, check the transmission fluid visually for signs of degradation, like a burnt smell or dark color. If the fluid seems to be in good condition, test the transmission oil life sensor using a scan tool.

Potential Solution: Replace degraded transmission fluid or repair/replace the defective transmission oil life sensor as needed.

Implement a Predictive Maintenance Approach

Are you unsure about the reason behind your vehicle’s check engine light? A system for managing fleet maintenance can streamline any confusion and save significant time by automatically classifying fault codes into crucial, significant, and minor risks. Pitstop’s software, for example, also will automatically close out minor DTC alerts, to keep you focused on the important ones. A proactive maintenance method can help in deciphering these fault codes and issuing alerts on likely breakdowns for each vehicle.

Pitstop is a software dedicated to fleet maintenance that scrutinizes key vehicle parts to deliver robust, proactive maintenance understanding. If you’re interested in learning more, consider scheduling a demo here.

💡 It’s important to keep in mind that while SPNs and DTCs can aid in identifying a vehicle’s issue, they don’t offer insights into the cause of the problem. To pinpoint and address the root cause of a problem, further diagnostics by a trained mechanic are required.