Top 10 Fault Codes in Truck Maintenance

Understanding your vehicle’s check engine light keeps your fleet ahead on maintenance and avoids unnecessary costs. Often, a dashboard alert can be avoided when the right precautions are in place, but with many different alerts coming in, it can be difficult for a fleet manager to know which to prioritize. That is why it is essential to find a fleet maintenance solution that can easily assist you in interpreting Suspect Parameter Numbers (SPN) to identify your vehicle’s problem and take proper action.

The SPN diagnostic fault code terminology is industry standard for heavy-duty trucks and buses using a J1939 CAN data link. The SPN is the fault location number assigned to a specific part or electric subsystem to assist in locating and repairing a problem within the vehicle.

These are the ten most common SPN error codes detected from over 17 million fault codes on Pitstop’s fleet maintenance platform, the symptoms linked with these fault codes, potential causes, and how to diagnose these prevalent vehicle faults using a simple interface:

1. Coolant Level: SPN 111
2. Aftertreatment 1 Diesel Exhaust Fluid Tank Volume: SPN 1761
3. Aftertreatment 1 Diesel Particulate Filter Differential Pressure: SPN 3251
4. Engine Coolant Temperature: SPN 110
5. Fuel/Rail System Pressure – Too Low Bank 1: SPN 135
6. Front Operator Wiper Switch: SPN 2863
7. Engine Exhaust 1 NOx 1: SPN 3216
8. Engine Fuel Delivery Pressure: SPN 94
9. Aftertreatment 1 Outlet NOx 1: SPN 3226
10. Engine Exhaust Gas Recirculation 1 Valve 1 Control 1: SPN 2791

1. Coolant Level: SPN 111

Of the 17 million fault codes, this SPN code was triggered over 12 thousand times. Coolant level indicates the ratio of the volume of liquid found in the engine cooling system to the total cooling system volume.

Coolant, often green or red in colour, contains chemicals that prevent the fluid from freezing in freezing weather and protect the engine from corrosion. Check the coolant level to prevent damages such as overheating the engine, blowing the head gasket, damaged engine block, and seized engine. Unregulated coolant levels are costly, so check your car’s coolant levels every time you refuel your vehicle.

Symptoms:

• Check engine light
• Rising temperature gauge
• Automatic engine cut-off
• Poor fuel economy
• Internal heating system not working properly
• Distinct sweet smell

Potential Causes:

• External coolant leak
• Faulty intake manifold gasket
• Faulty head gasket
• Faulty radiator cap
• Faulty EGR cooler

2. Aftertreatment 1 Diesel Exhaust Fluid Tank Volume: SPN 1761

The SPN 1761 fault code means the ratio of the volume of diesel exhaust fluid to the total volume of diesel exhaust fluid storage container for aftertreatment system 1 (exhaust bank 1) is below the operating range. This reduces emissions to a safe and lawful level while diminishing your vehicle’s performance. If you still have a long road to go and no refill, you may need to call for a tow. The vehicle may eventually refuse to start unless the DEF tank is full.

Symptoms:

• Yellow Check lamp illuminated
• Low DEF fluid lamp illuminated
• No Aftertreatment Diesel Exhaust Fluid (DEF) dosing
• DEF level gauge reading incorrectly
• Reduced power and possibly a 5MPH derate
• Warning Chimes for low DEF level or derate

Potential Causes:

• DEF tank pickup assembly/sensor failure
• Running out of DEF fluid

3. Aftertreatment 1 Diesel Particulate Filter Differential Pressure: SPN 3251

Exhaust differential pressure measured between the intake and exhaust of a diesel particulate filter in exhaust bank 1 is too high or low. This issue should be fixed quickly as it may cause reduced engine performance. Check the DPF differential pressure data if it’s within the normal range.

Symptoms:

• Check engine light
• Engine protection derate
• The aftertreatment diesel particulate filter lamp will be illuminated and will begin to flash as the severity of the soot load increases. 

Potential Causes:

• The soot load of the aftertreatment diesel particulate filter has exceeded recommended limits.

4. Engine Coolant Temperature: SPN 110

Similar to SPN 111, this fault code alerts when the temperature of liquid found in the engine cooling system is out of the engine protection limit range. To put it another way, your engine isn’t getting (or staying) hot fast enough. While this isn’t a significant issue, it can have a negative impact on emissions and fuel economy, so address it as soon as possible.

Symptoms: 

• Check engine light
• Malfunction indicator lamp (MIL)
• Engine may be hard starting
• Poor fuel economy
• Engine running extremely rich or lean
• Engine dying
• Engine overheating

Potential Causes: 

• Extreme driving conditions
• Coolant thermostat
• Engine cooling fan
• Radiator
• Engine Cooling System
• Coolant pump
• Cylinder Head
• Oil cooler 

5. Fuel/Rail System Pressure – Too Low Bank 1: SPN 135

Fuel Rail/System Pressure Too Low.” It may get logged by the PCM when the pressure inside the fuel rail or the fuel system dives below the minimum levels needed to supply the engine with enough fuel to run correctly. If this issue goes unattended, the engine’s performance may suffer due to insufficient fuel, leading to misfires from running lean, rough running, or reduced power during acceleration or high RPMs.

Symptoms: 

• Check engine light
• Engine stalling
• Fuel efficiency drops
• Vehicle might run rough
• Lack of power on acceleration

Potential Causes: 

• Weak fuel pump, clogged filter or screen, restricted fuel supply line
• Faulty fuel pump driver module
• Faulty fuel pressure sensor

6. Front Operator Wiper Switch: SPN 2863

SPN 2863 identifies an issue with the front operator wiper switch. The windshield wiper switch is placed on either the turn signal indicator or a separate indicator level near your steering wheel, depending on the make and model of your car.

There are numerous options, such as wiper speed and length and a switch for spraying windshield washer fluid. Quick access to a clean windshield is imperative, especially during rain or snow storms.

Symptoms:

• Windshield wipers won’t turn on or off
• Windshield wipers do not change speed or settings
• Turn signals are not working

Potential Causes:

• Malfunctioning switch
• Bad wiper module
• Broken wiper motor
• Stripped wiper motor linkage
• Damaged wiper arm

7. Engine Exhaust 1 NOx 1: SPN 3216

The amount of combined NO and NO2 in the exhaust, represented in NOx molecule parts per million non-NOx molecules, in exhaust bank 1. This sensor is located at the intake of the SCR system if an SCR system exists. NOx sensors monitor the level of nitrogen oxide being emitted by a diesel vehicle to ensure compliance with emissions regulations. Failure to address the problem might result in low engine power, erratic idling, unexpected engine misfire, and poor gas mileage. 

Symptoms: 

• The ECU will illuminate an engine warning light
• High fuel consumption
• Erratic acceleration
• Engine misfire and soot buildup
• Check engine light

Potential Causes:

• Soot buildup
• Burnt or damaged cables

8. Engine Fuel Delivery Pressure: SPN 94

Fuel pressure is a surprisingly delicate aspect of your vehicle, but it’s ultimately so critical to the performance and longevity of your vehicle. The wrong fuel pressure in the system as delivered from the supply pump to the injection pump can harm your vehicle’s power and fuel economy. 

Symptoms:

• Check engine light
• The exhaust gives off a fuel smell
• Low fuel economy and constant refuelling
• Poor engine performance
• Blackened spark plugs
• Spark plugs that are wet with fuel
• Restrictions in the return line

Potential Causes:

• Dirty fuel filter
• Weak pump
• Incorrect tank venting
• Restricted fuel lines
• Clogged return line
• Faulty electrical control

9. Aftertreatment 1 Outlet NOx 1: SPN 3226

An incorrect NOx sensor reading has been detected by the aftertreatment outlet NOx sensor causing an SPN 3226 fault code alert. Specifically, the amount of combined NO and NO2 in the exhaust exiting the aftertreatment system measured by a NOx sensor at the aftertreatment outlet, represented in NOx molecule parts per million non-NOx molecules in exhaust bank 1.

Symptoms:

• Check engine light
• Engine stalling or misfiring
• Engine performance issues
• Car not starting

Potential Causes:

• CCD Bus (-) circuit open between Powertrain Control Module (PCM) and Transmission Control Module (TCM)
• CCD Bus (+) circuit open between PCM and TCM
• Transmission CCD bus wiring damaged
• Faulty TCM

10. Engine Exhaust Gas Recirculation 1 Valve 1 Control 1: SPN 2791

A SPN 2791 fault code triggers when there is an undesired percentage of Exhaust Gas Recirculation (EGR) valve 1 opening. An EGR malfunction can create problems with the EGR system’s circulation and operation, resulting in performance concerns such as reduced power, slower acceleration, and lower fuel economy. It may also result in an increase in vehicle emissions.

Symptoms: 

• Check engine light
• Engine has a rough idle
• Poor vehicle performance
• Increased fuel consumption
• Frequently stalls when idling
• Smell of fuel
• Knocking noises coming from the engine

Potential Causes:

• Build-up of soot particles from urban driving conditions

Predictive Maintenance

Got a check engine light but not sure what it means? A fleet maintenance management platform can help translate fault codes and alert on high probability failures for each vehicle, organized by critical, major and minor risks. 

Pitstop is a fleet maintenance software that analyzes major vehicle components to provide powerful predictive maintenance insights. To learn more, book a demo here.

💡 It’s crucial to remember that while SPNs help diagnoses a vehicle’s fault, they’re not a tool to figure out why it happened. A mechanic must further diagnose a vehicle in order to determine and treat the source of the problem.