When a diesel engine throws a fault code, it’s easy to assume the problem is straightforward.
But in real-world diagnostics, that’s rarely the case.
👉 A fault code points you in a direction – it doesn’t tell you the root cause.
For repair shops, fleet managers, and owner-operators, understanding what these codes actually mean in practice is what prevents:
- Misdiagnosis
- Unnecessary part replacement
- Repeat failures
Below are some of the most common diesel engine fault codes – and what’s really behind them.
Fuel Rail Pressure Low (SPN 157 FMI 0 / 18)
What it Means: The engine is not maintaining the commanded fuel rail pressure.
What It Feels Like
- Loss of power under load
- Hard starting
- Engine derate
- Stalling
What Actually Causes It
This is one of the most misdiagnosed codes.
Common root causes include:
- Fuel restrictions (filters, lines, pickup tubes)
- Air entering the fuel system
- Injector return flow issues
- Weak or failing fuel pump
👉 The code does NOT automatically mean the pump is bad.
What Gets Misdiagnosed
Fuel pumps are often replaced first – when the real issue is restriction or injector leakage.
What To Check First
- Fuel filters and supply lines
- Air leaks in the system
- Injector return flow
Camshaft/Crankshaft Correlation (Code 0731)
What it Means: The ECM detects a mismatch between camshaft and crankshaft position signals.
What It Feels Like
- May run normally
- Possible hard start
- Intermittent fault after rebuild
What Actually Causes It
In many cases – especially after an overhaul – the issue is not timing itself.
Common causes include:
- Camshaft movement during assembly
- Improper installation sequence
- Tone wheel indexing issues
- Sensor signal inconsistencies
👉 Even slight movement can trigger this code.
What Gets Misdiagnosed
Sensors are often replaced unnecessarily when the issue is mechanical alignment or installation error.
What To Check First
- Verify cam/crank timing
- Confirm installation sequence
- Inspect tone wheel and sensors
👉 Read the full breakdown here:
ISX15 Camshaft and Crankshaft Misalignment Code 0731 After Overhaul
Turbo Boost Pressure Low
What it Means: The engine is not reaching the expected boost pressure.
What It Feels Like
- Loss of power
- Slow acceleration
- Excessive smoke
What Actually Causes It
This code is often blamed on the turbo – but that’s not always correct.
Common causes include:
- Boost leaks (charge air cooler, piping)
- Exhaust restrictions
- Fuel system issues limiting combustion
- Turbocharger failure
👉 Low boost can be caused by air, fuel, OR exhaust issues.
What Gets Misdiagnosed
Turbochargers are frequently replaced when the issue is actually a boost leak or fuel limitation.
What To Check First
- Charge air cooler and piping
- Exhaust system restrictions
- Fuel delivery performance
EGR / SCR System Faults
What it Means: There is a problem within the emissions system (EGR, DEF, or SCR).
What It Feels Like
- Engine derate
- Reduced power
- Warning lights
What Actually Causes It
Common causes include:
- Soot buildup in EGR system
- Sensor drift or failure
- DEF system issues
- Clogged components
👉 Many emissions codes are caused by sensor inaccuracies – not component failure.
What Gets Misdiagnosed
Entire components (EGR valves, SCR systems) are replaced when the issue is sensor-related or contamination-based.
What To Check First
- Sensor readings and calibration
- EGR passages for buildup
- DEF system operation
Engine Oil Pressure Low (SPN 100 FMI 1)
What it Means: The engine is not maintaining proper oil pressure.
What It Feels Like
- Warning light
- Engine shutdown
- Noise or knocking
What Actually Causes It
This is one of the most critical codes.
Common causes include:
- Worn engine bearings
- Oil pump issues
- Low oil level
- Internal engine wear
👉 Oil pressure issues often indicate internal damage – not just a sensor issue.
What Gets Misdiagnosed
Assuming it’s a bad sensor when it’s actually mechanical wear or oiling failure.
What To Check First
- Oil level and condition
- Oil pressure with mechanical gauge
- Internal engine condition
👉 For more on oil-related damage, read about:
The 6 Most Common Diesel Engine Oil Pump Failures
The Problem With Relying on Fault Codes Alone
Fault codes are a starting point – but they don’t tell the whole story.
👉 They identify a symptom – not the root cause.
Many diesel engine failures are the result of:
- Multiple systems interacting
- One failure affecting another
- Underlying issues not addressed
This is why replacing parts based on codes alone often leads to:
- Repeat failures
- Increased downtime
- Higher repair costs
Final Takeaway
The most common diesel engine fault codes all have one thing in common:
👉 They rarely point directly to the failed part.
Understanding what actually causes these codes is what allows you to:
- Diagnose correctly
- Fix the problem the first time
- Avoid unnecessary repairs
Need Help Diagnosing a Fault Code?
If you’re dealing with:
- Persistent fault codes
- Loss of power
- Repeat failures
At Highway and Heavy Parts, we help you identify the problem and get the right parts the first time.
Call 844-304-7688 or visit highwayandheavyparts.com to place your order today.
From diagnosis through delivery, we’re Highway and Heavy Parts.
