Low power complaints are one of the most common issues seen in diesel trucks—and also one of the hardest to diagnose. Whether you're working on a Cummins ISX, Detroit DD15, or a Paccar MX-13, this guide will walk you through the step-by-step process to pinpoint what’s robbing the engine of power using a diesel diagnostic laptop.

1. Check for Active or Inactive Fault Codes

Start with a full scan using your diesel laptop:

• Look for SPN and FMI codes related to boost pressure, EGR flow, or fuel delivery.

• Don’t ignore inactive codes—they can point to intermittent issues.

✅ Tip: Use software like Cummins INSITE, JPRO, or CAT ET to get detailed troubleshooting trees.

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2. Inspect Fuel Delivery System

Low rail pressure = low power. Check:

• Fuel rail pressure (FRP) under load

• Fuel filter restriction sensor readings

• Lift pump pressure at idle and WOT

⚠️ Low rail pressure with normal command = injector or rail leak.
⚠️ Low command = failed sensor, fuel actuator, or control issue.

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3. Evaluate Turbo and Boost Pressure

Live data you want to track:

• Desired vs. actual boost

• Turbo VGT position or actuator duty cycle

• MAP sensor readings

If boost never rises above 10–15 psi under load, you may have:

• Leaking charge air cooler (CAC)

• Sticky turbo vanes

• Failed VGT actuator

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4. Run a Cylinder Cutout Test

Use diagnostic software to:

• Disable one injector at a time and observe RPM drop

• No drop = weak or dead cylinder

🛠 Weak injectors or compression loss may not throw a code but will rob power.

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5. Check for DPF or EGR Restrictions

A clogged exhaust = choking the engine.

• Monitor DPF delta pressure

• Track soot load %

• Look at EGR valve position vs. command

If soot % is stuck high, run a forced regen and watch EGT sensors.

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6. Log a Road Test with Live Data

Create a log file while driving:

• Throttle %

• Engine load

• Boost vs. desired

• FRP vs. desired

• NOx sensors and EGR flow

Then review the log to find where power drops—on throttle tip-in, under load, or mid-regen.

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7. Bonus: Check for Throttle or Torque Derates

Some trucks will reduce torque output silently if:

• DEF quality is poor

• NOx sensors are misreporting

• EGR temp is outside spec

Use parameter view to check if torque limiters are active.

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Final Thoughts:

Low power can come from dozens of root causes, but a structured approach and proper diagnostic tools make the difference. Start with codes, verify sensor data, run key tests, and always compare actual vs. commanded values.

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