Diesel Trouble Codes

Loss of Power — Cummins ISX15

Diagnostic reference for loss of power on Cummins ISX15 (2010–2017) with CM2250/CM2350 ECM. Covers derate, fuel rail pressure, VGT turbo and EGR-related power…

Updated October 15, 2026

What’s going on

Loss of power on the ISX15 usually means the ECM is limiting torque due to an active fault or a real performance problem in fuel delivery, air/boost control, or EGR. Low power is often a derate from another fault, low fuel rail pressure, a sticking VGT turbo, or EGR system problems. Read all active and inactive codes.1

Most likely causes

  1. Engine protection torque derate — ECM is applying an engine protection torque derate, typically as an effect of another active fault. FC 3714 is informational; find and fix the companion code first.12
  2. Low fuel rail pressure — XPI high-pressure common rail cannot maintain commanded pressure. Typical triggers are fuel supply/filter restriction, air in fuel, leaking high-pressure relief valve, injector leakage, or a weak pump/actuator. Often sets FC 559.13
  3. VGT turbocharger performance issue — Variable-geometry turbo not producing expected boost. Sticking VGT actuator or vanes, boost leaks, or turbo speed sensor problem can reduce airflow. Often sets FC 687.14
  4. EGR system restriction or fault — Cooled EGR valve, cooler or differential pressure/temperature sensors causing incorrect EGR flow or high exhaust temperatures. Can contribute to power loss and may be associated with high turbine inlet temperature, e.g., FC 2346.15

How to narrow it down

  1. Read and record all active and inactive fault codes; note FC 3714, FC 559, FC 687, FC 2346, FC 1866 if present.
  2. Check ECM derate status and identify any companion codes that triggered engine protection torque derate; FC 3714 is an effect, not a root cause.
  3. Verify fuel supply health: lift pump operation, filter condition, signs of air or water in fuel, and rail pressure actual vs commanded at idle and load. Low rail pressure points to supply restriction, air ingress, relief valve or pump/actuator issues.
  4. Inspect VGT turbocharger operation: actuator response, vane sticking, boost pressure vs expected, and turbo speed data. Low turbo speed or boost suggests VGT sticking, leaks, or sensor fault.
  5. Evaluate EGR system: EGR valve command vs position, cooler restriction signs, differential pressure and temperature sensor readings, and turbine inlet temperature. Elevated turbine inlet temperature can indicate excess fueling, boost/air restriction or EGR faults.

Common mistake to avoid

Replacing the EGR valve without testing it. If the valve still moves freely, a carbon cleaning often fixes the flow code; and if it opens on command but the code stays, the blockage is in the passages or cooler, not the valve. Source: FleetRabbit — EGR valve problems and repair.

Parts and tools you may need

Tools you’ll need

Rough cost and difficulty

Likely repairPartsShop laborWho usually does it
Shop diagnosis—1–2 hrsShop
EGR valve/passage carbon cleaning$20–$1001–4 hrsDIY with tools and a manual
Fuel filter service$60–$2000.3–1 hrsEasy DIY
Electronic VGT actuator (needs calibration)$800–$1,8001–3 hrsDIY with tools and a manual

Typical US independent-shop figures; labor runs about $140–$200 per hour (dealers charge more). Confirm the diagnosis before buying parts.

When to see a pro

If high-pressure fuel system internal inspection is required, or if turbocharger removal/rebuild or EGR cooler service is needed, have a qualified diesel technician perform the work. Do not service high-pressure fuel components without proper training and safety equipment.

Check for recalls

Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.

Sources

  1. Curated symptom seed — Cummins ISX15 (curated) ↩︎ ↩︎ ↩︎ ↩︎ ↩︎

  2. Curated DTC seed — Cummins ISX15 (curated) ↩︎

  3. Curated DTC seed — Cummins ISX15 (curated) ↩︎

  4. Curated DTC seed — Cummins ISX15 (curated) ↩︎

  5. Curated DTC seed — Cummins ISX15 (curated) ↩︎