Fuel Rail Pressure Sensor - Voltage Low
The CAT-CID1797-FMI4 (Caterpillar) diesel fault code means: Fuel Rail Pressure Sensor - Voltage Low. This is a serious severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- advanced
- Estimated cost
- $300-$4500
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Common Symptoms
- Hard start or extended crank
- Low power, derate
- Black or white smoke
- Check Engine Light
- Stall at idle
Probable Causes (Ranked by Likelihood)
- Fuel filter restriction / old filters Very Likely
- HP fuel pump internal wear Likely
- Fuel rail regulator stuck Likely
- Injector internal leak dropping rail pressure Possible
- Air intrusion through supply plumbing or filter housing Possible
Step-by-Step Diagnostic Procedure
Confirm fault in CAT ET and record commanded vs actual rail pressure at idle and rated.
Change fuel filters and prime system; retest.
Check supply pump outlet pressure with a gauge; must meet HP pump inlet spec.
Test for air intrusion in low-pressure side (clear line).
Perform injector return flow test to identify leaking injector(s).
If supply and injectors test OK, HP pump wear is the likely cause.
Frequently Asked Questions
What does CAT CID1797-FMI4 mean?
CAT code CID1797-FMI4 (Fuel Rail Pressure Sensor - Voltage Low) is a serious-severity fuel system fault. Fuel rail pressure sensor shorted to ground. Common-rail C15/C13 engines rely on 30,000 psi+ rail pressure; even small supply issues or injector leaks can flag this.
Should I replace the HP pump right away?
No — start with fuel filters and a supply pressure test. HP pump replacement on a C15 is expensive ($3,000+) and often unnecessary. Confirm injector return flow and supply pressure first.
How much to fix CAT CID1797-FMI4?
Filter service is $80-$150. Supply pump replacement is $400-$900. Injector replacement runs $500-$800 each installed. HP pump replacement is $3000-$4500.
Sources
This page is built from documented references. Verify against your own service info before repair work.