Fuel Rail Pressure Sensor Voltage Too High
The MASSEY-FERGUSON-157-3 (AGCO Power) diesel fault code means: Fuel Rail Pressure Sensor Voltage Too High. This is a serious severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- moderate
- Estimated cost
- DIY sensor replacement $80-$180 for the sensor plus your time. Professional diagnosis and repair at an AGCO dealer typically runs $250-$600 depending on labor rate and whether wiring repair or ECM work is needed.
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Common Symptoms
- Amber or red fault light illuminates on the instrument cluster with SPN 157 FMI 3 displayed
- Engine derate kicks in, limiting RPM or power output noticeably during field work
- Hard starting or engine cranks longer than normal before firing
- Engine stumbles or surges under load, especially during heavy draft or PTO work
- Fuel consumption increases noticeably with no change in workload
- Black or excessive smoke from the exhaust at startup or under load
- AGCO EDT software logs an active or pending fault against the rail pressure sensor circuit
Probable Causes (Ranked by Likelihood)
- Open circuit in the fuel rail pressure sensor signal wire between the sensor and ECM connector Very Likely
- Short-to-power (5V reference or 12V battery feed) on the sensor signal wire causing voltage to read above the valid range Very Likely
- Failed fuel rail pressure sensor with internal signal pin open or shorted high Likely
- Corroded, spread, or backed-out pin at the ECM harness connector or sensor connector causing a poor signal ground reference Likely
- Damaged wiring harness where it routes near the exhaust manifold or injection lines, causing insulation breakdown and contact with a voltage source Possible
- ECM internal fault affecting the analog input channel for SPN 157 Less Likely
Step-by-Step Diagnostic Procedure
Start by pulling the fault with AGCO EDT or reading SPN 157 FMI 3 on the instrument cluster. Confirm the fault is active, not just stored, before chasing wiring.
With the ignition key in the RUN position and the engine OFF, unplug the fuel rail pressure sensor connector. Measure voltage between the signal pin and sensor ground pin at the harness side. You should see the ECM pull-up voltage, typically around 5V. A reading above 5.2V or at battery voltage (12-14V) points to a short-to-power on the signal wire.
With the sensor still unplugged, measure resistance from the signal wire back to the ECM connector. Use a wiring diagram for your specific series (MF 5700, 6700, 7700, or 8700) to identify the correct ECM pin. Resistance should be close to 0 ohms for a good wire. An open reading means a broken wire.
Inspect the harness from the fuel rail sensor back toward the ECM, paying close attention to where it routes near the exhaust manifold, injection lines, and any factory plastic looms. Look for melted insulation, chafe points, or pinched sections. Repair any damaged sections with properly rated wire before replacing the sensor.
Check the ECM ground reference wire for the sensor. Measure resistance from the sensor ground pin at the harness connector to a clean chassis ground. More than 1 ohm of resistance here can cause the signal voltage to read artificially high. Clean or repair the ground path as needed.
If wiring checks out clean, install a known-good replacement fuel rail pressure sensor. Clear the fault with AGCO EDT, start the engine, and check that SPN 157 FMI 3 does not return. Monitor live rail pressure data in AGCO EDT to confirm the sensor reads in the expected range, typically 200-1800 bar depending on engine load.
If the fault returns immediately after a sensor swap with verified-good wiring, suspect an ECM analog input failure. This step requires AGCO EDT and dealer-level access to confirm. Contact your AGCO dealer for ECM diagnosis and possible reprogramming or replacement.
Frequently Asked Questions
What does SPN 157 FMI 3 mean on a Massey Ferguson tractor?
SPN 157 FMI 3 tells you the ECM is reading a voltage that is too high on the fuel rail pressure sensor signal circuit. FMI 3 always means the signal is above the expected range. This usually points to a broken wire, a wire shorted to a voltage source, or a failed sensor, not an actual high-pressure condition inside the fuel rail.
Can I keep working with this code active?
Your tractor will likely stay running but in a derate condition, meaning the ECM limits power or RPM to protect the injection system. Continued operation with an unknown rail pressure signal risks injector damage and poor fuel metering. Finish the immediate job if you have to, but get it diagnosed before your next full workday.
How much does it cost to fix SPN 157 FMI 3?
If you can do the wiring inspection and sensor swap yourself, plan on $80-$180 for the sensor and a few hours of your time. If the fault turns out to be a wiring harness repair or ECM issue, an AGCO dealer will typically charge $250-$600 in parts and labor, depending on what they find.
Will replacing the sensor fix it every time?
Not always. FMI 3 is a circuit fault, so the wiring between the sensor and the ECM is just as likely to blame as the sensor itself. Check the harness first, especially near heat sources like the exhaust manifold. Replacing the sensor without checking the wiring can mean spending money on a part that does not solve the problem.