Updated: 2026-11-10
Safe to driveTo the nearest safe stop
J1939 code SPN 97. Below — how the fault is worded at the factory by Cummins ISB, DAF, Dongfeng, International, KAMAZ, MAZ, Renault Trucks, SITRAK, YaMZ and ZF AS-Tronic, and what the second half of the code (FMI) means in the standard.
SPN 97 is the water-in-fuel indicator. A sensor sits in the water separator (the sedimenter of the coarse fuel filter) and tells the engine computer when water collects at the bottom of the bowl. The engine computer then turns on a warning, and on some engines it reduces power.
Water is the enemy of the fuel system. It does not burn, so it causes poor combustion, rust inside the fuel pump and injectors, and it can feed microbial growth in the tank that clogs the filters. This is why the code should be taken seriously even when the engine runs normally.
The second half of the code, the FMI, tells you what the sensor reports. FMI 3 and FMI 4 point to the sensor circuit: voltage above normal or shorted high (3), voltage below normal or shorted low (4). When water is actually detected, the FMI shows how serious it is, and the exact number depends on the make: for example FMI 15 or 16 on many engines, and FMI 18 (critical level) on some DAF engines. The table below shows the exact wording for each make.
If the light comes on while the engine runs normally, drain the separator at the first safe stop, provided you have a drain valve, a clear container and the right tools. Water settles at the bottom, so drain until clean fuel comes out, not just a few drops.
If the engine loses power, stumbles or starts badly, stop as soon as it is safe and call for help. Note where you last refuelled. If the code comes back after draining, or the water returns within a day or two, do not keep driving on that fuel: go to a service point.
Check the sensor connector and the wiring first, using the wiring diagram for the make. Corrosion at the connector at the bottom of the filter is a common cause. Clean the sensor electrodes; a dirty sensor can give a false reading.
Drain the separator and the tank sediment, then inspect the bowl for cracks. Check the tank breather and the filler cap: a damaged cap or breather lets in water through condensation as the temperature changes. Ask where the fuel was bought, because many water cases come from one bad delivery.
Replace the filter element as the service schedule says, not only when a code appears. In winter the water in the bowl can freeze and block the drain: warm it up, and do not replace the sensor just because the reading is wrong.
Replace the sensor only after the connector, the wiring and the separator are confirmed good, and check the manufacturer manual for any reset or calibration after replacement.
SPN 94 - Fuel delivery pressure - if the fuel supply is also weak.
SPN 95 - Fuel Filter Differential Pressure - if the filter is clogged after water or dirt.
Fuel problems: symptoms and what to check
The water-in-fuel sensor sits in the sedimenter of the coarse filter (separator). Water is heavier than fuel and collects at the bottom; the sensor detects it by conductivity.
| Drain the sediment | at every service and immediately on the signal |
Cummins ISBDAFDongfengInternationalKAMAZMAZRenault TrucksSITRAKYaMZZF AS-Tronic
| FMI 3 | Water-in-fuel indicator sensor circuit voltage high |
| FMI 15 | Water-in-fuel indicator high, least severe |
| FMI 3 | Fuel water content sensor F655 short to power supply at ECU pin A34 |
| FMI 4 | Fuel water content sensor F655 short to ground at ECU pin A34 |
| FMI 15 | Fuel water content sensor F655 water detected |
| FMI 18 | Fuel water content sensor F655 water detected, critical level |
| FMI 3 | Short circuit to power or open circuit of the water-in-fuel sensor |
| FMI 4 | Short circuit to ground of the water-in-fuel sensor |
| FMI 15 | Water detected in fuel |
| FMI 16 | Water in fuel exceeds limit |
| FMI 4 | Water In Fuel Indicator |
| FMI 6 | Water In Fuel Indicator |
| FMI 3 | Fuel filter water separator sensor |
| FMI 4 | Fuel filter water separator sensor |
| FMI 5 | Water-in-fuel sensor circuit open |
| FMI 11 | Water in fuel above permissible limit (replace fuel) |
| FMI 3 | Fuel pre-filter water separator sensor circuit fault - Voltage above allowed. Blink code: 2-1-4. Repair: Check the pre-filter water trap sensor and its wiring. |
| FMI 4 | Fuel pre-filter water separator sensor circuit fault - Voltage below allowed. Blink code: 2-1-4. Repair: Check the pre-filter water trap sensor and its wiring. |
| FMI 11 | Water content in fuel above the allowed limit - Water detected in fuel. Blink code: 2-1-1. Power will be reduced. Repair: Drain the water from the fuel pre-filter. |
| FMI 8 | Engine operating mode signal fault. Repair: Run the diagnostic check of the EECU. Check the programming. Switch the ignition off, switch it on again and repeat the test. If the fault persists, replace the control unit. |
| FMI 12 | Water-in-fuel sensor self-test failed, upper threshold exceeded. Scan tool code: P2264. Effect: Torque may be limited. |
| FMI 3 | Fuel pre-filter water separator sensor circuit fault (Voltage above allowable limit). Blink code: 2-1-4. Repair: Check the pre-filter water trap sensor and its wiring. |
| FMI 4 | Fuel pre-filter water separator sensor circuit fault (Voltage below allowable limit). Blink code: 2-1-4. Repair: Check the pre-filter water trap sensor and its wiring. |
| FMI 11 | Water content in fuel above allowable limit (Water detected in fuel). Blink code: 2-1-1. Power will be reduced. Repair: Drain the water from the fuel pre-filter. |
FMI is per the block maker's own table, not the J1939 table below.
| FMI 8 | No CAN bus signal: "engine speed". |
Safe to drive. Below — how much time you have and what happens if you keep going.
Fuel is not arriving in the right amount or at the right pressure. It shows up first on climbs and under load.
You will most likely stall out on the road, and may not restart on the shoulder. A clogged filter or water in the tank you can nurse to a shop; a failing injection pump you cannot.
This is an electrical fault: an open, a short or a bad contact in the sensor circuit. The value it measures is most likely fine. You can drive. But until the circuit is repaired this system runs blind and cannot warn you about a real failure.
Check the fuel filter and water trap, drain the water. In cold weather, suspect fuel waxing.
KAMAZMAZYaMZSITRAKDongfengDAFRenault TrucksCummins ISBInternational
If the engine runs normally and there is no red lamp or power loss, you can drive to a service point, but drain the separator first if you can. If the engine loses power or stumbles, stop.
Not necessarily. Water often comes back from the tank, so check the tank sediment and the breather, and refuel at a trusted station.
Usually not first. Check the connector, the wiring and the separator bowl. Replace the sensor only when they are good.
Water in the bowl can freeze and block the drain, and the sensor can read wrongly while it is cold. Warm the bowl and check the drain before changing parts.
The FMI is the half of the code that says how the water sensor is failing. These are the values people search for most, based on SAE J1939-73 and the usual Cummins listings.
1. Drain the separator into a container until clean fuel with no cloudy layer comes out. Water is heavier than diesel and sits at the bottom.
2. Clear the code and run the engine. If FMI 15 returns within a day or two, take a sample from the tank bottom into a clear jar and let it settle: a separate water layer means the tank or the supplier is the problem.
3. For FMI 3, 4 or 2, look at the sensor connector first. Green corrosion, a loose latch or a wire rubbing on the bracket are the usual findings.
4. Only then test or replace the sensor.
Water matters beyond the warning: modern high-pressure common-rail pumps rely on diesel for lubrication and water shortens their life quickly, so a repeat code is worth chasing.
FMI 15 means the engine computer thinks there is water. FMI 3 means the electrical circuit is open or shorted high, and there may be no water at all.
Usually not on its own; it is a warning. But if water reaches the injection pump the engine can lose power, so drain it promptly.
Scanner threw several codes at once? Paste the whole list — we will show which one is the root cause and which ones simply followed.