John Deere Engines SPN620FMI4 Fault Code: Sensor Supply Voltage Low
Also called Sensor Supply 1 Voltage Low, Sensor Supply 2 Voltage Low
Sensor Supply Voltage Low · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
SPN 620 FMI 4 means the ECU cannot maintain the required 5 volts on one of its sensor supply circuits (Sensor Supply 1, Sensor Supply 2, or the general Sensor Supply Voltage circuit, depending on application). This can cause affected sensors to read wrong or fall back to default values, and in some cases the engine can derate or throttle response can be affected.
High severity. Engine operation may not be affected at all if only one lightly used sensor is impacted, but on some applications this code can cause numerous other faults to set, engine derate, and incorrect sensor readings, so it should not be ignored.
What does John Deere Engines error code SPN620FMI4 mean?
The ECU sends out a regulated 5 volt supply to power several sensors, such as the oil pressure sensor, throttle position sensors, MAP sensor, or fuel pressure sensor depending on the machine's setup. Each sensor pulls down that 5 volts by a specific amount depending on what it is measuring, and the ECU reads that voltage drop to calculate the actual sensor value.
At the same time, the ECU is watching the actual voltage level on the supply circuit itself to make sure it stays where it should. SPN 620 FMI 4 sets when the ECU sees that voltage sag below what is expected on the 5 volt sensor supply circuit, whether that is Sensor Supply 1, Sensor Supply 2, or the shared sensor supply circuit used on a given application.
Depending on which sensors share that particular supply circuit and what caused the drop, the effects range from no noticeable change up to multiple sensor faults setting at once, incorrect readings, default value substitution, and possible engine derate or altered throttle response.
What triggers a John Deere Engines SPN620FMI4 code?
On some ECU versions the exact threshold is defined: the code sets if the ECU detects a voltage below 4.59 volts on the 5 volt sensor supply circuit (DTC 000620.04), or on another version if voltage drops below 4.44 volts (DTC 620.04). Other versions state only that the code sets when the ECU detects a supply voltage lower than the expected 5 volts, without giving a separate numeric threshold.
Common causes of SPN620FMI4
- Dirt, damage, or poor terminal positioning in the ECU connector (including the ECU 30-way connector) or at any connector for a component using the sensor supply voltage
- A bad sensor on the affected supply circuit, such as the oil pressure sensor, analog throttle sensor A, B, or C, MAP sensor, or fuel pressure sensor
- An open or short in the wiring harness between the ECU and the sensors it supplies
- Sensor 5 volt supply circuit shorted to ECU ground circuit
- Sensor 5 volt supply circuit shorted to sensor ground circuit
- A short in the throttle circuit, identified if supply voltage reads normal once the IP harness is disconnected
- Bad jumpers in the performance program connector
- A bad ECU
How to troubleshoot John Deere Engines SPN620FMI4: first checks
- Without disconnecting anything first, visually inspect the ECU connector and all connectors for components using this 5 volt supply for dirt, contamination, damage, or terminals that are pushed back or poorly positioned
- Inspect the wiring harness between the ECU and the affected sensors for chafing, pinches, or exposed conductors that could cause a short to ground
- Use the JT07328 Connector Adapter Test Kit to check voltages at the connectors instead of back-probing or forcing test probes into terminals, since forcing probes into terminals can damage them
- Identify which sensors use the affected supply (Sensor Supply 1 or Sensor Supply 2) for your specific application, since the wiring and sensors involved differ between OEM and tractor applications
- If voltage reads normal with the IP harness disconnected, suspect a short in the throttle circuit rather than the ECU itself
- Check the performance program connector jumpers if equipped, since bad jumpers there are a listed cause
- If all wiring, connectors, and sensors check out, the ECU itself may be at fault
How the code clears
No separate clearing step is listed. Standard practice is to repair the wiring, connector, or sensor fault found during diagnosis, then clear the code and confirm the sensor supply voltage stays within range with the ECU running before returning the machine to service.
Frequently asked questions
What does SPN 620 FMI 4 mean on a John Deere engine?
It means the ECU could not keep the required 5 volts on one of its sensor supply circuits. This circuit powers sensors like the oil pressure sensor, throttle position sensors, MAP sensor, or fuel pressure sensor, and when the voltage sags the ECU either logs incorrect readings or substitutes default values.
Can I keep running the machine with SPN 620 FMI 4 active?
It depends on which sensors are affected. Engine operation may not be affected at all, or the engine may derate and throttle response may change, and other sensor faults may set at the same time. Because the outcome is unpredictable, it is safest to diagnose the fault before continuing normal work.
What voltage does the ECU expect on the sensor supply circuit?
The circuit is a regulated 5 volt supply. On some ECU versions the fault sets specifically below 4.59 volts, and on others below 4.44 volts. Other versions just state the code sets when voltage drops below the expected 5 volts without a separate published threshold.
What is the most common cause of low sensor supply voltage?
Dirty, damaged, or poorly seated terminals at the ECU connector or at a sensor connector are commonly listed causes, along with an open or short in the wiring harness and a short to ground on the supply circuit itself.
Do I need special tools to check this circuit?
Yes, it is recommended to use the JT07328 Connector Adapter Test Kit to take measurements at the connectors. Forcing test probes directly into connector terminals can damage them and create a new problem.
Why would multiple fault codes set at the same time as SPN 620 FMI 4?
Because several sensors can share the same 5 volt supply circuit, a low voltage condition can cause each of those sensors to send bad readings to the ECU, which can trigger separate fault codes for each affected sensor in addition to SPN 620 FMI 4.
Is SPN 620 FMI 4 the same on every John Deere engine application?
No. The exact sensors tied to Sensor Supply 1 or Sensor Supply 2 differ between OEM and tractor applications, and the exact voltage threshold that triggers the code also varies by ECU level, so the wiring and affected sensors should be confirmed for your specific application.