Bobcat M6402 (64-02) Fault Code: Switched Power Relay Error On
Also shown on the panel as 64-02
Switched Power Relay Error On · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
M6402 (panel code 64-02) sets when the gateway controller sees voltage on feedback wire 3450 or wire 8020 (depending on model/harness variant) when the switched power relay is not commanded on. It only occurs at startup with key switch (run/enter) on, and it only shows up in diagnostic software, not as a dash warning.
Medium severity. M6402 is a startup diagnostic code that does not itself disable a system, but it points to a relay or wiring fault that could feed unintended voltage into HVAC and other fused circuits. It is not an emergency stop condition, but it should be run down before it masks a bigger electrical short.
What does Bobcat error code M6402 mean?
M6402 is logged by the gateway controller when it detects voltage present on a feedback wire (wire 3450 on some harness/model variants, wire 8020 on others) at a time when that circuit was not commanded to be powered. This feedback wire tells the controller whether the switched power relay is actually on or off.
The switched power relay itself supplies voltage to the HVAC relay and to several fused circuits (fuses grouped under FRC1, with the exact fuse numbers varying by model and harness variant: fuse 1, 2, 3, 4, 5, 6, 7, 8, 10, or 12 depending on version, or simply fuses 6, 7, and 8 on some variants). If the controller sees voltage on the feedback wire when it did not turn the relay on, something downstream is feeding voltage back where it should not be.
This code is a startup-only check. It is only displayed in diagnostic software, it is not something the operator will see flash on the dash, so it is normally caught during a service scan rather than during daily operation.
What triggers a Bobcat M6402 code?
Key switch (run/enter) on. The code is only evaluated and set at startup.
Common causes of M6402
- Feedback wire 3450 or wire 8020 (depending on harness/model variant) shorted to supply voltage.
- One of the circuits controlled by the FRC1 fuses tied to the switched power relay (fuse numbers vary by variant, commonly among fuse 1, 2, 3, 4, 5, 6, 7, 8, 10, or 12, or fuses 6, 7, and 8 on some builds) shorted to supply voltage.
- A faulty switched power relay.
- A faulty gateway controller.
- Moisture, corrosion, or pushed-back pins in the gateway controller connectors, auxiliary controller connectors, ACS controller connectors, drive/drive plus controller connectors, switched power relay connector, mainframe harness, ACS harness, AWS/SJC harness, engine harness, or cab harness.
- Intermittent faults: if the code is not active at the time of testing, the cause may not be found right away. Loose connections, corrosion, pushed-back pins, the controller, the wire harness, or a combination of these can all be involved.
How to troubleshoot Bobcat M6402: first checks
- Connect diagnostic software and confirm the code is active or check when it last set, since this code only logs at startup and will not show as a dash warning.
- Inspect the switched power relay and its connector for corrosion, moisture, or pushed-back pins.
- Check the feedback wire (3450 or 8020, depending on your model/variant) for a short to supply voltage, tracing it back through the mainframe harness, cab harness, engine harness, ACS harness, and AWS/SJC harness as applicable.
- Check the FRC1 fuse circuits tied to the switched power relay for shorts to supply voltage. Confirm which fuse numbers apply to your specific model and harness version, since this varies.
- Inspect connectors at the gateway controller, auxiliary controller, ACS controller, and drive/drive plus controller for corrosion or pushed-back pins.
- If the code will not reproduce, treat it as intermittent: recheck all connections, look for corrosion, and wiggle-test harness sections rather than assuming no fault exists.
How the code clears
Cycle power (turn the key switch off, then back on) once the repair is complete. No separate clearing step is listed beyond cycling power.
Affected models and serial ranges
M6402 appears in our records across 16 Bobcat models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| S510 | SN A3NJ11001-99999, SN A3NK11001-99999, SN ATZC11001-ATZC99999 |
| S530 | SN A7TV11001-99999, SN ATZD11001-99999 |
| S550 | SN A3NK11001-A3NL99999, SN A3NM11001-99999 |
| S570 | SN A7U711001-799999, SN A7U811001-899999 |
| S590 | SN ANMN11001-99999, SN ANMP11001-99999 |
| S630 | SN A3NT12370-99999, SN A3NU11001-11111, SN A3NU11112-99999 |
| S650 | SN A3NV11001-13098, SN A3NV13099-99999, SN A3NW11001-11248, SN A3NW11249-99999 |
| S750 | SN A3P211001-299999 |
| S770 | SN A3P411001-99999 |
| T550 | SN A7UJ11001-AJZV12276 |
| T590 | SN ALJU11001-999999, SN B37811001-999999, SN B3Z711001-999999 |
| T630 | SN A7PU11001-11663, SN A7PU11664-99999 |
| T650 | SN A3P012214-099999, SN A3P111242-199999 |
| T750 | SN ANKA11001-A99999 |
| T770 | SN A3P811001-899999, SN A3P911001-999999 |
| T870 | SN A3PG11001-99999, SN A3PH11001-99999 |
Frequently asked questions
What does Bobcat code M6402 mean?
It means the gateway controller detected voltage on a feedback wire (wire 3450 or wire 8020 depending on your model) tied to the switched power relay, when that relay circuit was not commanded on. It is a startup-only diagnostic code.
Will M6402 show up on the dash display?
No. This code is only displayed in diagnostic software. It does not trigger a dash warning light or message for the operator.
Which fuses are affected by the switched power relay in this code?
It depends on your model and harness variant. Some versions tie the relay to FRC1 fuses 1, 2, 3, 6, 7, 8, and 10, others to FRC1 fuses 1 through 5, 7, 8, and 12, and some simpler harnesses to just fuses 6, 7, and 8. The relay also feeds the HVAC relay in all versions.
How do I clear M6402 after repair?
Cycle power by turning the key switch off and back on. No other clearing procedure is listed.
Why can't I find M6402 active during testing?
This code only sets at startup with the key switch in run/enter, so if it is intermittent it may not be present when you connect diagnostic software later. Suspect loose connections, corrosion, pushed-back pins, the controller, or the wire harness.
Is M6402 safe to keep driving with?
The code itself is a startup diagnostic flag rather than an active fault warning, but it points to a wiring or relay issue that can affect HVAC and other fused circuits. It is best to have it diagnosed rather than ignored, especially if it recurs.
What models does M6402 apply to?
It applies across Bobcat's S510, S530, S550, S570, S590, S630, S650, S750, S770, T550, T590, T630, T650, T750, and T770 loader lineup, though the exact wire and fuse numbers involved can differ by model.