DiagnosticsMedium severity

Bobcat M6403 (64-03) Fault Code: Switched Power Relay Error Off

Also shown on the panel as 64-03

Switched Power Relay Error Off · ai-assisted, editor-reviewed · Last updated 2026-07-13

TL;DR

Bobcat code M6403, panel form 64-03, sets when the gateway controller commands the switched power relay on but does not see voltage return on the feedback wire (wire 3450 on some machines, wire 8020 on others). It sets with the key switch in run/enter, and points to a wiring, connector, or relay problem in the switched power circuit rather than a specific sensor failure.

Medium severity. M6403 does not describe an engine shutdown or a lifting lockout by itself, but the switched power relay it monitors feeds circuits that other systems may depend on. Treat it as a diagnose-this-shift issue rather than something to ignore, especially if other electrical symptoms show up alongside it.

What does Bobcat error code M6403 mean?

M6403 is a gateway controller diagnostic on the switched power relay circuit. The gateway controller commands the relay to switch on, then checks a feedback wire to confirm the relay actually closed and voltage is present. If that voltage isn't seen when expected, the controller logs M6403.

There are two documented feedback paths for this fault. On some machines the controller monitors feedback wire 3450, and on others it monitors feedback wire 8020. Both versions of the fault work the same way and share the same possible causes, they just point at different wires depending on the machine's wiring configuration.

Because this is a feedback-based diagnostic, the fault does not necessarily mean the relay is dead. It means the controller commanded power and didn't get confirmation back, which can be caused by an open circuit, a short to ground, a bad connector, or the relay or controller itself.

Common causes of M6403

Feedback wire 3450 or 1790 (or 8020/1790 on the other wiring variant) open or shorted to ground
Supply wire 1050 open
Switched power relay failure
Gateway controller failure
Moisture, corrosion, or pushed-back pins in the gateway controller connectors, switched power relay connector, or the mainframe wire harness
Intermittent fault: if the code is not active at the time of testing, the root cause may not be found on that visit. Probable causes in this case include loose connections, corrosion, pushed back pins, the controller, the wire harness, or a combination of these problems

How to troubleshoot Bobcat M6403: first checks

With the key switch in run/enter, check whether the code is currently active or was only logged historically, since an intermittent fault changes the diagnostic approach
Inspect the gateway controller connectors and the switched power relay connector for moisture, corrosion, or pins that have been pushed back out of position
Trace supply wire 1050 for an open circuit
Trace the feedback wire for the machine's variant (wire 3450 or wire 8020, along with wire 1790) for an open circuit or a short to ground
Check the mainframe wire harness along the relay and controller circuit for chafing, corrosion, or loose connections
Test or swap the switched power relay if wiring and connectors check out clean
If wiring, connectors, and relay all test good, suspect the gateway controller itself

How the code clears

Bobcat lists cycling power as the step to clear M6403 once the repair is complete. No separate clearing procedure or reset sequence is listed beyond turning the key off and back on after the fault has been corrected.

Affected models and serial ranges

M6403 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

Safety-critical: confirm before use

Electrical safety: stop the engine before inspecting wiring, fuses, connectors, or drive belts, and avoid shorting power terminals. Thresholds, component locations, wire numbers, fuse locations, and reset procedures can vary by model, configuration, and serial number. Confirm them in the service documentation for your exact machine.

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Frequently asked questions

What does Bobcat code M6403 mean?

It means the gateway controller told the switched power relay to turn on but did not detect the expected voltage on the relay's feedback wire. Depending on the machine, that feedback wire is either wire 3450 or wire 8020.

What is the panel display code for M6403?

The numeric panel form is 64-03, which shows on the display panel or as an alternate reference for the same fault.

Will M6403 shut down my Bobcat machine?

The documentation does not describe an automatic shutdown or lockout tied to this specific code. It is a diagnostic fault on the switched power relay circuit, but any real-world impact depends on what other systems share that switched power feed on your machine.

Which Bobcat models can show code M6403?

This code is documented across the S510, S530, S550, S570, S590, S630, S650, S750, S770, T550, T590, T630, T650, T750, T770, and T870 models.

How do I clear M6403 after fixing it?

Cycle the key power off and back on. That is the only clearing step listed once the underlying wiring, connector, relay, or controller issue is repaired.

Why does M6403 come and go instead of staying on?

Intermittent behavior usually points to loose connections, corrosion, or pushed-back pins that only cause an open or short under certain vibration, temperature, or connector movement conditions. If the fault is not active when you test, the exact cause may not show up until it happens again.

Is M6403 caused by a bad relay or bad wiring?

It can be either. The documented causes include an open or grounded feedback wire, an open supply wire (wire 1050), a failed switched power relay, a failed gateway controller, or corrosion and connector problems anywhere along that circuit. Checking wiring and connectors first is usually faster than replacing parts.