Bobcat H4721 (47-21) Fault Code: 8 Volt Sensor Supply Out of Range High
Also shown on the panel as 47-21 · Also called 8 Volt Sensor Supply Out Of Range High
8 Volt Sensor Supply Out of Range High · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
H4721 sets when the auxiliary controller sees more than 8.25 volts on the 8 volt supply wire 4300, which feeds the right handle thumb paddle switch on manual and ACS controlled machines. It usually points to a short to battery voltage on wire 4300, a bad auxiliary controller, or corroded/damaged connector pins.
Medium severity. H4721 is self resetting and does not by itself force a shutdown, but it means the auxiliary controller is getting bad supply voltage data, which can affect attachment or auxiliary hydraulic control accuracy through the right handle thumb paddle switch. Diagnose it before relying on auxiliary functions for precise or safety-critical work.
What does Bobcat error code H4721 mean?
H4721 is an 8 volt sensor supply fault. The auxiliary controller runs an internal 8 volt reference supply out on wire 4300 to power the right handle thumb paddle switch on manual and ACS controlled machines. The controller constantly monitors that supply so it can trust the switch signal it reads back.
When the controller measures more than 8.25 volts on wire 4300, it knows the supply is running higher than it should and flags H4721. This is not a low-voltage or open-circuit fault, it is specifically a high reading, which usually means an unwanted voltage source is getting onto that wire.
Because this circuit feeds a control switch on the joystick, a bad reading can affect how auxiliary hydraulic functions respond, even though the machine will likely still run in other respects.
What triggers a Bobcat H4721 code?
The code sets when the auxiliary controller detects voltage greater than 8.25 volts on the 8 volt supply, wire 4300, with the key switch in the run/enter (on) position.
Common causes of H4721
- 8 volt supply wire 4300 shorted to battery voltage, which pushes voltage on the circuit above the expected 8 volt reference.
- A fault in the auxiliary controller itself.
- Moisture, corrosion, or pushed-back pins in the auxiliary controller connectors, the right handle connector, or the mainframe harness.
- An intermittent condition that is not present at the time of testing. In these cases the root cause may not be found right away, and probable causes may include loose connections, corrosion, pushed-back pins, the controller, the wire harness, or a combination of these problems.
How to troubleshoot Bobcat H4721: first checks
- With the key switch on, check for the code and note whether it is active now or only stored, since intermittent faults may not show a fault condition during testing.
- Inspect the right handle connector and the auxiliary controller connectors for moisture, corrosion, or pins that have been pushed back out of their sockets.
- Inspect the mainframe harness along the run of wire 4300 for chafing, pinches, or contact points where it could short to a battery-voltage wire.
- Check wire 4300 for a short to battery voltage using appropriate electrical test procedures.
- If connectors and wiring check out, suspect the auxiliary controller itself as the source of the fault.
How the code clears
H4721 is self resetting, so no separate manual clearing step is listed once the underlying wiring or connector problem is repaired. Because intermittent conditions can hide the fault during testing, confirm the repair by cycling the key switch and re-checking for the code before returning the machine to service.
Affected models and serial ranges
H4721 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 ATZC11001-99999 |
| 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 A39511001-99999, 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 H4721 mean?
It means the auxiliary controller detected more than 8.25 volts on the 8 volt sensor supply wire 4300, which feeds the right handle thumb paddle switch on manual and ACS controlled machines.
Is H4721 the same as fault 47-21?
Yes. 47-21 is simply the numeric panel display form of the same code, H4721.
Can I keep working with H4721 active?
The code is self resetting and may not stop the machine outright, but since it affects the supply for the right handle thumb paddle switch, auxiliary hydraulic control behavior could be affected. It is best to diagnose it before relying on precise auxiliary function control.
What usually causes H4721 on Bobcat skid steers and track loaders?
The most common causes are wire 4300 shorted to battery voltage, a faulty auxiliary controller, or moisture, corrosion, or pushed-back pins in the auxiliary controller, right handle, or mainframe harness connectors.
Why does H4721 come and go instead of staying on?
Intermittent connector or wiring issues, such as loose connections, corrosion, or pushed-back pins, can cause the short or bad connection to appear only sometimes, which is why the code may not be present at the moment you test the machine.
Do I need to manually clear H4721 after fixing it?
No separate clearing step is listed. The code is self resetting, so once the wiring, connector, or controller issue is repaired, it should clear on its own.
Which Bobcat machines can show H4721?
H4721 applies across models including the S510, S530, S550, S570, S590, S630, S650, S750, S770, T550, T590, T630, T650, T750, and T770.