JCB Fault Code

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P0721

Harsh Shifts / Transmission Drops Drive

System

Transmission / Speed

Symptom

Harsh Shifts / Transmission Drops Drive

Diagnosis & fix steps

LOGIC: Transmission Output Shaft Speed (OSS) Sensor Signal Erratic. ACTION: Remove magnetic pickup sensor from rear gearbox casing. Clean accumulated iron fuzz off the magnetic tip. Check air gap (spec: 0.8–1.2 mm) and measure coil resistance (~900–1,200Ω).

Red Seal field insights

Drivetrain Fines: Planetary gear and clutch pack break-in generates fine metallic wear paste in the oil that clings to the magnetic sensor face, attenuating square-wave pulses.

Same code keeps coming back?

That's a program problem, not a part problem. Book a 30-minute uptime review with Grant, a Red Seal Heavy Duty Technician.

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Read before you touch anything

  • This is general reference information, not a repair manual. It is for trained, qualified technicians.
  • Fault codes, meanings and procedures vary by model, serial number and software version. Confirm the code and the procedure against the manufacturer's service manual before acting.
  • Follow the OEM service manual, torque specs and your site's safety procedures: lockout/tagout, block raised equipment, relieve hydraulic, fuel and air pressure, and let hot systems cool.
  • You use this information at your own risk. Adventure Mechanical and AM Fleet Integrity accept no liability for damage, injury, downtime or loss arising from its use.
  • Not affiliated with or endorsed by any manufacturer. Brand names are used only to identify the equipment a code applies to.

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