JCB P0651 – ECU: Circuit 'B' of the Reference Voltage of the Sensor is Open

The fault code P0651 indicates a malfunction in the 5-volt reference circuit 'B' provided by the Engine Control Unit (ECU) to various engine sensors (such as pressure, temperature, and speed sensors). An open circuit means there is a break or interruption in the electrical path, preventing the necessary voltage from reaching the sensors. This leads to incorrect sensor readings, failure to communicate, or a complete loss of signal, which can cause a range of engine performance issues including the engine cutting out or failing to fire.

• JCB EcoMAX engine
• 2018 JCB 413 T4F wheel loader
• Tier 4 engines

1. 1

   Scan for other fault codes using a diagnostic tool, as P0651 is often accompanied by specific sensor codes that help pinpoint the affected circuit.

2. 2

   Verify battery voltage, as low or unstable battery voltage can trigger electrical fault codes.

3. 3

   Check fuses and relays related to the engine control system and sensor circuits for blown fuses or faults.

4. 4

   Visually inspect the engine wiring harness and connectors for damage, frayed or burnt wires, loose connections, corrosion, or exposure to heat and abrasion.

1. 1

   Isolate faulty circuit or sensor

   With the ignition on and engine off, use a digital multimeter to check for the 5-volt reference signal at the connector of each sensor on the 'B' circuit. Disconnect sensors one by one, clear codes, and check if P0651 returns. If the code does not return after disconnecting a specific sensor, that sensor is likely shorted internally and pulling the circuit voltage down.

2. 2

   Repair wiring and connectors

   Repair any damaged wiring found during the visual inspection, ensuring all repairs are properly insulated and secured. Clean corroded terminals or replace loose connectors to re-establish a secure connection.

3. 3

   Replace faulty sensor

   If a specific sensor is identified as shorted or faulty during the isolation test, replace it with a genuine OEM part.

4. 4

   Test Engine Control Unit (ECU)

   If all wiring and sensors test good, the ECU may be faulty (this is a rare cause). Exhaust all other diagnostics first. If possible, temporarily swap an ECU from a similar machine with caution to confirm the diagnosis before replacing.

5. 5

   Final verification

   Clear the P0651 fault code using a diagnostic tool. Start the engine and perform a test drive or operate the machine to ensure the fault does not reappear.