ECM Communication Lost on CAN Network

Common Symptoms

• Platform LCD on the upper control box shows fault code 71 and the machine stops responding to drive or lift commands
• Engine may crank and start normally but the platform controller immediately disables functions
• No RPM readout appears on the platform display even when the engine is running
• Machine enters a lockout state and will not allow boom or drive movement
• Fault code 71 reappears immediately after clearing and cycling the key switch
• Intermittent faults may allow brief operation before locking out again, especially after vibration or heat
• Ground control box may also display fault 71 or show a blank engine data screen

Probable Causes (Ranked by Likelihood)

• Damaged, corroded, or loose CAN Hi/Lo wiring connector at the ECM or TCON/GCON controller Very Likely
• Failed or missing 120-ohm CAN termination resistor at one or both ends of the J1939 CAN bus Very Likely
• Engine ECM lost power or ground due to a blown fuse or loose battery cable on the engine harness Likely
• Faulty or failed engine ECM that is no longer transmitting J1939 messages on the CAN bus Likely
• Chafed or shorted CAN wiring in the main harness caused by rubbing on the chassis or boom structure Possible
• TCON or GCON controller firmware version mismatch or internal CAN transceiver fault Less Likely
• Water intrusion into a harness connector causing intermittent CAN signal dropout Possible

Step-by-Step Diagnostic Procedure

1. Step 1. Turn the key switch to OFF and lock out the machine. Walk the full length of the engine harness from the ECM to the TCON/GCON controller. Look for obvious damage, pinched wires, rubbed-through insulation, or connectors that have backed out of their housings.

2. Step 2. Unplug the main J1939 CAN connector at both the ECM and the TCON. Inspect the pins for corrosion, pushed-back terminals, or moisture. Clean with electrical contact cleaner and reseat firmly. This alone clears fault 71 in a significant number of field cases.

3. Step 3. Measure CAN bus resistance. With the key OFF and both ends of the harness connected, place your multimeter across the CAN Hi and CAN Lo wires at any accessible connector. You should read approximately 60 ohms. A reading of 120 ohms means one termination resistor is missing or open. A reading near 0 ohms means the bus is shorted. Either condition will cause fault 71.

4. Step 4. Check ECM supply voltage. Locate the ECM power fuse in the engine fuse block (refer to your specific model wiring diagram). With the key in RUN, confirm you have battery voltage (typically 12-13.5 V DC or 24-26 V DC depending on model) at the ECM supply pin. A dead fuse here will pull the ECM offline and cause fault 71.

5. Step 5. Verify ECM ground integrity. Measure resistance between the ECM ground pin and a known-good chassis ground point. You want under 0.5 ohms. A high resistance ground is a common overlooked cause of CAN dropout.

6. Step 6. If wiring checks out, attempt a key-cycle reset: key OFF for 30 seconds, then back to RUN. If fault 71 clears and stays clear for normal operation, you likely had an intermittent dropout. Monitor closely and check for vibration-related connector movement.

7. Step 7. Connect the Skyjack diagnostic harness via the EZcal / Pcal handheld if available. Navigate to the active fault log to confirm fault 71 is the only active code. If additional codes are present alongside 71, address those as they may point to the root cause of the ECM going offline.

8. Step 8. If all wiring is good, fuses are intact, resistance is correct, and fault 71 persists, the ECM itself may have failed internally. ECM replacement or reprogramming requires a Skyjack-authorized service technician with the appropriate factory software. Do not operate the machine until communication is restored.

Frequently Asked Questions