Resolving Error Code 21-02 on the Bobcat 425 Mini Excavator

[MikePhua]

The Bobcat 425 and Its Diagnostic System
The Bobcat 425 mini excavator was introduced in the early 2000s as part of Bobcat’s compact equipment expansion. With an operating weight of around 5,000 pounds and a 27-horsepower diesel engine, the 425 was designed for trenching, grading, and utility work in confined spaces. Bobcat, founded in 1947, has long been a leader in compact machinery, and the 425 helped solidify its reputation for reliability and operator-friendly design.
Like many modern machines, the 425 features an onboard diagnostic system that monitors electrical and engine functions. When a fault is detected, the system displays an error code to help technicians identify the issue. One such code—21-02—has become a common point of confusion for operators and mechanics alike.
Terminology Notes

• Glow Plug: A heating element used to warm diesel engine cylinders during cold starts
  
• Relay: An electrically operated switch that controls high-current circuits
  
• ECM (Engine Control Module): The computer that manages engine performance and diagnostics
  
• Ground Fault: An unintended electrical path to ground that can disrupt circuit behavior
  
• Harness: A bundle of wires and connectors that distribute power and signals throughout the machine
  

What Error Code 21-02 Means
Error code 21-02 on the Bobcat 425 typically indicates a glow plug circuit fault. This may involve:

• Failed glow plug relay
  
• Damaged or corroded wiring
  
• Faulty glow plugs
  
• Poor ground connection
  
• ECM miscommunication
  

Glow plugs are critical for cold starts, especially in diesel engines operating in low temperatures. If the system detects abnormal voltage or current in the glow plug circuit, it triggers the 21-02 code to prevent further damage or failed ignition attempts.
Diagnostic Steps and Inspection Strategy
To resolve error code 21-02:

• Locate the glow plug relay in the fuse and relay panel
  
• Test the relay for continuity and coil resistance
  
• Inspect wiring harness for pinch points, corrosion, or loose connectors
  
• Disconnect the wire at the glow plug and observe whether the code changes
  
• Check voltage at the glow plug terminals during key-on
  
• Examine the ECM for additional fault codes and verify grounding integrity
  

In one case, a Bobcat 334 excavator showed the same error. After replacing the glow plugs and relay, the issue persisted. Further inspection revealed a corroded negative cable at the starter, hidden inside the insulation. Replacing the cable resolved the fault completely.
Repair Recommendations and Component Replacement
If the relay is faulty:

• Replace with a matching OEM or high-quality aftermarket unit
  
• Clean all terminals and apply dielectric grease to prevent future corrosion
  
• Verify that the glow plugs draw correct current (typically 8–12 amps each)
  

If wiring is damaged:

• Cut back to clean copper and install new connectors
  
• Use heat-shrink tubing and electrical tape to seal connections
  
• Route wires away from heat sources and moving parts
  

If the ECM is suspected:

• Consult a Bobcat diagnostic tool or dealer service technician
  
• Check for software updates or calibration errors
  
• Ensure all ground points are clean and tight
  

Preventative Measures and Long-Term Reliability
To avoid future glow plug faults:

• Inspect electrical connections quarterly
  
• Replace glow plugs every 2,000 hours or as recommended
  
• Keep the fuse panel dry and clean
  
• Use battery terminal protectant to reduce corrosion
  
• Monitor cold start behavior and address sluggish ignition early
  

In 2023, a fleet manager in Minnesota added glow plug circuit checks to his winterization checklist. As a result, cold start failures dropped by 80% across his compact excavator fleet.
Conclusion
Error code 21-02 on the Bobcat 425 mini excavator points to a glow plug circuit issue that can be resolved with methodical inspection and targeted repair. Whether it’s a failed relay, damaged wire, or grounding fault, the solution lies in understanding the system’s role in cold starts and maintaining electrical integrity. In compact excavation, reliability starts with ignition—and the glow plug system is where that reliability begins.