Resolving Fault Codes on the Bobcat T870 SCR System

[MikePhua]

Bobcat T870 Overview and Emissions Architecture
The Bobcat T870 is the largest compact track loader in Bobcat’s lineup, introduced in the early 2010s to meet Tier 4 Final emissions standards while delivering high horsepower and hydraulic performance. Powered by a turbocharged diesel engine and equipped with a Selective Catalytic Reduction (SCR) system, the T870 uses Diesel Exhaust Fluid (DEF) to reduce nitrogen oxide emissions. The SCR system includes temperature sensors, dosing modules, and pressure monitors—all of which are prone to fault codes if components drift out of spec or are tampered with.
Terminology Note

• SCR (Selective Catalytic Reduction): An emissions system that injects DEF into the exhaust stream to neutralize NOx gases.
  
• DOC (Diesel Oxidation Catalyst): A pre-treatment chamber that oxidizes hydrocarbons and carbon monoxide.
  
• Pre-treatment Sensor: A temperature sensor located before the SCR unit, used to monitor exhaust conditions.
  
• Tampering Fault: A diagnostic code triggered when emissions components are altered, bypassed, or misread.
  
• U004360-03: A fault indicating the SCR pre-treatment sensor is reading too high.
  
• E005246-25: A tampering fault code often linked to sensor inconsistencies or system modifications.
  
• U005435-12: A general pressure check fault, typically related to DEF dosing or exhaust backpressure.
  

Sequence of Faults and Diagnostic Strategy
A Bobcat T870 with serial number ASWT12057 was disassembled for hose and AC line replacement. Upon reassembly, three fault codes persisted:

• E005246-25 Tampering fault
  
• U004360-03 SCR pre-treatment sensor out of range high
  
• M6803 HVAC relay error off
  

The operator drained and refilled the DEF tank to eliminate fluid contamination. The exhaust system, radiator, and cooling fan were removed and reinstalled. Initial confusion arose over which sensor was the pre-treatment unit—some dealers pointed to the sensor near the manifold, others to the one at the muffler outlet.
Sensor Testing and Resolution
Resistance checks revealed one SCR temperature sensor was open (failed) and the other had measurable resistance. After further research, it was confirmed that:

• The system uses one temperature sensor in the SCR
  
• The second sensor is located at the DOC inlet
  
• The dosing module injects DEF into the center of the SCR, making the inlet sensor critical for temperature monitoring
  

Replacing the pre-treatment sensor resolved both the tampering and high-temperature fault codes. RPM control was restored, indicating that the engine management system had resumed normal operation.
Recurring Tampering Fault and Pressure Check Error
Despite resolving the initial sensor issue, the tampering fault returned alongside U005435-12, a general pressure check fault. This suggests that:

• The SCR system may still be detecting irregular pressure readings
  
• DEF dosing may be inconsistent due to clogged lines or a faulty pressure sensor
  
• The tampering fault could be residual or triggered by incomplete system resets
  

Recommendations for Further Troubleshooting

• Inspect DEF lines and dosing module for blockage or leaks
  
• Verify SCR pressure sensor calibration and wiring integrity
  
• Use Bobcat diagnostic software to clear stored faults and reinitialize the SCR system
  
• Confirm that all exhaust components are correctly seated and sealed
  
• Replace HVAC relay if M6803 persists, as it may affect system voltage distribution
  

Field Insight and Operator Experience
In Michigan, a technician noted that early Tier 4 Bobcat machines often triggered tampering faults after sensor replacement due to incomplete resets. In New Jersey, a road mechanic emphasized that SCR systems are sensitive to exhaust leaks and sensor mismatches—especially when aftermarket parts are used.
Conclusion
The Bobcat T870’s SCR system is a complex network of sensors and dosing components that must operate in harmony to meet emissions standards. Fault codes like E005246-25 and U004360-03 often stem from sensor failure or misidentification. Replacing the correct pre-treatment sensor can restore functionality, but lingering faults may require deeper pressure diagnostics and system resets. With careful testing and OEM parts, the T870 can return to full performance without triggering emissions-related shutdowns.