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Home › Hummer › 2008 › H3 Alpha › Repair and Diagnosis › Engine Mechanical › Cooling System (Mechanical) › Engine Cooling › Diagnostic Information and Procedures › DTC P1258 › Circuit/System Testing

Circuit/System Testing

  1. Test the thermostat for correct operation. Refer to Thermostat Diagnosis .
  2. Test the engine cooling system for overheating. Refer to Engine Overheating .
  3. Inspect the water pump and coolant flow for correct operation. Refer to Water Pump Replacement (With LH8)  or Water Pump Replacement (With LLR) .
  4. Inspect the engine for worn/leaking/cracked cylinder heads and engine block. Refer to Coolant in Combustion Chamber and Coolant in Engine Oil for the 5.3L engine
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