P0128
Coolant Thermostat
Description
Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) This body system diagnostic trouble code indicates an issue with the vehicle's comfort, safety, or convenience systems.
OBD Codes Details
Possible Causes
Blown fuse, faulty switch, damaged wiring harness, poor electrical connection
Solution
Check fuses and relays, inspect wiring harness, test switches and sensors, repair damaged circuits
Symptoms
- Check engine light illuminated
- Engine takes longer than normal to warm up
- Poor heater performance in cold weather
- Reduced fuel economy
- Temperature gauge reads lower than normal
Diagnostic Recommendations
- Verify coolant level and check for a stuck-open thermostat
- Test thermostat operation by monitoring warm-up time
- Inspect ECT sensor for correct readings
- Check cooling system for proper operation
- Replace thermostat if it fails to close properly
Repair Cost Estimate
- Thermostat replacement: $100-300
- Coolant flush and refill: $100-200
- ECT sensor replacement: $20-100
- Diagnostic scan: $80-150
- Total diagnostic and repair: $150-550
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Fact-checking completed
Verified by:
Pavel Volkov
Created:
Last updated:
Information verified against official sources
Official Sources
Society of Automotive Engineers
Official OBD-II diagnostic trouble codes standard (SAE J2012)
Automotive engineering standards organization
International Organization for Standardization
International standard for vehicle diagnostic systems
International standards for automotive diagnostics
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