Yes, a failed thermostat can cause a smog check failure. The failure can be a result of either a visual observence, engine emission output, or both.
If the thermostat is defective and the thermostat housing is leaking radiator fluid, the smog technician can fail the vehicle based on a visual defect. If there is no leak and no visual failure, a faulty thermostat may cause high engine operating temperatures which in turn can cause increased NO emissions. Nitrogen Oxide or NO is created when an engine's combustion chamber temperature reaches over 2500F.
posted by SmogTips Support
