"While Tesla hasn't changed the Model S in terms of how it looks, it keeps changing the vehicle underneath, so the reliability fluctuates," said Jake Fisher, director of auto testing at Consumer Reports.
Fisher added that many Tesla owners reported a number of issues, especially with the vehicle's air suspension system.
Tesla said the suspension issues for the Model S have been cut by 65 percent since last year.
"The suspension issues that some Model S customers experienced primarily in 2017 were due to a supplier-related issue that did not pose any threat to vehicle safety or drivability, and presented itself only when the car was parked," the company said in a statement sent to CNBC. "The issue has already been addressed for customer vehicles in the field and resolved at the source with fundamental design improvements."
While the reliability of the Model S has slipped in the eyes of Consumer Reports, it says the Model 3 has "average" reliability.
"The lack of complexity in the Model 3 is why we thought it would have average reliability, and that's what our members have told us," said Fisher.
The Consumer Reports annual auto reliability report is based on more than a half-million members of Consumer Reports reviewing their own vehicles. In essence, the owners report what's wrong with the cars, trucks and SUVs they drive every day.