A group of current and former Tesla employees claim that the electric automaker is having trouble assembling its vehicles on time because a large portion of the parts received from suppliers are sub-standard.

CNBC claims that, according to one current Tesla engineer, roughly 40 per cent of the parts made or received at Tesla’s Fremont factory require rework. The employee also states that the need to review and rework parts is a factor in Model 3 delays.

What’s more, another current employee asserts that Tesla is struggling to reach production targets because of the defective parts. Teams of technicians and engineers from its remanufacturing lines and service centers have also been brought in to aid in the rework and repairs, staff claim.

Moreover, Tesla is allegedly sending flawed and damaged parts from Fremont to its remanufacturing facility in California.

In response, Tesla released a strongly-worded statement denying quality issues with its parts and vehicles:

“Every Model S or Model X on the assembly line must pass through hundreds of inspection and test points. Towards the end of the line, every vehicle is then subjected to an additional quality control process involving more than 500 other inspections and tests. The majority of issues identified at the end of line inspection are extremely minor, and are resolved in a matter of minutes.”

It is common procedure for large automakers to evaluate and repair used vehicle parts in remanufacturing facilities to make them as good as new. These parts are typically used by certified pre-owned vehicles or those being repaired. Unlike most, Tesla doesn’t outsource remanufacturing, but instead does it in-house.