Desertion of the Dinobots (1) - S2-E19
Plot hole: Perceptor discovers that the reason for their malfunctioning is because their Cybertonium has deteriorated after millions of years of being exposed to Earth's atmosphere. The key word here being 'millions'. This explains why the original Transformers who crash landed on Earth (4 million years ago) are affected, but newer characters like Perceptor, Powerglide, Hoist, etc. are affected despite the fact that they have only been on Earth recently.
Plot hole: Omega Supreme transforms his arm into a rocket and blasts off to the moon, but later, he transforms fully, but he left his body on earth, how was he able to transform?
Plot hole: When the Quintesson scientist is tricking Cyclonus into going inside his ship he disguises it as a character named Broadside who then says that he has come to take Cyclonus to see Galvatron. The problem is, Broadside is an Autobot yet Cyclonus simply walks inside without any reaction. Most likely the writers intended it to be Astrotrain but wrote the wrong name down.
Answer: This was actually one of many errors in Transformers, the Constructicons MUST have been built on Cybertron, the only transformers genuinely built on earth were the dinobots, reason being Vector Sigma has to give personalities to each and every robot, else they end up like the dinobots, very simple and dumb. For reference watch the episode "The Key to Vector Sigma" which introduces the Stunticons and Arielbots - both Megatron and Optimus Prime have to go to Vector Sigma to get decent personality traits installed, prior to this the new robots are controlled by remote controllers and have no AI.