Tesla is preparing to release the next version of its Autopilot software, according to a pair of tweets from Tesla CEO Elon Musk. In October, Tesla updated the self-driving hardware in its cars and, since then, buyers of new Teslas have been without the advanced self-driving features of Autopilot because the company was still working on the related software.

Musk says the update will roll out to a 1,000-car strong test fleet on Monday, with the rest of the Tesla fleet running the update in “shadow mode” where the computer simulates what it would have done if active, and gives Tesla a chance to test the software in more conditions without taking physical control of vehicles. “If all looks good,” Musk says, they’ll switch from shadow to active mode for all cars by the end of this week.

New rev for Autopilot HW2 rolling out Mon to first 1000 & to rest of fleet in shadow mode. Also improves HW1 and enables Ludicrous+. — Elon Musk (@elonmusk) January 8, 2017

If all looks good, HW2 Autopilot functionality will switch from shadow to active mode by end of week for cars beyond initial 1000 — Elon Musk (@elonmusk) January 8, 2017

The company says the hardware is capable of supporting fully autonomous driving, but the software is still years away from a general release. However, the first goal is to bring “parity” between the Autopilot Hardware 1 and Hardware 2 sets. Tesla has had some false starts when predicting releases of the new Autopilot software. Back in December, Musk said the company “might be ready” to roll out the software, but that update never landed.

Ensuring that the system is reliable and working as intended is essential as the company needs to avoid any questions about the safety of the system. A fatal crash last year with Autopilot active delivered a round of very bad PR for Tesla, though it appears driver inattention was the primary contributor. The Autopilot system is (aside from the all-electric powertrain) one of the most compelling competitive advantages for Tesla buyers and other companies are getting close to releasing cars with similar capabilities.

We have reached out to Tesla for additional information on the update.