The German car manufacturer is chasing an eighth consecutive F1 title double this season, having been undefeated during the turbo hybrid era.
But as it ramps up work for the start of the 2021 campaign, its engine chief Hywel Thomas has revealed that it is facing some early year challenges.
"We have got some issues with the power units," he said in a video released by Mercedes. "We know we have issues but we have plenty of plans in place to fix all of those issues. I'm sure it will all be ready for the first race."
Mercedes is no stranger to facing dramas in the pre-season build-up, and 12 months ago its then engine chief Andy Cowell admitted that the team was facing troubles in its preparations for the new campaign.
While the team endured some reliability dramas in testing, it was eventually able to get on top of the matter by the time the delayed season started in July.
Mercedes is currently constructing its 2021 engines and has sent out the first batch of power units to its customer teams.
And while it now runs through durability testing, new restrictions that limit dyno use have added to its difficulties.
Furthermore, engine homologation rules that rule out in-season power unit modifications mean it has to get everything sorted by the season opening race in Bahrain.
Thomas added: "This year there have been a few extra additional things thrown at us. It's been the first winter where we've had to react to a change to the regulations, which limits the amount of dyno hours we're able to run.
"This means we have to make every single dyno run count and must make sure that we are entirely productive, to learn about the performance and the reliability of the Power Unit during each hour
"Also, it's the first winter where we have prepared for only one single upgrade for the whole season.
"So, we have to get all of our performance into the first PU that goes to the first race, whereas in previous seasons we have been able to split that up into different packages for each PU that gets introduced.
"So, with that challenge, we have to get absolutely everything there for the first race and make sure it's completely reliable."