2013 GL450 91,000 miles First start of the date the vehicle starts immediately after the vehicle is erratic in that it may or may not start immediatly or take a few turns before it does. It always starts on the second attempt immediately. If the car sits for a few hours its 50/50 on starting immediately or a long crank to start. The battery is original and I am thinking of replacing it, but I'm thinking its more of sensor issue. Any ideas?

 

Having this intermittent issue as well. My battery is new. No codes on it. All my MB techs say the M278 starter is a very rare failure - and this doesn't seem like a starter dying. So, I'm just as lost as you.

 

Starters will wear off. The wear rate is higher for cars which are driven shorter distances over the same mileage. They will be started more often and wear the starter quicker. The wear items in the starters are often the brushes which can be replaced and the starter rebuilt, but the whole unit is usually replaced as it's faster and less work for the mechanics, and this translates to less money out of the owner's pocket unless you're DIYing the job.

Try diagnosing the voltage when the car starts immediately as well as when it does not. Lots of threads here which have a wealth of info about diagnosing and replacing the starter.

 

You have a 2013 GL and it's now 2021. Yeah, it's time a new battery anyway. Since you need to replace the battery you might as well do that first and see if it helps, or not, with the starting issue before throwing $$$ at the starter.

 

Replaced the battery. No change. The vehicle starts as it should most of the time. The vehicle is coming due for an A3 service and I’d rather not pay the service department for trouble shooting I may be able to accomplish myself. I’m thinking a Camshaft Position sensor or Knock sensor. Any thoughts would be appreciated.

 

You and I have the same issue. All my cam sensors are new, still noticed it every now and then. I’m not chasing it anymore.