Tripac APU. Code 17. Engine failed to crank

Either starter (95% probability) or engine (5% chance).

Listen if you her the shaft extending from the starter (if not - the starter is bad). 

Replacing the starter is one of the hardest jobs with this APU as there's not much space for you to work around. You either have to know what you do or have good mechanics skills and 2-5 hrs of time for this project. 

1. Disconnect 12V from APU (by taking + cable from the battery).

2. Remove top and bottom lids from APU.

3. Remove alternator belt (12 mm bolt at the bottom and 17 mm bolt at the top).

4. Disconnect exhaust pipe at the back by taking four 13mm nuts off. Disconnect exhaust pipe at the bottom from muffler and remove it (so that it's not on your way).

5.  Remove the back of air filter housing and air filter. 

6. Remove the 12 mm bolt that holds the bracket attached to starter on very top part (next to exhaust pipe's 13 mm nuts). 

7. Locate two 15mm bolts/nuts around starter shaft in the alternator belt area. The top bolt (with a wierd round nut/washer) is easier to see from the top of APU. The bottom bolt is very hard to see, you can see where it is by looking at the new starter). The bottom bolt has a 15 mm nut on the other end. You can use an extension and a 15 mm socket from the right side of APU to reach to the bottom bolt/nut on one side and use a rachet with a 15 socket on the other side (left) either from top or bottom. 

8. After you remove these two 15 mm bolts with nuts you can pull starter up a bit from the top, rotate it and expose 12V cable connection at the bottom with another electrical connection next to it and disconnect them. It's much easier to do it this way then from the bottom on the installed starter. 

Here are 2 pics with starter shaft in rest and active spinning/engaging position: