I have not had those codes come back since I replace the crank position sensor. It's worth it to replace it. It's only $30 at AutoZone or O'Reillys

 

If you look up the code descriptions for the 35x codes it tells you coil primary/secondary circuit errors.
These are electrical performance problems and are not related to timing intervals like the 30x codes are.
If the crank sensor was the fix, all cylinders would have been affected.
They were not.
If the timing ring was the issue a different code would have been generated such as 314, 315, 316, 336, 337, 338, 339 and the 301/306 misfire codes etc.
The program and computer board hardware was designed to monitor every possible issue and cannot make a mistake between a 35x and a 30x fault because they are so different. One is timing, the other is current flow signature.

Good luck.

 

Well, I had done everything else that everybody else had done already to get these codes to go to way. I found a video showing the same thing my truck was doing . Different vehicle but still a Ford product on a V6 model. They had to replace the crank position sensor and it fixed it so I bought one put it in and I have not had two codes since. Replacing the crank position sensor fixed my truck the p0351 and P0356. I'm not a certified Ford mechanic but if you're having these two codes it's worth spending $30 instead of spending over 400 on a PCM

 

It likely was a dirty connector responding to vibration at the same points each time.
If the circuit happens to go open at the specific points in rotation, the PCM loses the signal then picks it up again.
You will never know now.
It might just have been solved by exercising the connector to clean the dirt off the contacts and not needing to replace the sensor.
After all it did run the rest of the engine functions and cannot by itself be selective for malfunction.
Good luck.

 

Ok, I replaced the crank positioning sensor. I'll report back in a week or two whether the p0351 and p0356 codes come back. Fingers crossed.
Andy

 

I know this is an old thread, but same issue as above, same codes, new crank position sensor and problem solved. I thought adding to the thread would help someone else. The codes thrown do not always point to the problem.

 

Actually, if you think about it, P0351 and P0356 ARE connected to the Crank position sensor, sort of... The Crank Position sensor is deciding where the first (and 5th) firing position in an 8 cylinder engine... Cylinder 6 IS the 5th in the firing order. I have been having the same issue after I replaced the (looked original 220,000 mile Motorcraft) plugs and coils. Those plugs had gaps wider than a hillbilly smile! But then I got the P0351 and P0356 faults, I blamed and replaced those 2 coils, and no difference. Mine seems to run fine at high RPMs, but throws the faults at idle. If I reset the faults as I drive, they won't return until I idle again. I will try replacing the sensor, see how it does.

 

If you understand the fault codes and their definition, you'd know they are each an electrical circuit fault specific to the indicated cylinder.