Good day,
hoping someone can help us out with a situation we are having mind aches over. My son performed an M54B25 auto swap to M52B28 & ZF5 manual trans, had a guy perform ews delete & reprogram. everything paired up nice, car cranks on 1st turn, idles perfect, but appears to be in failsafe mode will not rev over 4k rpms nor go over 55mph. after hooking up code reader BM700 only code that displays is P0601. The new/used swap came out of a 2000 528i bmw. My sonns car is originally a 2001 525i BMW, any possible ideas or point me in the right direction will tremendously be appreciated.
Thank you

Afternoon Adrian

Welcome to the Forum

The code you have can be a number of things so checking time 

1. Is the Battery fully charged and in good order (low Voltage can trigger this)

2. A faulty ECM or connection to the ECM

3. ECM Harness has a fault some connection open or short to earth 

4.ECM control module circuit has poor connection (check for corroded connectors or pins)

6. PCM my be the cause it may need replacing or re-flashing 

7. Same process as above 

The fault is triggered when the cars self test systems check each module and the check sum is wrong

Get the guy who reset the EWS to check if the PCM is OK as it clearly isn't communicating with the EWS and ECM resulting in Limp Mode

Hope this helps not hinders

Dave