I have a 2006 boxster S 3.2 Broke a valve spring, motor has been out of the car since then while I did all the work it needed- new clutch, all new valves and valve springs, new timing chain, basically a full motor rebuild by the time I was done. Put the motor back in the car- it idles perfectly smooth, but the gas pedal only gets about 5% response. error code is 638. I found a couple of articles about a 1502 error but I'm not getting that. I'm looking into the relay in the trunk as I saw that as a possible issue also- anyone else hear of this?

 

My info is the error arises if target throttle position doesn't agree with the actual throttle position when the engine is running.

You can check related fuses and related relays, even replace these just in case. I do not think you'll find anything amiss but it never hurts to check/eliminate these before moving on to more expensive things.

Check the throttle body wiring for damage. Engine compartment hardware including wiring is always at risk of damage whenever the engine is removed/installed. Check the connector for damage, a bent pin, broken pin, maybe not even fully connected.

If no damage or connector problems found the source of the problem can reside in the drive by wire system (gas pedal and its electronics), the throttle body and the hardware that controls the butterfly valve, the butterfly valve can be damaged/sticking, or even the DME can be at fault.