What is the meaning of codes:
SID 251 Power Supply--Invalid data
PID 71 Idle Shutdown Timer Status
Any knowledge I could glean would be after reading through any number of documents on the web, the same documents anyone could search for. Having no special engine knowledge, I'll be guessing as to the significance of these particular errors. We build electronics and read voraciously but are not engine mechanics.
Generally speaking, the Power Supply reference is relative to, very specifically, the engine's ECU. It has chassis voltage applied to some pin on a plug to power it up; this error is claiming that the voltage being measured by its own internal volt meter doesn't jive with reality ie a voltage well outside a specified/ expected range. Maybe its a zillion....maybe -256v dunno. Whatever the value, the monitoring logic gave it a thumbs-down; calling foul.
Often times, if a temp sensor (as an example) varies wildly...but is still within range...it will cause the ECU to throw an error such as "Sensor within range but erratic." --An amber code; sensor is likely failing. We see this with turbos sometimes. However, your error is not relating to being wild, just only being way off from the expected. What that is I can't know... but, if it were an occasional or situationally-thrown error then something may be pulling the ECU's supply down. Or, it may be, if the engine is running and the alternator had some blown diodes in the regulator, the resulting voltage would both appear very high and noisy. So noisy as to appear to be, maybe 30 or 40Vdc...when it's just a noisy 12.8vdc. That's what I can suppose for code 251. There's a second number, an FMI, which gives the more specific picture. It may be helpful to know this as you're searching.
As for 71 (without a specific fmi) I can only suppose the clock or knowledge of runtime is screwie. The " Idle Shutdown Timer Status" is, I believe, the timer used to automatically shut down the engine after so-many idle minutes in compliance with EPA guidelines. This error relates to that process.
I hope this helps,