Battery Kept Running Down So I Installed a Isolation Switch

The simplest fix is to remove, disconnect or bypass the battery isolation switch. OBD II trouble code P2508 indicates the PCM (power control module) is losing battery power. Even though you turn your vehicle off after driving, it should not lose 12v power to the PCM. Having installed a disconnect switch does just that; stops current from reaching the computer at all times. A small current of battery voltage is needed by the PCM to store data, mainly emissions data (trouble codes and readiness flags).So the fix is to remove the isolation switch and ensure the PCM always gets current.

Once you've done this you'll need to drive your vehicle through a "drive cycle"... hint: just search #your vehicle# drive cycle in the search box. A drive cycle is a specific driving pattern you'll need to follow in order to make "ready" or complete the emission monitors (readiness flags). Readiness flags are used to indicate certain PCM self-tests have passed. These are system tests used to ensure the engine is running efficiently. If any of the monitors are not complete the vehicle can not pass the smog test (with the exception of the EVAP monitor). It should be noted it is not always required to perform a specific drive cycle. Manufacturers have designed drive cycles (driving patterns) in such a way where during the course of driving over a few days and in both city and highway traffic conditions, the parameters required to run the emission monitors are normally met. What we mean is... driving your vehicle for the next few days (after you fix the voltage problem and at least 100 miles or so) might be all that's need to make the monitors ready and clear trouble code P2508 on its own. You can always use a scan tool to check your vehicle's emission status before the smog test.