How long are fault codes kept in memory and are they erased if the battery is disconnected? I fear I have lost them because the fault has not reoccurred for 2-3 months and I briefly had the battery out recently.

I have an aircon fault and jury rigged the electrics to get around it. After disconnecting the jury rig I took the car to the garage to get the codes read and they found no fault codes at all. Stupidly I had not run the aircon again since removing the jury rig so, if the fault code had been lost, it was not refreshed by running the aircon.

I have a feeling they are erased if the problem doesn't happen after 3 (?) ignition cycles..... But Ross can probably clarify

Chris