Hello , thanks for your question in the autoaid Community.
Your described fault Code means: P120D - SECONDARY FUEL INJECTOR EXCESSIVE FLOW, BANK 2
If there is a Problem with the wirings, the engine runs, because the injector stands opened.
Look for the wirings and measure this for a plus or mass circuit.
You could check the fuel pressure, if there is a fault, the engine still runs immediately.
Sometimes the injector stands fault opened, check this also out.