The car was misfiring and front pre-cat broke and pieces went into the downstream cat. I took them apart and cleared out the debris from the downstream cat. The front pre-cat is empty. But when I started it again the misfiring persisted and there was an exhaust leak from the front manifold. I re-tightened the manifold bolts and the leak seemed to decrease but the car only idled for a few seconds and engine stopped. Now it cranks and seems to fire and then stops immediately. A rapid clicking sound continues till I remove the keys. The car was low on fuel so I put a gallon in. I can hear the fuel pump when I put in the keys. What could be the issue? There was a code related to heated O2 sensor circuit open but I assumed that was old since I started the car once with O2 sensor unplugged. I cleared that code and there are no more codes. How to I diagnose this no start?