I recently put new head gaskets on my 99 Disco 128K. After the rebuild and initial debugging I have gotten to this point. I start up my Disco, it runs awesome. After 60-90 seconds the SES begins to blink and the idle speed drops below 1000 and the engine begins running rough. It spits me a P1300 engine code: Catalyst Damaging misfire, multiple cylinders Drive cycle C signal out of range (above). Besides the engine gasket set I also changed: Thermostat, air filter, plugs and wires which the parts person insisted were factory spec but after much research were revealed to be only 7mm. The truck also has a significant exhaust leak near the back box. Could it just be the wires? o2 sensor? Fuel filter? All of them? Any and all input is greatly appreciated.