P0420 pops up after Header kit install

[Longshot270]

Could be one of the possible causes in bold, probably not anything in grey.

P0420,"Catalyst System Efficiency Below Threshold (Bank 1).
--Possible Causes--
- Use of leaded fuel
- Spark timing retarded below spec
- Oil contamination
- Cylinder misfiring
- Damaged HO2S
- Damaged ECT senor
- Downstream HO2S wires improperly connected
- Fuel pressure too high
- Damaged exhaust system pipe
- Damaged exhaust manifold
- Damaged muffler/tailpipe assembly
- Damaged catalytic converter
Note: The Powertrain Control Module(PCM) tests the catalyst efficiency by monitoring the rear Heated Oxygen Sensor. The Diagnostic Trouble Code(DTC) is set when the catalyst efficiency is below an acceptable level."


Check and make sure all of your sensors are intact by rolling under and following the tailpipe up to the engine.