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.

[martinoledad]

well i got under the truck today and noticed that it wasnt welded around the cats. they are just clamped to the exhaust pipe. i think thats the problem. im getting it looked at sat. morning.

[Longshot270]

Before or after the cats? Anything past the last sensor doesn't matter. If it is before you have to have a real leak. Many exhaust systems clamp together before anyways I think.

[jmt0645]

are they original or aftermarket cats? some aftermarket cat aren't as efficient as OE. (fyi: i have no cats at all and periodically have the same code even with my rear HO2S turned off by bill)

[martinoledad]

they are the hi flow cats that come with the header kit. So do you think this can be helped by tuning. The funny thing is i still hear the sound but the light isnt on anymore.

[Longshot270]

You need a retune after just LT headers or highflow cats. You definitely need one after both.

[martinoledad]

I'm running the truck stock right now. But the new customs are on their way.