Got the engine rebuilt a couple months ago.
Did a tune up and basically replaced every filter, fuel injectors, everything.
I have been pleased with how it's been running and the MPG.
Day before yesterday I get P0430.
The truck is a 2001 screw 5.4 4x4. WITHOUT CATS!
What could cause this after three months of owning it?
Also I should say I know the gas in it is crappy right now. I got it at a station that has been known to get their diesel and petrol mixed up from time to time. I didn't know this until I asked around.

 

With no cats, the PCM has to be retuned to shut off the cat monitors, whether the rear O2's are still there or not. The Gotts O2 mod may or may not work.

 

I understand what you're saying, I just don't know why it would just now throw this code after 3 months of no code?
Is it likely to affect performance?
From what I've read so far, the rear O2s don't affect that.

 

A deeper investigation of the 430 code would reveal a detection by the PCM of an out of limit switch ratio for cat monitoring on bank 2.
Since the cat is missing, and a certain amount of time passes, the PCM will zero out the code because nothing is useful by going back to it and keep looking at the condition that does not change.
This can be verified with a Scanner by looking at the output of the bank 2 rear sensor.
You must have had the battery disconnected for the other work causing the PCM to reboot. This will initiate the 420/430 codes again and left go long enough may not result in a CEL again.
You see there is more behind all this than most know about.
These codes are emission codes and do not normally affect engine performance, only the air quality behind the end of the tail pipe when the engine is running.
Good luck.

 

You have only had it 3 months, and how many miles are on it since you have owned it and/or the cats were pulled? Are all 4 cats gone?

It could be that the sensors finally reached readiness since the last code clear.

 

Should have had immediate heater faults unless something was jerry-rigged.

 

glc yes all four are gone.
I believe bluegrass hit it right on the nail head, I have had the battery disconnected for an extended period while we swapped radios.
I just went and did the gotts mod per glc's suggestion. 11 dollars and literally 20 minutes with a drill and unitbit.

 

use a scan tool to clear the code. Maybe it wont come back.

 

Anytime a code clear is done the PCM goes right into full diagnostics and will detect anything that does not pass testing.
Clearing codes does not just make them go away into never never land.
The design and the federal gov. made sure you cannot hide a fault for inspection purposes or otherwise.
Good luck.