I drive an '03 with a 4.8, and after a huge fiasco with the dealership, and swapping and cleaning MAFs, my truck is throwing the P1133 code. I've checked the Haines manual and it states that it is "Oxygen sensor insufficient switching (pre-converter sensor, left bank). What does this mean exactly? Ive checked the other threads related to this, and I noticed the cats were mentioned. Only problem is I have none, and Nelson Performance adjusted the tune accordingly. Any suggestions as to what is causing this? Many thanks.

I drive an '03 with a 4.8, and after a huge fiasco with the dealership, and swapping and cleaning MAFs, my truck is throwing the P1133 code. I've checked the Haines manual and it states that it is "Oxygen sensor insufficient switching (pre-converter sensor, left bank). What does this mean exactly? Ive checked the other threads related to this, and I noticed the cats were mentioned. Only problem is I have none, and Nelson Performance adjusted the tune accordingly. Any suggestions as to what is causing this? Many thanks.

bank 1 sensor 1 O2 sensor is up stream of the cats driveside,,,you having no cats leads me to believe that the driver side O2 sensor is damaged...or the connections to it are ...

swap the driver side O2 with the passenger side see if the code moves if so it is just the O2 sensor..

many of these vehicles have had O2 wiring issues driver side ,,,very hard to access this side..

this operation [no cats]is illegal and you are lucky the dealership did not report you...if this is a registered for road use...

you sure it's not p0133?
http://www.obd-codes.com/p0133

I just checked this morning, and made sure it is P1133.

some help from other sites

P1133 - Heated Oxygen Sensor (HO2S) Insufficient Switching Bank 1 Sensor
You will need to check the output signal from this O2 sensor, if the voltage low (.10-.25 volts) try spraying propane into the air cleaner to see if the O2 volts go up, it should go to around .75 volts, if nothing happens then the O2 sensor is bad, bank #1 O2 is on the same side as #1 cylinder

Check the connections to the four O2 sensors first to make certain they are not corroded. There is no way to tell which sensor is bad by the code. You need a scan tool to monitor the individual voltages to know which is bad. This falls into the category of take it to the dealer which may not be a bad idea. The O2 sensors are covered under warranty. My Vette, a 2003 Z06 just had one of the catalytic converters replaced under warranty. Warranty on emissions for 2003 year cars is 8 years from delivery or 80,000 miles. It doesn't matter if you are the original owner or not, it is still covered. Newer model cars are covered for ten years or 100,000 miles. I don't recall the year the length of warranty changed.

and they say that your o2 sensors still might be under warranty
http://www.epa.gov/obd/warranties.htm

Would it be a bad idea to just replace the sensor when I got to my muffler guy? Exhaust has to be redone because it'll be cut off for my lift kit. Thanks for the information.

That's what I would do, if you've got over 100k miles on it they should be replaced anyway, plus that code almost has to be a bad sensor. If the voltage was high/low there would be a code for that, so that rules out a bad connection, low switching (from what I have seen) is always the sensor. Have both upstream sensors replaced, having one new and one old one can cause a slight power imbalance between the banks that can cause it to run worse than with two old sensors.

Actually, how difficult would this be to do myself? I'm askin now cause I haven't had a chance to go look and find them. Again, thanks for the help and quick responses