Our 6 cyl trailblazer has the check engine light on with these three codes: P0411, P2431 and P2433. Is this something critical, and is it something we can fix ourselves? If a shop would have to repair, estimated cost?
Thanks in advance! BS
Why create another new thread for the same problem? Last time, you didn’t even know which engine you had. Have you at least gotten that far?
Sorry, I didn’t know I was breaking the rules. Rookie here.
A guy at work said the engine is a 6 cyl 4.3 vortex.
Well, you shouldn’t take that person’s advice anymore because there is no such thing as a 4.3L vortex in a Trailblazer. You either have the 4.2L inline Atlas I6 or the 5.4L Vortec V8.
When you have engine codes, knowing the engine type is very important to resolving the issue and the first step.
a black thing on the top of the motor reads Vortec 4200. An oil change receipt reads 6 cyl 4.2L FI 2wd. The book says stock # 627001, VIN 1GNDS13S562101019. Does that help?
Yes, now we know you have the I6 which has known problems with its secondary air injection system. Specifically, the check valve is a known issue since 2002 on. They did improve the valve in 2006 but I hear lots of people still experiencing issues with them.
Your codes indicate-
Incorrect flow
Pressure sensor circuit high
Pressure sensor circuit range
There are really three components at risk; the relay to drive the pump, the pump itself and the check valve.
When the engine is off for 6 hours or so, it should be cold enough for the system to activate on the next start. Each time you start the engine cold, the pump should run to do a system diagnostic. It lasts for upwards of 20 seconds, depending on how well the system is performing.
When you start the engine, there is a pump located under the truck on the driver’s side that should run. It’s right below the seat, mounted to the frame rail. It may sound like a leaf blower but if you can’t hear it, get down near it. If you still can’t hear it or feel it running with your hand on it- then check the fuse for the pump. Fuse OK? check the relay.
If the pump runs, then the most likely suspect is the check valve. It is located on the right side of the engine and has a ~1" diameter hose going to it from the pump. These check valves are known for seizing up. Typically, the owner notices that the pump is louder and runs longer when the check valves start to stick.
The check valve in your truck is a bit more complex than prior incarnations in that it also has a pressure sensor. They used to run around $80 online but yours may be slightly more expensive. They will be 2-3x that at the dealer.
It’s fairly easy to change yourself. Takes about 1 hour the first time 
but can be done in 30 minutes.
Post back if you have more questions-