New member with a 02 C230K coupe (2.3L same as SLK)about 51K miles and several questions:

1. P0410 secondary Air Injection Failure code.
I cleaned the related system, reset codebut did not fix the problem.
I have read previous posts and it looks like the most common problem is the check valve. Does it screw onto the metal tube? How do you replace it? Is there any way to test this system other than replacing possible parts, then driving it for a week waiting for second air to pass?

2. What is a normal range for the front O2 sensor, Mine fluctuates from 0.2 to 0.9 while running, with the average being about 0.6. It will read 0.00 while decelerating. The rear O2 sensor always reads around 0.73, except when decelerating. What is a good reading? Is this rich or lean?

3. Fuel mileage to way down also, don't know if it is related. Use to be able to get near 30 MPG, now I'm lucky to get 23 MPG. Read about the crank position sensor, what happens when it starts going out?

4. How do you clear the Service A/B messages?

5. Any performance upgrades out there at reasonable cost? Love this car, but it really needs about 40 more ponies to be fun to drive.
My 2.0L turbo talon had 200 HP factory, and makes over 300HP with simple low cost modifications. It seems that with 2.3L, the output should be higher. ????

6. What is most trusted online parts store?

Thanks for any help you can provide. I need to get this trouble code fixed so I can pass the emissions test!!!!