What if your rear derailleur is one gear up and shift into 1st even it shifted any goes how do you fix this problem?

If your rear derailleur is one gear up and shifts into 1st even when it shifts any gear, it could be due to several factors. Here's a step-by-step troubleshooting guide to help you fix this issue:

1. Check Cable Tension:

- Make sure the derailleur cable has enough tension. If the cable is too loose, the derailleur may not shift properly.

- Tighten the cable tension by turning the barrel adjuster on the derailleur or brake lever until the derailleur moves smoothly and precisely.

2. Check Derailleur Limit Screws:

- The derailleur has two limit screws: the high limit screw and the low limit screw.

- Ensure the low limit screw is adjusted correctly to prevent the derailleur from overshifting into the spokes.

- Rotate the low limit screw clockwise until the derailleur stops at the smallest cog (highest gear).

3. Inspect Derailleur Hanger Alignment:

- The derailleur hanger is a critical part that connects the derailleur to the frame. If it's bent or misaligned, it can cause shifting problems.

- Check the derailleur hanger's alignment by placing a derailleur hanger alignment tool between the derailleur hanger and the frame. If it doesn't fit snugly, the hanger needs to be straightened.

4. Verify Chain Length:

- Ensure that the bike chain has the correct length. If the chain is too long, it may cause shifting problems.

- To determine the proper chain length, wrap it around the largest chainring and smallest rear cog. If the ends of the chain overlap by about 1/8 inch with light tension, the chain length is correct.

5. Clean and Lubricate the Drivetrain:

- A dirty or dry drivetrain can also lead to poor shifting performance.

- Clean the chain, cassette, and chainrings using a chain cleaner and brush. Apply a small amount of bike-specific lubricant to the chain.

6. Examine Cable Housing:

- Inspect the cable housing for damage or kinks. Damaged cable housing can hinder smooth shifting.

- Replace any damaged cable housing with new ones.

7. Inspect the Derailleur:

- Look for signs of damage or excessive wear on the derailleur. Ensure that the derailleur pivots smoothly.

- If the derailleur is damaged, replacing it may be necessary.

8. Consult Professional Help:

- If you've tried all the above solutions but the problem persists, it's best to consult a professional bike mechanic. They can diagnose and fix the issue accurately.

By following these steps, you can troubleshoot the issue causing your rear derailleur to shift into the first gear unexpectedly and ensure smooth and precise shifting throughout the gears.