Hi there! I understand you're seeing an "internal fault" message when trying to add a promo code and pay in the Santander Cycles app on your iPhone running iOS 18.6. Let's work together to get this sorted.

Just to clarify before I guide you through some steps:
- Have you successfully used promo codes in the app before?
- Is this issue happening only with this specific promo code or with any code?
- Have you tried closing and reopening the app or reinstalling it already?

Meanwhile, here is a step-by-step approach you can try:

1. Restart the App: Fully close the Santander Cycles app by swiping it away in the app switcher, then open it again. This can clear temporary glitches.

2. Check for App Update: Open the App Store, tap your profile icon, then scroll to see if there's an update available for Santander Cycles. Running the latest version can fix bugs causing errors.

3. Verify Network Connection: Ensure your iPhone has a stable internet connection via Wi-Fi or cellular data. Try switching connections to test.

4. Clear Cache by Reinstalling: Delete the app by pressing and holding its icon and choosing "Remove App," then reinstall from the App Store. This clears stored data that might cause "internal fault" errors.

5. Retry Entering Promo Code: After reinstall, open the app and carefully enter the promo code again, making sure there are no extra spaces or errors.

6. Payment Method Check: Ensure your payment details in the app are up to date and valid. Try a different payment method if possible.

If these steps don't fix the issue, the "internal fault" message may indicate a backend or server problem on our side. TfL has acknowledged ongoing maintenance and app issues recently, which could cause such errors5.

If you continue to face this problem, I recommend contacting Santander Cycles support directly at [email protected] with details of the promo code and your device. They can check if the promo code is currently active or if there's a system fault affecting payments.

Please give these steps a try, and let me know how it goes or if you need further help. Are you satisfied with this guidance so far?