Fix Existing Mobile App
Fixing crashes, bugs, API issues and performance problems in existing iOS and Android apps, including apps built by other developers.
Fix an Existing App Built by Another Developer
If your mobile app already exists but is crashing, unstable, outdated or difficult to maintain, I can help investigate and fix it.
Specialist in debugging and stabilising existing mobile apps
A lot of mobile app work is not about building something new. It is about getting an existing app working properly again.
I help companies fix existing mobile apps where there are problems such as:
- app crashes
- slow performance
- broken API calls
- sync and data issues
- problems after iOS or Android updates
- App Store or Play Store submission failures
I am James Thornley, a senior software engineer with over 30 years of development experience and 15 years in mobile development, including iOS, Android and backend systems.
Common Existing Mobile App Problems I Fix
These are the kinds of issues that often appear in live apps and inherited codebases.
App Crashes
Investigating and fixing crashes in live apps, including hard-to-reproduce problems, production-only issues and intermittent failures.
Performance Issues
Fixing apps that feel slow, freeze, consume too much memory or behave poorly on newer devices and operating systems.
API and Network Problems
Debugging issues with REST APIs, authentication, failed requests, bad payload handling and backend integration problems.
Problems After OS Updates
Resolving problems that appear after iOS or Android upgrades, SDK changes, deprecated APIs or store policy updates.
Data and Sync Issues
Fixing data corruption, offline sync problems, duplicate uploads, missing records and inconsistent state between app and server.
Store Submission Issues
Fixing existing apps so they can be successfully submitted to the App Store or Google Play again.
Apps Built by Other Developers
Many of the apps that need fixing were built by someone else originally.
One of the most common situations is being asked to fix an app built by another developer, freelancer or agency. Sometimes the app was handed over badly. Sometimes the original developer is no longer available. Sometimes the code works, but only just.
I regularly work on inherited projects with:
- incomplete documentation
- limited or no handover
- legacy Objective-C, Swift, Java or Kotlin code
- deployment steps known only to the previous developer
- urgent business pressure to get fixes out quickly
If the original developer is completely gone, see also what to do if your mobile app developer disappeared.
Fix Existing iOS App or Android App
I work across both Apple and Android platforms, including legacy and modern codebases.
Fix Existing iOS App
I can fix existing iOS apps that crash, fail App Store submission, behave badly on newer iOS versions or contain legacy code that needs modernising.
This includes Objective-C, Swift and SwiftUI projects, and apps with backend/API dependencies.
Fix Existing Android App
I can fix existing Android apps with crash issues, SDK update problems, API failures, performance issues and Play Store submission problems.
This includes Java and Kotlin projects, including apps that need updating for newer Android platform requirements.
How I Approach Debugging Existing Apps
Inherited app problems are usually solved by disciplined investigation rather than guesswork.
When fixing an existing mobile app, I typically work through:
- understanding the current architecture and data flow
- reproducing the issue reliably where possible
- analysing crash logs, requests, responses and state transitions
- identifying the real root cause rather than masking symptoms
- testing the fix carefully against the live business workflow
- improving maintainability where needed so the same issue is less likely to return
My background in large production apps, server-side systems and long-running business software is particularly useful here because many mobile problems are not just UI bugs. They often involve concurrency, state, sync, backend integration and deployment processes as well.
Relevant Experience
My experience is particularly suited to inherited, business-critical and technically awkward app projects.
My background includes:
- maintaining large production mobile apps used internationally
- taking over existing projects with minimal handover
- debugging complex mobile and backend issues
- modernising legacy Objective-C and Android codebases
- delivering fixes quickly where the business depends on the app
You can see examples on the homepage portfolio, including TeamHaven, CHEP, AT Creative and Miele.
Fix Existing Mobile App FAQ
Common questions from companies with an app that already exists but is causing problems.
Can you fix an app built by another developer?
Yes. I regularly work on existing apps built by freelancers, agencies and internal teams, including projects with poor documentation or incomplete handover.
Can you debug mobile app crashes?
Yes. I can investigate crash reports, stability issues, performance problems, API failures, sync bugs and other production issues.
Can you fix old iOS and Android apps?
Yes. I can fix and modernise older iOS and Android apps, including legacy Objective-C, Swift, Java and Kotlin codebases.
Do you only fix the problem, or can you maintain the app afterwards as well?
I can do both. After stabilising the app, I can also provide ongoing maintenance and support.
What if nobody properly understands the code anymore?
That is common with inherited apps. I can analyse the codebase, understand how it works, identify the root causes of issues and get the project back under control.
Need Help Fixing an Existing Mobile App?
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Typical Enquiries
App crashes, legacy iOS or Android updates, App Store or Play Store issues, and projects where the original developer is no longer available.