How do I test failed transactions in the PayPal Sandbox?

We allow developers to test failed transactions and other error conditions by enabling Negative Testing in the sandbox test environment. 

Perform failed transaction testing using one of the following methods: 

• Simulate negative response with request headers
• Simulate negative responses with test values
  

Negative testing in the sandbox test environment is currently in beta. 

See Also: 
PayPal Sandbox Testing Guide