How can I troubleshoot and resolve the issue of “invalid grant”?

In the context of Salesforce, you can solve and troubleshoot the issue of the “invalid grant” error in Salesforce OAuth 2.0 integration by using these steps which are given below:-

Checking of OAuth Configuration

You can try to verify whether the OAuth Configuration in Salesforce is working properly or not.

Review the Authorization request

Try to double-check the parameters of the Authorization request which are sent to the server of authorisation.

Inspect Authorization response

You can try to inspect the responses that are received from the Authorization server after the authorization request. You should look for the error message or additional details provided in the response body which possibly can indicate the cause of this issue.

Debugging code

If you are enjoying custom code for handling the OAuth authentication in Salesforce, then you can try to review the code logic to ensure that it correctly handles token exchange, error handling, and validation of the response.

Here is an example given of how you can handle the “invalid grant” error in Salesforce apex code when using OAuth 2.0:-

HttpRequest req = new HttpRequest();
Req.setEndpoint(‘https://authorization_server.com/token’);
Req.setMethod(‘POST’);
Req.setBody(‘grant_type=authorization_code&code=invalid_code&client_id=my_client_id&client_secret=my_client_secret&redirect_uri=my_callback_url’);
Http http = new Http();
HttpResponse res = http.send(req);
If (res.getStatusCode() == 200) {
    // Successful response
    // Parse access token from response body
} else {
    // Error response
    String errorMsg = ‘Error: ‘ + res.getStatusCode() + ‘ ‘ + res.getStatus();
    If (res.getBody() != null) {
        errorMsg += ‘, ‘ + res.getBody();
    }
    System.debug(errorMsg);
}