A method is provided for isolating errors which occur when code supplied by independent software vendors (ISV code) interacts with code supplied by a primary vendor (host code). Code suspected of containing ISV code with host code is executed. If the results are in error, the code is scanned and then compared with a master host code. Any differences are stored in a delta file, including locations in the host code where ISV code hooks into the host code. The code is then modified to by-pass the hooks to ISV code to generate a side-executable code and both codes are executed and the results are compared. A lack of substantial difference in the results is indicative that the ISV code adversely affects the host code while a difference between the results is indicative of an error in the modified code.