Code generation leverages artificial intelligence technologies, particularly Large Language Models (LLMs), to automatically produce source code, enhancing software development efficiency and reducing repetitive tasks. However, the LLMs' generated code often fails to pass test cases and requires substantial human effort to fix errors. Previous studies focused on better prompts or improving LLMs' capability but ignored why LLMs failed. In this paper, we first reproduced 14 LLMs, including GPT-3.5-turbo and 13 open-source LLMs, on the HumanEval dataset. We extracted 12,837 code generation errors and conducted an in-depth analysis of their causes, which led to the identification of 19 distinct error causes. Our empirical analysis indicated that three of these causes can be directly fixed. Consequently, we proposed a fixing method called LlmFix, which addresses these three types of errors through a three-step process: filtering code for indentation correction, truncating redundant generated code, and importing missing modules. Experimental results demonstrate that LlmFix can fix these three types of errors, significantly improving the performance of 14 LLMs on HumanEval and MBPP datasets with average increases of 9.5% and 5.4%, respectively.

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