2001 Toyota Celica GTS PO172 Code: Rich Condition, Injector Leak, and Recurring Check Engine Light

Summary: My 2001 Toyota Celica GTS (stock setup) recently started showing a persistent PO172 code. I had previously replaced the spark plugs, but that was before the issue appeared. Last month, I replaced the air/fuel ratio (A/F) sensor after it failed during a test at a shop — the seller had cleaned it to clear the PO172 code. However, I’ve since learned that cleaning this sensor can damage it permanently, especially in older Toyota models. The PO172 code clears for a few days and then returns repeatedly. A fuel pressure test came back normal. A leak-down test revealed significant leakage from cylinder 4’s injector, so I replaced that injector. Despite the replacement, the check engine light returned. I’ve confirmed that the ECT (engine coolant temperature) sensor is functioning properly, and the MAF (mass air flow) sensor was replaced by the seller two weeks prior to my purchase — though I’m unsure whether it was OEM or aftermarket. Additionally, when revving at idle, the engine hesitates and then jumps unexpectedly. This behavior has been consistent. During a recent 3-day trip where the car sat unused, the CEL (check engine light) turned off and remained off for about 30 miles before returning. I’m now completely confused — any insights or troubleshooting steps would be greatly appreciated. Technical note: PO172 indicates a rich air-fuel mixture in the exhaust system, which may stem from injector issues, sensor inaccuracies, or fuel delivery problems.