Batch-to-batch variability of commercial PbI2 precursors, even with metal purity levels of >99.99%, is a major limitation toward achieving intra- and interlab reproducibility of lead halide-based perovskite solar cells. This work identifies that Pb(OH)I is the primary impurity that causes these adverse issues. Through a systematic investigation into the role of synthesized Pb(OH)I within Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3 perovskite films and solar cells, it is shown that this impurity induces segregated PbI2 and δ-FAPbI3 phases and reduces the ensuing solar cell performance characteristics. To avoid these issues, a facile, high-yield, remediation process amendable to any PbI2 precursor batch is introduced. Through its use, reliably high performing metal halide perovskite solar cells are achieved.