Granville‑Phillips 303 is a microprocessor‑controlled vacuum process controller (VPC) that drives two ionization gauge tubes over the range 5 × 10⁻¹¹ to 1 × 10⁻¹ Torr and includes emission current selection, leak‑test indicators, and filament‑deterioration warnings. Common failure modes for this unit are power supply degradation (over‑voltage or ripple), gauge tube filament burnout or contamination, microprocessor reset loops, and indicator LED failures due to internal board damage. First‑check diagnostics: verify 12 V/24 V input voltage and current rating; inspect the gauge tubes for visible soot or cracked filaments; review any fault codes displayed on the panel or via serial port; and perform a visual inspection of the PCB for burn marks, loose solder joints, or damaged connectors.[oss-gpt-20b]