Thermistor
Common temperature-sensing fault part. Confirm resistance range and connector.
Penn Commercial Controls Fault Codes — Quick Reference
Penn controls from Johnson Controls protect compressors, fans, and pumps in refrigeration and HVAC systems. Many Penn controls are mechanical safeties, so the “fault code” is often just an open control, a blank output, or a digital sensor error on electronic models.
| Fault | Meaning | Quick Fix |
|---|---|---|
| High Pressure Open | Safety opened on high head pressure | Check condenser fans and airflow |
| Low Pressure Open | Safety opened on low suction pressure | Check charge and evaporator airflow |
| Sensor Failure | Electronic sensor open or shorted | Check probe resistance and wiring |
| Manual Reset Locked Out | Safety latched after trip | Fix cause before reset |
| No Output | Control relay not closing | Check power and relay contacts |
| Wide Temperature Swing | Sensor placement or differential issue | Reposition sensor and review settings |
Most Common Faults
High Pressure Open
This is the classic condenser problem. Check condenser fan motors, blade rotation, and coil cleanliness. On water-cooled systems, check condenser water flow and tower performance.
Low Pressure Open
Low suction safety opens when refrigerant pressure drops too low. Check for low charge, iced evaporator, failed evaporator fan, or a restricted TXV. If the box is warm and suction is low, suspect low charge or restriction.
No Output
On electronic Penn controls, a blank or dead output often comes from loss of control voltage. Verify the input power at the control first. Then check whether the relay closes when the setpoint is crossed.
Common Penn Families
| Family | Use |
|---|---|
| P70 / P72 | Refrigeration pressure controls |
| P78 | Manual reset high and low pressure safeties |
| A19 / A28 | Temperature controls |
| A350 / A421 | Electronic temperature controls |
Need Replacement Parts?
Before ordering, match the refrigeration part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
Jump to Fix
- High pressure open → Check fan operation → Clean condenser → Check head pressure
- Low pressure open → Check evaporator airflow → Check charge → Inspect TXV and drier
- No output → Check control power → Check relay action → Replace control if needed
When to Call a Pro
Penn controls protect expensive compressors. If a safety keeps opening, the equipment is telling you something. Find the refrigeration or airflow problem before you reset and restart.