Jandy AquaPure Low Chlorine Output: Codes 120, 121, 123

What These Codes Mean

The AquaPure control board monitors the DC current flowing through the electrolytic cell in both forward and reverse directions. The cell reverses polarity every 3 hours as part of its automatic cleaning cycle. When the measured current drops below expected thresholds, the board generates a service code.

Code 120 indicates low current during the forward cycle only. Code 121 indicates low current during the reverse cycle only. Code 123 indicates extremely low current in both directions, which is the most serious of the three and usually points to a cell near end of life or completely blocked with scale.

These codes do not immediately shut down chlorine production but indicate the cell is not operating at full capacity. Chlorine output will be reduced, and the pool will gradually drift toward low chlorine levels.

Step-by-Step Troubleshooting

Owner-Level Checks

Check the salt level

1. Press the SALINITY button (C) on the user interface. The reading should be between 3.0 and 3.5 gpl.
2. If the reading is below 2.5 gpl, low salt is likely contributing to low current. Add salt per the manual's Table 2 and retest after 24 hours of circulation.
3. Verify with an independent salt test (Myron L meter calibrated for NaCl or titration kit). The AquaPure's salinity sensor can drift if it has scale buildup.

Check the water temperature

1. Press the POOL TEMP/BOOST button (D) to read the current water temperature.
2. Cold water below 60°F significantly reduces cell efficiency and current flow. Below 54°F, the unit displays "Lo" and stops production entirely.
3. In winter or early spring, reduced current is expected behavior, not necessarily a cell failure.

Inspect the DC cord connection

1. Open the power center door and verify the DC cord is firmly plugged into the two spade connectors on the wiring harness.
2. At the cell end, verify the DC cord plug is fully seated on the cell stud terminals. Look for corrosion, discoloration, or a loose fit.
3. If contacts are corroded, clean them with a wire brush or fine sandpaper and reseat firmly.

Tech-Level Checks

Inspect and clean the cell

1. Turn off the pump and release system pressure.
2. Remove the cell from the plumbing by unscrewing the coupling nuts.
3. Look through the cell. If you see white scale buildup between the plates or debris, it needs cleaning.
4. Perform an acid wash: mix 1 pint of muriatic acid with 2 quarts of tap water in a plastic bucket. Always add acid to water.
5. Plug the two adjacent ports and pour the acid solution into the cell. Vigorous foaming means scale is dissolving. Leave until foaming stops, but no more than 30 minutes.
6. Rinse thoroughly with clean tap water and inspect. Repeat if deposits remain.

Check for cell end-of-life

1. If the cell is clean, salt is correct (3.0-3.5 gpl), water is warm, and the DC cord is solid, the cell plates may be depleted.
2. Many pool professionals report 3-7 years as a typical range for cell lifespan, though actual results depend on pool size, production rate, and water chemistry.
3. If recurring low-current codes persist after thorough cleaning, the cell has likely reached end of life and should be replaced.
4. If codes 120/121/123 persist after acid washing and confirming all parameters, replace the cell. See the cell replacement guide.

Measure DC voltage at the cell

1. With the pump running and the cell energized (CELL ON indicator lit), measure DC voltage across the cell stud terminals.
2. Normal operating voltage is approximately 20-27 VDC depending on salt level and temperature.
3. Voltage significantly below 20V with good salt and warm water may indicate a power board issue.

Code 124: Higher Than Normal Current

Code 124 is the opposite: the board detects higher current than expected. This can indicate a short in the cell, a backboard relay issue, or an over-salted pool. Check salinity first. If salt is within range, contact a pool professional for board-level diagnosis.

Common Parts That Fix This Problem

• Replacement electrolytic cell (3-port, with universal 2"-2.5" unions)
• DC power cord if contacts are corroded or damaged
• Muriatic acid for cell cleaning (1 pint per cleaning)
• Cell O-rings (replace when removing the cell)

How to Prevent Low Current Codes

• Maintain salt at 3.0-3.5 gpl. Low salt accelerates cell plate erosion.
• Keep water balanced: pH 7.4-7.6, alkalinity 80-120 ppm, calcium hardness 175-400 ppm.
• Inspect the cell every 2 months initially. If no scale is found, extend to biannual inspection.
• Do not run the cell at 100% continuously unless necessary. Running at 90% allows rest that extends cell life.
• Never use dry acid to lower pH in arid areas with high evaporation.