Reverse current to the cell is lower than the board expects, so you're making less chlorine. Jandy says: clean the cell and check the DC cord.
Your cell isn't getting enough current in reverse. Usually it's scale or a loose cord, and usually you can fix it yourself.
Code 121 means the AquaPure control is seeing low current going to the salt cell in the reverse direction. Same causes as 120: a scaled cell, a bad DC cord connection, or a cell near the end of its life.
Reverse current to the cell is lower than the board expects, so you're making less chlorine. Jandy says: clean the cell and check the DC cord.
Use a strip, not the board's number. Under 3.0 gpl? Add salt, wait 24 h, recheck.
Confirm salt and temperature first.
Don't trust the board's salt number if the sensor is dirty. Use a strip or a meter. If salt is under 3.0 gpl, add salt per the manual's chart, brush it in, let it circulate 24 hours, then clear the code and recheck. Cold water lowers current the same way it does for code 120.
Power off. Unplug the cord at the cell and check pins for corrosion or burn marks.
Inspect and reseat the DC cord.
Power off. Unplug the cord at the cell, look at the pins and the socket. Corroded or blackened pins, a loose fit, or a cord that brings the code back when you wiggle it is your culprit. Clean light corrosion with a contact-safe brush; a burned connector means a new cord.
Acid step. Goggles and gloves on. Acid into water, never the reverse. Hose first; acid only if scale stays.
Clean the cell.
Try a hard stream from a garden hose first. If scale stays, pull the cell, plug the two ports, and fill it with the manual's mix: 1 pint muriatic acid into 2 quarts water in a plastic bucket. Let it foam until it stops (don't soak it for hours, that eats the plate coating), dump, rinse well, reinstall. Don't put the flow/temp/salinity sensor in the acid.
Power back up. Let it run forward and reverse for a few hours. Gone? You're done.
Clear the code and watch a full cycle.
Power back up and let it run through both forward and reverse polarity (a few hours). If 121 stays gone, you're done. Recheck in a week.
Remove the pin cover plate. Each pin should be tight to its plate. Loose or corroded means the cell is going.
Look for loose cell pins.
If it still throws 121 (or 120), remove the cover plate over the cell's power pins and check that each pin is tight to its plate. A loose or corroded pin is a known failure, and it means the cell is on its way out.
Everything checks out and it's still 121 on an older cell? The cell is worn.
Call it: cell vs. board.
Clean cell, good cord, good salt, warm water, and still 121? On an older cell (many last roughly 3-7 years, depending on use) that's a worn cell. Swap the cell. If you see 126/127 or 124 alongside it, that's a power/board issue, not the cell.
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When everything else checks out, the cell is worn.
When salt, temperature, cord and cleaning all check out and 120/121 keeps coming back, the cell's plates are worn and replacing the cell is the fix.
No. The unit keeps running at reduced output. Your chlorine will drift down, so test and supplement with liquid chlorine until it's fixed.
120 is low current forward, 121 is low current reverse, 123 is extremely low current overall. 123 usually means the cell is done.
Raising salt to the top of the 3.0-3.5 gpl range can buy a little time on a weak cell, but it doesn't fix a worn one. Don't go above 3.5 gpl.
Once the cause is fixed, cycling power clears it. If it comes back within a cycle or two, the problem is still there.
Source: Jandy AquaPure/PureLink Power Center and Cell Kit, Installation and Operation Manual (H0325600), Section 7.2 Service Codes, Section 6.3, Section 9.3. Written by a Florida pool service owner. Updated 2026-10-14.