Straight up front: I have never found error 402 in a Clack manual. Not the WS1 service manual, not the CI, EI, EE or SP programming manuals. The only place it's defined is a one-page error chart a water treatment distributor put out for Clack valves. What follows is the best available explanation, labeled as such, not Clack's official word.
Short answer: that chart lists Error 402 (4002 on one control family) as a checksum error from the power-down section of the control's EEPROM memory, a nonvolatile memory failure where the microcontroller can no longer operate properly. In plain terms, the saved memory failed its own integrity check. It's an electronics fault, not a motor or piston fault. Reset it once; if it comes back, the control board is the suspect.
What the chart says, and what it doesn't
What I could verify, claim by claim
402 is a checksum error in the power-down section of EEPROM
- No
- Clack's own manuals: Not listed in any Clack manual I've read.
- Yes
- Distributor error chart: Listed, with 4002 as the form on the SD controller family.
Clack's manuals do confirm the board uses nonvolatile EEPROM memory; they just don't list a 402.
The control board's memory is nonvolatile EEPROM
- Yes
- Clack's own manuals: Quick-reference specs: 'PC Board Memory: Nonvolatile EEPROM.'
- Not published
- Distributor error chart: Not addressed.
WS1 and WS1.25 service manual, quick reference specifications.
402 shows on CI and EI controls
- No
- Clack's own manuals: Neither manual lists 402.
- Partly
- Distributor error chart: Chart columns are CS/WS1, CH/EE/SP, CD/CU, system controller and SD. No CI or EI column at all.
I can't confirm what a WS1CI or WS1EI shows for this fault.
A dead CR2032 backup battery causes 400-series errors
- Partly
- Clack's own manuals: Manuals confirm the battery type (3-volt lithium coin cell, type 2032) and how to seat it. They don't tie it to any error code.
- Not published
- Distributor error chart: Not on the chart. The claim comes from an unattributed repost online.
Treat the battery link as unverified. Replacing the cell is cheap and harmless, so I do it anyway on an older board.
Nearby codes 403 and 404 are memory checksum errors too
- No
- Clack's own manuals: Not listed.
- Yes
- Distributor error chart: 403 = program-setting section, 404 = diagnostic section. For 405, 407 and 408 the chart says the control stays out of error mode and keeps running. It does not say that for 402.
That last point is why I take 402 seriously: the chart treats it as a stop, not a note.
A sensible check order
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Write down the code and pull the error log. This part is Clack guidance. CI controls keep the last ten errors in Valve History Step 6VH; EI and EE controls keep them in Diagnostics Step 6D. A string of repeated 4xx codes is exactly what the next tech, or the board supplier, will want to see.
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Reset once. Also Clack guidance. On EI, EE and SP controls: disconnect the power supply at the PC board and reconnect, or press NEXT and REGEN together for 3 seconds. Other controls are on the reset page.
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Check the settings after the reset. Field practice. A memory fault can take your programming with it. Confirm time of day and the system settings before you leave, or the homeowner's next call is about a softener that regenerates at 2 p.m.
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If 402 comes back, suspect the control board. Field practice. The chart describes 402 as a memory failure where the microcontroller can no longer operate properly. That's the board's electronics, not the valve's mechanics. Clack publishes no board-replacement rule for 402, so this is judgment, and mine is: a repeat 402 on a board you've reset and re-powered is a board.
On the backup battery: there's an unattributed repost going around that says to replace the CR2032 coin cell for 400-series errors. I can't trace it to Clack. What Clack does document is the battery itself: CI, EI and EE manuals specify a 3-volt lithium type 2032, installed with the positives aligned and pushed down to seat fully. Swapping it costs a dollar and five minutes, so on an older board I do it while I'm in there. Just don't expect it to be the fix.
For motor-type codes, like the motor timing out on its way home, see the Error 104 page. Error 402 isn't one of those.
Questions I get on this one
What does error 402 mean on my Clack softener? A distributor's Clack error chart lists 402 as a checksum error in the power-down section of the control's EEPROM memory, a nonvolatile memory failure. None of the Clack manuals I've read list 402.
Is Clack error 402 the board or the motor? The chart describes it as a memory fault where the microcontroller can no longer operate properly, which points at the control board's electronics, not the motor. That's the chart's description, not a Clack manual's.
How do I clear error 402 on a Clack valve? On EI, EE and SP controls, disconnect power at the PC board and reconnect, or press NEXT and REGEN together for 3 seconds. If 402 returns, have a technician check the board.
Does a dead battery cause Clack error 402? Unconfirmed. An unattributed repost links 400-series errors to the CR2032 backup battery, but it doesn't trace to Clack. Clack's manuals do confirm the board uses a 3-volt type 2032 lithium coin cell.
Does 402 show up on a WS1CI or WS1EI? I can't confirm it. The chart lists 402 for CS, WS1, CH, EE, SP, CD, CU and the system controller, plus 4002 on SD. It has no CI or EI column.
If you're the one who gets these calls
The tech who installs the system is the one the homeowner calls when a code shows up, including the odd ones. Aquonyx builds city-water treatment systems and sells them only through dealers, so the installer keeps the customer. If that's the kind of manufacturer you'd rather work with, become an Aquonyx dealer.