Pool pilot Auto Pilot Salt reading way too low, what could be the reasons?

[Mayday]

I check the pool pilot fairly regularly. I keep the salt level around 3200 ppm. I think I checked a few weeks ago and the pool pilot was saying salt was down to 3000, so I added about a bag and brought it back up.
This morning, I checked the pool pilot display and it said "Error: Purify off. Add Salt 324 Lb". It thinks the salt level is at 400ppm.

This has never happened before. Salt cell is about 3 years old. I checked, It is clean. It is this: Autopilot Pool Pilot Digital Chlorine Generator Replacement Cell Only With Unions - PPC1 (Formerly RC35/22)

There is a pipe that splits off from the main pipe (main pipe is the one that takes the water from the filter back to the pool). This other pipe pipe takes some water to the salt cell, and it has it's own little filter that I clean when I clean the main cartridge filter. That filter was very dirty and mostly blocked. I cleaned that and put it back. I don't know if that may have been the reason for this reading. It has been a few hours and the reading has not changed. I even reset the breaker switch to make the system restart.

What other reasons might there be for a bad reading? What can I check or do? Should I try to clean the Tri-Sensor Assembly that sits before the water cell? I also have an old cell that this one replaced 3 years ago. I think that cell was still working, if that can be used for testing.

Since it says "Purify off" I guess it is not trying to generate chlorine right now. I checked chlorine level in the pool. I had shocked when closing the pool some months ago, and the chlorine had stayed around 16 (32 drops of the Taylor FAS DPD kit) for a long time during biweekly checks, now it is down to 11 (22 drops). So it's lower than it was, still high though. I can keep it high manually with liquid chlorine while I figure out what to do.

Or should I call in a pool equipment repair person? They may just replace everything....

 

[HermanTX]

Have you used the Taylor K1766 salinity test kit to confirm your actual salt level?
most salt systems use an amp reading or some similar method to calculate a salt level but it is rarely that accurate.
However it is important to keep the salt cell happy so it turns on.
So having the actual salt level helps determine the next steps.
Have you cleaned the sensor that reads the salt level for the cell?

 

[Mayday]

Have you used the Taylor K1766 salinity test kit to confirm your actual salt level?
most salt systems use an amp reading or some similar method to calculate a salt level but it is rarely that accurate.
However it is important to keep the salt cell happy so it turns on.
So having the actual salt level helps determine the next steps.
Have you cleaned the sensor that reads the salt level for the cell?

I don't have a salt test kit so I will take it to the store today.
I seriously doubt that salt level would drop from 3100 to 400 in a few weeks with no rain, no manual water added, etc. That's why I am wondering if the salt reading is bad.
I searched a bit and found this in the pool pilot manual:

"Tri-sensor Assembly

The Tri-sensor Assembly is used to measure water flow, salt level, and water temperature.

When the water flow reaches a minimum flow rate of 20 gpm (76 L/m), the flow paddle magnet closes a micro-switch that is monitored by the Control Unit.

The Control Unit uses dedicated salt sensor blades to measure the level of salt in the water"

So I need to check on that, but that things is really tough to get out. I already tried with some pliers to move it and it didn't. I will try again later today.

My plan is to get some liquid chlorine and raise the Cl level in the pool, in case I break something and the pool sits while I get a pro. So it won't grow algae while I wait. And then I will try really hard to get that sensor out.
It's this: Autopilot Tri-Sensor Assembly with 12' Cord-APA0003
There is a youtube video on it:

I am afraid I will break something but I guess I have to try. It is definitely not something I can do with my hands.

 

[HermanTX]

It looks easy enough to remove but that oring properly has some friction on it.
Do you have a strap wrench? That could wrap around the sensor housing and allow you to rotate it after you remove the holding screws. You just need to break the friction and then try to pull it out once it rotates slightly.
The idea of 2 people with using pump pressure is good - just need to be careful as it will pop loose once it breaks the friction bond .

 

[Mayday]

It looks easy enough to remove but that oring properly has some friction on it.
Do you have a strap wrench? That could wrap around the sensor housing and allow you to rotate it after you remove the holding screws. You just need to break the friction and then try to pull it out once it rotates slightly.
The idea of 2 people with using pump pressure is good - just need to be careful as it will pop loose once it breaks the friction bond .

I actually didn't understand what they were suggesting with the pump pressure- can you help? They say open the air valve on the filter to break the vacuum and let air in. OK, got it. Then they say have someone else turn on the pump to help push out the sensor assembly as you twist it. But if the air valve on the filter is open will you not just have water gushing out of it? And if they meant to close the air valve before turning on the pump, that will just recreate the "prime" will it not?

Or will water not gush out of the open air valve if you turn on the pump for a bit, because the filter is not full yet? I didn't understand that part.

I didn't know what a strap wrench was, so I looked it up and I can get one delivered from amazon tomorrow. I have a 10 inch channel lock pliers that barely grips from the bottom, I've been trying to get the assembly off using that but no luck so far. I already ordered a bigger channel lock style which arrives tomorrow. I'll keep trying.

 

[HermanTX]

I actually didn't understand what they were suggesting with the pump pressure- can you help? They say open the air valve on the filter to break the vacuum and let air in. OK, got it. Then they say have someone else turn on the pump to help push out the sensor assembly as you twist it. But if the air valve on the filter is open will you not just have water gushing out of it? And if they meant to close the air valve before turning on the pump, that will just recreate the "prime" will it not?

Or will water not gush out of the open air valve if you turn on the pump for a bit, because the filter is not full yet? I didn't understand that part.

They did explain that part very well. The goal of opening up the filter valve is because as you pull out the sensor it is creating a vacuum and you want air to be coming into the system. This is with pump off.
However, if you plan to use the pump pressure to push out the sensor, - then close the filter bleed valve. As they say, you need 2 people so one can quickly turn off the pump as the sensor is pushed out.

 

[Mayday]

They did explain that part very well. The goal of opening up the filter valve is because as you pull out the sensor it is creating a vacuum and you want air to be coming into the system. This is with pump off.
However, if you plan to use the pump pressure to push out the sensor, - then close the filter bleed valve. As they say, you need 2 people so one can quickly turn off the pump as the sensor is pushed out.

I was able to remove the sensor. I cleaned it for 15 minutes in a 20% muratic acid solution. The two sensor blades looked flat black but there was yellowing on the plastic housing. After the cleaning, all the yellow was gone. I put in the sensor, and instead of reading 400 ppm, it read 500, but not higher (it should be around 3000, I had the water tested). I removed it and dis a 50% solution for almost an hour. Still, no change.
At this point, I think I should get a replacement sensor right? It's generally around $250, but discountsaltpool.com has it for $190.

Or is there another step before I spend the money? I just don't want to get a new sensor, and it turns out the issue is something else. But I just don't see what else to do if the sensor is not reading correctly.
By the way, this sensor is at least 8 years old. We changed the salt cell three years ago, but not anything else in this module.
What do you think?

Edit: I found out out that there is something called an AutoPilot Pool Pilot Tri-Sensor Simulator 986-ST. It is a plug that I guess you use to test whether the board is good or something. I think it sends signals to the control board that simulate the correct levels for everything so you can calibrate. So if I use this and check the reading, and if the reading is correct salt levels, then I will know the sensor is bad. If it still reads incorrectly, then I don't know what to do. I guess get a whole new system. I ordered one off ebay.

 

[HermanTX]

I am not familiar with that system but it is general best practice to change the sensor when you change the salt cell if it has been several years.
The only other piece is the board that the sensor connects to in the control box.
That simulator is interesting. That would tell you if the board was reading correctly, if so, then it would be the sensor that is bad.