# Sensor B is downgraded, but Sensor A is stuck at zero

**URL:** https://community.purpleair.com/t/sensor-b-is-downgraded-but-sensor-a-is-stuck-at-zero/22816
**Category:** Sensors
**Created:** [July 20, 2026, 3:48pm UTC](https://community.purpleair.com/t/sensor-b-is-downgraded-but-sensor-a-is-stuck-at-zero/22816 "2026-07-20T15:48:29Z")
**Posts on this page:** 3
**Page:** 1

<div class="post-metadata">

### Author: ![Gordon](https://avatars.discourse-cdn.com/v4/letter/g/3da27b/32.png) [@Gordon](https://community.purpleair.com/u/Gordon)
#### Post date: [July 20, 2026, 3:48pm UTC](https://community.purpleair.com/t/sensor-b-is-downgraded-but-sensor-a-is-stuck-at-zero/22816/1 "2026-07-20T15:48:29Z")

</div>

![image](https://us1.discourse-cdn.com/flex020/uploads/purpleair/original/2X/1/1d1e6a24c874ef1077b8ea51c0977602e49b2a2c.png)

 ![image](https://us1.discourse-cdn.com/flex020/uploads/purpleair/original/2X/7/7c0ffdc30c58a1a7a250b2f2cfe87ba4af15eb76.png)

---

<div class="post-metadata">

### Author: ![ShawnBoyle](https://sea2.discourse-cdn.com/flex020/user_avatar/community.purpleair.com/shawnboyle/32/6816_2.png) [@ShawnBoyle](https://community.purpleair.com/u/ShawnBoyle)
#### Post date: [July 23, 2026, 9:58pm UTC](https://community.purpleair.com/t/sensor-b-is-downgraded-but-sensor-a-is-stuck-at-zero/22816/2 "2026-07-23T21:58:10Z")

</div>

Hi Gordon,

Thanks for letting us know! Judging by the constant 0s being reported, it’s likely that the fan for Channel A is dead. Our auto-downgrade algorithm is quite simple at this point, and it can miss sensors that have a dead fan, often incorrectly identifying the higher-reading channel as the unhealthy one. We’ve manually downgraded Channel A to override this false flag applied by the automated downgrade system, so only Channel B’s readings will show up on your map marker now.

You can visually confirm if the fan is dead with the following steps:

1. Remove the power cable to power off the sensor
2. Identify the fan inside the laser counter (Channel A is the one opposite from the USB port)
3. Reconnect the power cable and identify if the fans begin spinning. It may seem to “disappear” if it does

If the fan is indeed dead, the long-term solution would be to replace your laser counters. Since your sensor is a Classic SD, you’ll need to [retrofit](https://www2.purpleair.com/products/classic-pa-ii-retrofit-kit) it to be compatible with our [Classic Plus laser counters](https://www2.purpleair.com/products/replacement-laser-counters?variant=46220052136033), as we no longer sell Classic SD laser counters; only Classic. Here’s a couple of videos to [walk you through the retrofit process](https://www.youtube.com/watch?v=xsMEceaav5U), as well as [the laser counter replacement](https://www.youtube.com/watch?v=muaPSyiZ_8k) for a Classic Plus once retrofitted (this isn’t for the SD, but the process is similar).

Note that we will have to manually remove the downgrade if you do proceed with replacement, so you’ll want to reach out and let us know if you do so.

Happy to help if you need any assistance with this process.

---

<div class="post-metadata">

### Author: ![Gordon](https://avatars.discourse-cdn.com/v4/letter/g/3da27b/32.png) [@Gordon](https://community.purpleair.com/u/Gordon)
#### Post date: [July 23, 2026, 11:23pm UTC](https://community.purpleair.com/t/sensor-b-is-downgraded-but-sensor-a-is-stuck-at-zero/22816/3 "2026-07-23T23:23:35Z")

</div>

Hi Shawn,

The sensor is in a remote area and I’m not there and I am unable to check the fan now.

If the problem is the fan, does your warranty cover the cost of the laser counters and retrofit kit?

Replacement laser counters were purchased 08/10/25.

Thanks,

Gordon Pratt