---
title: "Bad, Missing, or Frozen Tag Values"
description: "The device is running but a tag reads bad quality, no value, or the wrong value"
source_url: https://ai-ops.com/docs/troubleshoot/tag-values
---

# Bad, Missing, or Frozen Tag Values

Use this article when a device is **Running** but one of its tags is wrong: the value is missing, flagged bad quality, non-numeric, or clearly incorrect. Because the connection is up, the problem is almost always specific to that one tag's address, data type, or the value it reads back, not the device or the network.

If **every** tag on the device is failing, or the device itself is Failed, start one layer up at [Troubleshoot a Connection](https://ai-ops.com/docs/troubleshoot/connection.md) instead.

> [!NOTE] Read the tag
> Every tag records an **error code**, **error message**, and **error detail**, plus a **quality** string and a **status**. Those fields tell you what failed and where. This article assumes you already know how to read them — see [Reading Status, Quality & Errors](https://ai-ops.com/docs/troubleshoot/reading-status-and-errors.md) for the shared explainer, the status/quality legend, the full code lookup, and the auto-clear/stuck checklist.

## First: is the device actually stopped?

If a tag is enabled but its parent device is stopped, the tag's overview shows a blue **info banner** (rather than a red error) telling you the device is stopped and the tag won't update until the device is enabled and running. This is **not a tag error**. The tag has nothing to read because its device isn't scanning. Enable and start the parent device, and the tag will begin collecting on the next scan. Only continue with the codes below once the device is Running.

## Symptom → cause → code

When the device is Running but the tag reads bad, match the symptom to the tag error code. All of these are tag-level codes in the `100`–`110` band; they clear automatically on the next successful scan.

| Symptom | Code | Name | What to do |
|------|------|------|------------|
| Bad OPC-UA quality (server returned a non-Good status) | 107 | **Bad: Read Quality** | The read succeeded but the source reported poor quality (e.g. `Uncertain` or a `Bad_*` OPC-UA status). The value may be stale or unreliable. Check the field device / OPC-UA server health for that node. |
| Node / register / variable not found on the device | 102 | **Bad: Node Not Found** | The addressed node ID, register, or variable doesn't exist on the device. Re-select it with the **Browse** button on the tag's **Configuration** tab — a namespace index or identifier may have changed. |
| No value / empty register / null response | 105 | **Bad: No Value** | The protocol returned nothing for this tag. The data point may not exist, the register may be empty, or the device returned null. Verify the address and data type on the Configuration tab. |
| Value came back but isn't a number | 103 | **Bad: Not a Number** | The device returned text, a special character, or an unexpected type where a numeric value was expected. Fix the configured data type so it matches what the device actually returns. |
| Read fails partway through the scan | 106 | **Bad: Failed To Read** | A read operation for this tag failed (invalid address, permission issue, or the device dropped this point mid-scan). The **error detail** carries the underlying exception. |
| Output write to the device failed | 101 | **Bad: Failed To Write** | Koios could not write an output value back. Confirm the address is writable, the credentials allow writes, and the value is within the device's accepted range. |
| Address, data type, register, bit position, or node ID is wrong | 104 | **Bad: Configuration Error** | The tag's protocol-specific settings are invalid or incomplete (e.g. a Modbus tag with no register or bad bit position, a malformed OPC-UA node ID, an invalid BOSS variable code). Review and correct the **Configuration** tab. This usually surfaces at device initialization. |
| General bad-quality condition, no better fit | 100 | **Bad: General** | A catch-all bad-quality state. Read the error detail for specifics. |
| Unexpected failure processing the tag | 2 | **Generic Exception** | An unhandled exception occurred while processing the tag. The error detail contains the specific exception. |

> [!TIP] Wrong value, not a bad-quality flag?
> If the tag reads a *good* value that is simply wrong — off by a factor, inverted, or in the wrong units — the collection is fine and the problem is in a transform. That's a value-mapping or expression issue, not a read error. See [Expression & Value-Mapping Errors](https://ai-ops.com/docs/troubleshoot/expressions-and-mapping.md). Codes `108` (Mapping Error) and `110` (Bad: Expression) also route there.

## Isolate it with a Test read

You can attempt a single read for one tag without enabling it. On the tag's **Configuration** tab, each protocol section has a **Test** button.

1. Open the tag's **Configuration** tab and **save** any changes (Test uses the tag's *saved* configuration, unlike the device connection test which uses the live form values).
2. Click **Test**. Koios performs a one-time read from the device for this tag.

| State | Meaning |
|-------|---------|
| **Test** | Idle, ready to test |
| **Testing...** | Read attempt in progress |
| **Success** | A value was read; the result message shows the value returned |
| **Failed** | The read failed; an error message appears below the form |

The device must be powered on and reachable, but it does not need to be enabled in Koios — the test uses a one-time connection separate from the scan cycle. A **Success** here with a **Failed** state during scanning points at scan timing or an intermittent device, not the tag config.

## The device-side view of a bad tag

A single misconfigured tag can also raise an error on the **device**, because the device is what performs the read and write:

| Code | Name | What it means for the tag |
|------|------|---------------------------|
| 5 | **Failed To Read from Device** | The connection is up but a tag read failed — often a tag pointing at an invalid address or node. Check which tag is failing and correct its address. |
| 6 | **Failed To Write to Device** | An output write failed. Verify the output tag's address is writable and the credentials permit writes. |
| 16 | **Failed To Validate Tags** | One or more tags have invalid or missing protocol settings (e.g. a Modbus tag with no register), caught before scanning. Fix the flagged tag's Configuration tab. |

If instead the tag shows code `1` **Parent Device Failed**, the device itself is down and the tag can't be collected until it recovers — troubleshoot the device at [Troubleshoot a Connection](https://ai-ops.com/docs/troubleshoot/connection.md). Code `109` **No Active Device** (redundancy: no device in the set is active) also belongs there.

> [!WARNING] Overscanning starves reads
> If the device is Running with a **Warning** strip and tags update sluggishly or intermittently, the scan group may be **overscanning** — execution is taking longer than the configured scan rate, so reads fall behind. That's a scan-group timing problem (scan-group error `1`, Overscan), not a per-tag fault. See [Service Health & Resource Alarms](https://ai-ops.com/docs/troubleshoot/services-and-alarms.md).

## Live value updates but history is flat or gappy

If a tag's live value is moving but Trends show flat lines or far fewer points than expected, the read is fine and the issue is historization, usually a mismatch between the tag's **Range Min / Max** and the compression settings. See [Data Is Stale, Frozen, or Has Gaps](https://ai-ops.com/docs/troubleshoot/stale-data.md) and [Tag Range and Compression](https://ai-ops.com/docs/system/retention.md#tag-range-and-compression). A code `200` **Failed To Historize** on the tag means the live value is available but that period will have a gap in historical storage.

## If a tag stays stuck

Tag errors clear on the next successful read or write — you don't acknowledge them manually. If a tag won't recover once the device is Running:

1. Read the **error message** and **error detail** for the specific cause.
2. Set the device's log level to **Debug** and review its [logs](https://ai-ops.com/docs/system/logs.md) for tag-level detail.
3. Verify the tag's **protocol settings** on the Configuration tab (address, data type, register, bit position, node ID).
4. Use **Test** to attempt a single read and see if the error reproduces.
5. Toggle the tag off and on with the **Enabled** switch.

For the shared stuck-tag checklist and how auto-clear works across all entities, see [Reading Status, Quality & Errors](https://ai-ops.com/docs/troubleshoot/reading-status-and-errors.md).

## What's Next

- [Reading Status, Quality & Errors](https://ai-ops.com/docs/troubleshoot/reading-status-and-errors.md) — the shared diagnostic-fields reference, status/quality legend, and master code lookup
- [Troubleshoot a Connection](https://ai-ops.com/docs/troubleshoot/connection.md) — when the device is Failed or every tag reads Parent Device Failed
- [Expression & Value-Mapping Errors](https://ai-ops.com/docs/troubleshoot/expressions-and-mapping.md) — when the value reads good but is transformed wrong (codes 108, 110)
- [Data Is Stale, Frozen, or Has Gaps](https://ai-ops.com/docs/troubleshoot/stale-data.md) — when live values update but history or Trends don't
- [Service Health & Resource Alarms](https://ai-ops.com/docs/troubleshoot/services-and-alarms.md) — overscanning scan groups and service health
- [Tag Value Mapping](https://ai-ops.com/docs/tags/value-mapping.md) and [Tag Expressions](https://ai-ops.com/docs/tags/expressions.md) — the transforms that own the mechanics
- [System Logs](https://ai-ops.com/docs/system/logs.md) — set Debug level and stream device logs
- [Tag Range and Compression](https://ai-ops.com/docs/system/retention.md#tag-range-and-compression) — how the tag range drives historization