# Troubleshoot control > Find the cause when a schematic, an Arc automation, or a script cannot control hardware. Each entry starts from what you see, then gives the cause and the fix. ## Schematics ### The schematic has no control button * Cause Your role is Viewer, or the schematic is a snapshot. A viewer cannot control hardware, and nobody can control a snapshot. * Fix Ask an administrator for the Operator role or higher. For a snapshot, open the schematic that the snapshot came from. ### A click on a valve does nothing * Cause One of these: * Editing is on, so the click selects the symbol. * The schematic does not have control. Another writer with a higher authority holds the channel, or one with the same authority got it first. * The command channel has no write task, or the symbol has no state channel, so the valve never shows as open. * Fix 1. Turn off editing, and take control. 2. Look at the [control legend](https://docs.synnaxlabs.com/reference/control/manual#control-legend) to see which writer controls the channel. To take over, see [take absolute control](https://docs.synnaxlabs.com/reference/control/manual#take-absolute-control-of-one-channel). 3. Check the channels of the symbol. See [what each symbol writes](https://docs.synnaxlabs.com/reference/control/manual#what-each-symbol-writes). ### `No symbols control any channels.` * Cause No symbol on the schematic has a command channel. The notification shows “Failed to acquire control”, with this text in the description. * Fix Turn on editing, and set a channel in the **Control** tab of each symbol. ### The schematic has no Control tab * Cause Your role is Operator, or the schematic is a snapshot. Operators cannot edit schematics, so they write at authority 1. * Fix Ask an administrator for the Engineer role. ## Authority ### Synnax drops commands with no error * Cause Another writer controls the channel. Synnax drops writes from a writer that does not have control. It reports an error only when the writer opens, and only if the writer asks for it. * Fix [See which writer controls the channel](https://docs.synnaxlabs.com/reference/control/authority#see-which-writer-controls-a-channel). Then lower its authority, or raise yours above it. ### ` has no control authority - it is currently held by ` * Cause The writer asked for an error when it opens without control, and another writer holds the channel. The Python `client.write` method asks for it. * Fix Stop the other writer, or open yours with a higher authority. ### Authority 255 does not take control * Cause On a channel with an index, a tie goes to the writer that got control first. A writer at 255 does not take control from another writer at 255. On a virtual channel, every writer at the controlling authority or above it writes. * Fix Give automations a lower authority, such as 100, so that operators can take over at 255. See [control authority](https://docs.synnaxlabs.com/reference/control/authority). ### An operator takes a channel from an automation, but commands do nothing * Cause The automation sets a per-channel authority with no default, such as `authority (vent_vlv_cmd 100)`. Its index channels stay at 255. The schematic also writes the index channel and loses it, so Synnax drops its writes to every channel on that index. * Fix Give the authority line a default: `authority (100 vent_vlv_cmd 200)`. ## Arc automations ### The stage indicator shows a running sequence, but the valves do not move * Cause The automation has a lower authority than the schematic. A click on any button takes control of every actuated symbol on the schematic, valves included. Synnax then drops the commands from the sequence. * Fix Keep the automation authority above the schematic authority, or put the start button on a separate schematic. See [start a sequence from a schematic](https://docs.synnaxlabs.com/reference/control/arc/start-from-schematic#start-the-test). ### The abort stage runs at the end of a normal run * Cause A write, a wait, or a gate step moves on to the next step in the code, also when the next step is a named stage. An abort stage right after such a step runs when the steps before it finish. * Fix Put the abort stage where no step leads into it, such as after a stage that only leaves by name. See [steps of a sequence](https://docs.synnaxlabs.com/reference/control/arc/execution-model#steps-of-a-sequence). ### The sequence runs once and never starts again * Cause The sequence ends in a stage, so it never stops. A start command to a running sequence does nothing. * Fix End the sequence with a write, a wait, or a gate, such as `0 -> vent_vlv_cmd`. See [when a sequence stops](https://docs.synnaxlabs.com/reference/control/arc/execution-model#when-a-sequence-stops). ### A transition does not fire when the stage starts * Cause A stage reacts only to values that arrive after it starts. A condition such as `tank_pt > 300` waits for the next `tank_pt` value, even when the last value was 400. * Fix No fix is necessary when the sensor streams. See [entering a stage](https://docs.synnaxlabs.com/reference/control/arc/execution-model#entering-a-stage). ### `The node failed: WASM execution failed in node at sample /: ...` * Cause A runtime error, such as an index out of bounds. On a Driver, the status shows “Failed to run Arc program” with this text in the description, and the automation stops. On the Core, the status shows “Node failed”, and the Core drops the cycle and keeps running. * Fix Fix the code. See [errors while it runs](https://docs.synnaxlabs.com/reference/control/arc/execution-model#errors-while-it-runs). ### The automation list shows Stopped for a running automation * Cause A known bug. The list shows **Stopped** when a running automation has a warning. * Fix Open the automation to see its real status and the warning. ### A Driver is not in the list * Cause The list shows only Drivers with the `arc` integration turned on. All Drivers have it by default, but a Driver configuration can remove it. * Fix Add `arc` to the integrations of the Driver. **Deploy** stays off until you choose a Driver. ### `undefined symbol: ` * Cause No channel, variable, or function has that name. Channel names are case sensitive. * Fix Create the channel, or correct the name. ### `numeric conditions are deprecated; use an explicit comparison like x != 0` * Cause A number is used as a condition, such as `start_cmd => press`. * Fix Compare it: `start_cmd == 1 => press`. ## Graph editor ### A block appears in the top-left corner * Cause A click on a block in the menu puts it at the top-left corner of the graph. * Fix Drag the block from the menu to where you want it. ### An Add, Subtract, Multiply, Divide, or Not block does not compile * Cause A known bug in these blocks. * Fix Write the expression in a text automation. See [compute values from channels](https://docs.synnaxlabs.com/reference/control/arc/compute-values). ### A Change status block fires on every value * Cause Graph edges pass every value, true or false. * Fix Connect a **Select** block, and connect each Change status block to its true or false output. See [build an alarm](https://docs.synnaxlabs.com/reference/control/arc/graph-editor#build-an-alarm). ## Python ### Controller commands do nothing and raise no error * Cause Another writer controls the channel. A `Controller` does not ask for an error, so Synnax drops its writes. * Fix See [Synnax drops commands with no error](https://docs.synnaxlabs.com/reference/control/troubleshooting#synnax-drops-commands-with-no-error). ### `authorities: authority count must be 1 or equal to channel count` * Cause `write_authorities` has more than one value. The controller adds the index channels to its write channels, so the counts do not match. * Fix Give one authority for all channels. Change it per channel with `set_authority` after the controller opens. ### `Channel ... not found in controller state` * Cause The code reads a channel that is not in the `read` list of the controller, or that has no value yet. * Fix Add the channel to `read`, or wait for its first value with `wait_until_defined`. ### The sequence goes on before the condition is true * Cause `wait_until` returns `False` at the timeout. It raises no error. * Fix Check the return value, and abort when it is `False`.