diff --git a/Cargo.lock b/Cargo.lock
index ab074128a..aafbe4117 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -6891,6 +6891,7 @@ dependencies = [
  "cfg-if",
  "cortex-m",
  "critical-section",
+ "hubpack",
  "num-derive 0.4.2",
  "num-traits",
  "paste",
diff --git a/drv/transceivers-server/src/main.rs b/drv/transceivers-server/src/main.rs
index 1a299be09..637c75645 100644
--- a/drv/transceivers-server/src/main.rs
+++ b/drv/transceivers-server/src/main.rs
@@ -49,7 +49,7 @@ task_slot!(THERMAL, thermal);
 include!(concat!(env!("OUT_DIR"), "/i2c_config.rs"));
 
 #[allow(dead_code)]
-#[derive(Copy, Clone, PartialEq, Eq, Count)]
+#[derive(Copy, Clone, PartialEq, Count)]
 enum Trace {
     #[count(skip)]
     None,
@@ -79,6 +79,8 @@ enum Trace {
     DisableFailed(usize, LogicalPortMask),
     ClearDisabledPorts(LogicalPortMask),
     SeqError(SeqError),
+    ModuleTemperatureCritical(u8, Celsius),
+    ModuleTemperaturePowerDown(u8, Celsius),
 }
 
 counted_ringbuf!(Trace, 16, Trace::None);
@@ -335,7 +337,7 @@ impl ServerImpl {
                     model: ThermalProperties {
                         target_temperature: Celsius(65.0),
                         critical_temperature: Celsius(70.0),
-                        power_down_temperature: Celsius(80.0),
+                        power_down_temperature: Some(Celsius(80.0)),
                         temperature_slew_deg_per_sec: 0.5,
                     },
                 })
@@ -429,7 +431,18 @@ impl ServerImpl {
                 // will return a `NotInAutoMode` error if the thermal loop is in
                 // manual mode; this is harmless and will be ignored (instead of
                 // cluttering up the logs).
-                match self.thermal_api.update_dynamic_input(i, m.model) {
+
+                let model = ThermalProperties {
+                    // We do *not* want the thermal loop to power down the whole
+                    // system in response to a transceiver overheating. Instead,
+                    // we will just disable the individual transceiver here.
+                    // Thus, disable power-down on the version of the device's
+                    // thermal properties by setting the
+                    // `power_down_temperature` to `None`.
+                    power_down_temperature: None,
+                    ..m.model
+                };
+                match self.thermal_api.update_dynamic_input(i as u32, model) {
                     Ok(()) | Err(ThermalError::NotInAutoMode) => (),
                     Err(e) => ringbuf_entry!(Trace::ThermalError(i, e)),
                 }
@@ -453,6 +466,25 @@ impl ServerImpl {
                     // We got a temperature! Send it over to the thermal task
                     self.sensor_api
                         .post_now(TRANSCEIVER_TEMPERATURE_SENSORS[i], t.0);
+
+                    if m.model.should_power_down(t) {
+                        // If the module's temperature exceeds the power-down
+                        // threshold, add it to the list of things to disable.
+                        ringbuf_entry!(Trace::ModuleTemperaturePowerDown(
+                            port.0, t
+                        ));
+                        // TODO(eliza): ereport
+                        // TODO(eliza): debounce
+                        to_disable.set(port);
+                    } else if m.model.is_critical(t) {
+                        ringbuf_entry!(Trace::ModuleTemperatureCritical(
+                            port.0, t
+                        ));
+                        // TODO(eliza): ereport
+                        // TODO(eliza): track over critical duration...
+                    }
+                    // TODO(eliza): see if it's nominal again and turn it back
+                    // on...?
                 }
                 // We failed to read a temperature :(
                 //
diff --git a/idl/thermal.idol b/idl/thermal.idol
index 5505b6bcf..dab15236e 100644
--- a/idl/thermal.idol
+++ b/idl/thermal.idol
@@ -80,13 +80,14 @@ Interface(
         "update_dynamic_input": (
             doc: "Provides a thermal model for a dynamic sensor",
             args: {
-                "index": "usize",
+                "index": "u32",
                 "model": "ThermalProperties",
             },
             reply: Result(
                 ok: "()",
                 err: CLike("ThermalError"),
             ),
+            encoding: Hubpack
         ),
         "remove_dynamic_input": (
             doc: "Removes the given dynamic input, so it is no longer used in the control loop",
diff --git a/sys/userlib/Cargo.toml b/sys/userlib/Cargo.toml
index f6d621645..1a0f2f69c 100644
--- a/sys/userlib/Cargo.toml
+++ b/sys/userlib/Cargo.toml
@@ -13,6 +13,7 @@ critical-section = ["dep:critical-section"]
 bstringify = { workspace = true }
 cfg-if = { workspace = true }
 critical-section = {workspace = true, optional = true, features = ["restore-state-none"]}
+hubpack = { workspace = true }
 num-derive = { workspace = true }
 num-traits = { workspace = true }
 paste = { workspace = true }
diff --git a/sys/userlib/src/units.rs b/sys/userlib/src/units.rs
index 005250595..f92ac2dde 100644
--- a/sys/userlib/src/units.rs
+++ b/sys/userlib/src/units.rs
@@ -6,6 +6,8 @@
 //! Tuple structs for units that are useful in the real world
 //!
 
+use hubpack::SerializedSize;
+use serde::{Deserialize, Serialize};
 use zerocopy::{FromBytes, Immutable, IntoBytes, KnownLayout};
 
 /// Degrees Celsius
@@ -19,6 +21,9 @@ use zerocopy::{FromBytes, Immutable, IntoBytes, KnownLayout};
     IntoBytes,
     Immutable,
     KnownLayout,
+    Serialize,
+    Deserialize,
+    SerializedSize,
 )]
 #[repr(C)]
 pub struct Celsius(pub f32);
diff --git a/task/thermal-api/src/lib.rs b/task/thermal-api/src/lib.rs
index 9c5cc75ef..806aa6aae 100644
--- a/task/thermal-api/src/lib.rs
+++ b/task/thermal-api/src/lib.rs
@@ -11,7 +11,7 @@ use drv_i2c_api::ResponseCode;
 use hubpack::SerializedSize;
 use serde::{Deserialize, Serialize};
 use userlib::{units::Celsius, *};
-use zerocopy::{FromBytes, Immutable, IntoBytes, KnownLayout};
+use zerocopy::{Immutable, IntoBytes, KnownLayout, TryFromBytes};
 
 #[derive(
     Copy, Clone, Debug, FromPrimitive, Eq, PartialEq, IdolError, counters::Count,
@@ -81,7 +81,7 @@ pub enum ThermalAutoState {
 }
 
 /// Properties for a particular part in the system
-#[derive(Clone, Copy, IntoBytes, FromBytes, Immutable, KnownLayout)]
+#[derive(Clone, Copy, Serialize, Deserialize, SerializedSize)]
 #[repr(C)]
 pub struct ThermalProperties {
     /// Target temperature for this part
@@ -93,7 +93,10 @@ pub struct ThermalProperties {
 
     /// Temperature at which we drop into the A2 power state.  This should be
     /// below the part's nonrecoverable temperature.
-    pub power_down_temperature: Celsius,
+    ///
+    /// If this is `None`, the system will not be sent to A2 due to this part's
+    /// temperature.
+    pub power_down_temperature: Option<Celsius>,
 
     /// Maximum slew rate of temperature, measured in °C per second
     ///
@@ -108,7 +111,11 @@ pub struct ThermalProperties {
 impl ThermalProperties {
     /// Returns whether this part is exceeding its power-down temperature
     pub fn should_power_down(&self, t: Celsius) -> bool {
-        t.0 >= self.power_down_temperature.0
+        if let Some(power_down_temperature) = self.power_down_temperature {
+            t.0 >= power_down_temperature.0
+        } else {
+            false
+        }
     }
 
     /// Returns whether this part is exceeding its critical temperature
@@ -229,4 +236,11 @@ impl From<drv_i2c_devices::pct2075::Error> for SensorReadError {
     }
 }
 
+#[derive(Clone, Copy, IntoBytes, TryFromBytes, Immutable, KnownLayout)]
+#[repr(u8)]
+pub enum PowerDownMode {
+    System = 1,
+    Transceiver,
+}
+
 include!(concat!(env!("OUT_DIR"), "/client_stub.rs"));
diff --git a/task/thermal/src/bsp/cosmo_ab.rs b/task/thermal/src/bsp/cosmo_ab.rs
index 608c7c897..3bd57e02d 100644
--- a/task/thermal/src/bsp/cosmo_ab.rs
+++ b/task/thermal/src/bsp/cosmo_ab.rs
@@ -165,7 +165,7 @@ impl Bsp {
 const U2_THERMALS: ThermalProperties = ThermalProperties {
     target_temperature: Celsius(65f32),
     critical_temperature: Celsius(70f32),
-    power_down_temperature: Celsius(75f32),
+    power_down_temperature: Some(Celsius(75f32)),
     temperature_slew_deg_per_sec: 0.5,
 };
 
@@ -175,7 +175,7 @@ const U2_THERMALS: ThermalProperties = ThermalProperties {
 const M2_THERMALS: ThermalProperties = ThermalProperties {
     target_temperature: Celsius(65f32),
     critical_temperature: Celsius(70f32),
-    power_down_temperature: Celsius(75f32),
+    power_down_temperature: Some(Celsius(75f32)),
     temperature_slew_deg_per_sec: 0.5,
 };
 
@@ -186,7 +186,7 @@ const M2_THERMALS: ThermalProperties = ThermalProperties {
 const CPU_THERMALS: ThermalProperties = ThermalProperties {
     target_temperature: Celsius(80f32),
     critical_temperature: Celsius(90f32),
-    power_down_temperature: Celsius(100f32),
+    power_down_temperature: Some(Celsius(100f32)),
     temperature_slew_deg_per_sec: 0.5,
 };
 
@@ -194,7 +194,7 @@ const CPU_THERMALS: ThermalProperties = ThermalProperties {
 const T6_THERMALS: ThermalProperties = ThermalProperties {
     target_temperature: Celsius(70f32),
     critical_temperature: Celsius(80f32),
-    power_down_temperature: Celsius(85f32),
+    power_down_temperature: Some(Celsius(85f32)),
     temperature_slew_deg_per_sec: 0.5,
 };
 
diff --git a/task/thermal/src/bsp/gimlet_bcdef.rs b/task/thermal/src/bsp/gimlet_bcdef.rs
index d81e0c1e4..b77840eba 100644
--- a/task/thermal/src/bsp/gimlet_bcdef.rs
+++ b/task/thermal/src/bsp/gimlet_bcdef.rs
@@ -208,7 +208,7 @@ impl Bsp {
 const DIMM_THERMALS: ThermalProperties = ThermalProperties {
     target_temperature: Celsius(80f32),
     critical_temperature: Celsius(90f32),
-    power_down_temperature: Celsius(95f32),
+    power_down_temperature: Some(Celsius(95f32)),
     temperature_slew_deg_per_sec: 0.5,
 };
 
@@ -223,7 +223,7 @@ const DIMM_THERMALS: ThermalProperties = ThermalProperties {
 const U2_THERMALS: ThermalProperties = ThermalProperties {
     target_temperature: Celsius(65f32),
     critical_temperature: Celsius(70f32),
-    power_down_temperature: Celsius(75f32),
+    power_down_temperature: Some(Celsius(75f32)),
     temperature_slew_deg_per_sec: 0.5,
 };
 
@@ -233,7 +233,7 @@ const U2_THERMALS: ThermalProperties = ThermalProperties {
 const M2_THERMALS: ThermalProperties = ThermalProperties {
     target_temperature: Celsius(65f32),
     critical_temperature: Celsius(70f32),
-    power_down_temperature: Celsius(75f32),
+    power_down_temperature: Some(Celsius(75f32)),
     temperature_slew_deg_per_sec: 0.5,
 };
 
@@ -244,7 +244,7 @@ const M2_THERMALS: ThermalProperties = ThermalProperties {
 const CPU_THERMALS: ThermalProperties = ThermalProperties {
     target_temperature: Celsius(80f32),
     critical_temperature: Celsius(90f32),
-    power_down_temperature: Celsius(100f32),
+    power_down_temperature: Some(Celsius(100f32)),
     temperature_slew_deg_per_sec: 0.5,
 };
 
@@ -252,7 +252,7 @@ const CPU_THERMALS: ThermalProperties = ThermalProperties {
 const T6_THERMALS: ThermalProperties = ThermalProperties {
     target_temperature: Celsius(70f32),
     critical_temperature: Celsius(80f32),
-    power_down_temperature: Celsius(85f32),
+    power_down_temperature: Some(Celsius(85f32)),
     temperature_slew_deg_per_sec: 0.5,
 };
 
diff --git a/task/thermal/src/bsp/grapefruit.rs b/task/thermal/src/bsp/grapefruit.rs
index bfdf13ec5..7afa71502 100644
--- a/task/thermal/src/bsp/grapefruit.rs
+++ b/task/thermal/src/bsp/grapefruit.rs
@@ -128,7 +128,7 @@ impl Bsp {
 const LM75_THERMALS: ThermalProperties = ThermalProperties {
     target_temperature: Celsius(60f32),
     critical_temperature: Celsius(70f32),
-    power_down_temperature: Celsius(80f32),
+    power_down_temperature: Some(Celsius(80f32)),
     temperature_slew_deg_per_sec: 0.5,
 };
 
diff --git a/task/thermal/src/bsp/sidecar_bcd.rs b/task/thermal/src/bsp/sidecar_bcd.rs
index 68c4b8688..c297a32c0 100644
--- a/task/thermal/src/bsp/sidecar_bcd.rs
+++ b/task/thermal/src/bsp/sidecar_bcd.rs
@@ -212,7 +212,7 @@ impl Bsp {
 const TF2_THERMALS: ThermalProperties = ThermalProperties {
     target_temperature: Celsius(60f32),
     critical_temperature: Celsius(70f32),
-    power_down_temperature: Celsius(80f32),
+    power_down_temperature: Some(Celsius(80f32)),
     temperature_slew_deg_per_sec: 0.5,
 };
 
@@ -221,7 +221,7 @@ const TF2_THERMALS: ThermalProperties = ThermalProperties {
 const VSC7448_THERMALS: ThermalProperties = ThermalProperties {
     target_temperature: Celsius(85f32),
     critical_temperature: Celsius(95f32),
-    power_down_temperature: Celsius(105f32),
+    power_down_temperature: Some(Celsius(105f32)),
     temperature_slew_deg_per_sec: 0.5,
 };
 
diff --git a/task/thermal/src/main.rs b/task/thermal/src/main.rs
index d92dd6380..56b07ed7c 100644
--- a/task/thermal/src/main.rs
+++ b/task/thermal/src/main.rs
@@ -269,14 +269,14 @@ impl<'a> idl::InOrderThermalImpl for ServerImpl<'a> {
     fn update_dynamic_input(
         &mut self,
         _: &RecvMessage,
-        index: usize,
+        index: u32,
         model: ThermalProperties,
     ) -> Result<(), RequestError<ThermalError>> {
         if self.mode != ThermalMode::Auto {
             return Err(ThermalError::NotInAutoMode.into());
         }
         self.control
-            .update_dynamic_input(index, model)
+            .update_dynamic_input(index as usize, model)
             .map_err(RequestError::from)
     }