diff --git a/firmware/fault.h b/firmware/fault.h
index 7b4967a..b65813c 100644
--- a/firmware/fault.h
+++ b/firmware/fault.h
@@ -7,6 +7,7 @@ enum class Fault
     // First fault code at 3 so it's easier to see
     SensorDidntHeat = 3,
     SensorOverheat = 4,
+    SensorUnderheat = 5,
 };
 
 void setFault(Fault fault);
diff --git a/firmware/heater_control.cpp b/firmware/heater_control.cpp
index 2cd564a..4a85652 100644
--- a/firmware/heater_control.cpp
+++ b/firmware/heater_control.cpp
@@ -59,12 +59,16 @@ static HeaterState GetNextState(HeaterState state, float sensorEsr)
 
             break;
         case HeaterState::ClosedLoop:
-            // TODO: handle departure from closed loop
-            if (sensorEsr < HEATER_FAULT_ESR)
+            if (sensorEsr < HEATER_OVERHEAT_ESR)
             {
                 setFault(Fault::SensorOverheat);
                 return HeaterState::Stopped;
             }
+            else if (sensorEsr > HEATER_UNDERHEAT_ESR)
+            {
+                setFault(Fault::SensorUnderheat);
+                return HeaterState::Stopped;
+            }
 
             break;
         case HeaterState::Stopped: break;
diff --git a/firmware/wideband_config.h b/firmware/wideband_config.h
index b0730af..e75a3f0 100644
--- a/firmware/wideband_config.h
+++ b/firmware/wideband_config.h
@@ -38,4 +38,5 @@
 
 #define HEATER_CLOSED_LOOP_THRESHOLD_ESR 2000
 #define HEATER_TARGET_ESR 300
-#define HEATER_FAULT_ESR 200
+#define HEATER_OVERHEAT_ESR 200
+#define HEATER_UNDERHEAT_ESR 3000