diff --git a/src/libslic3r/GCode.cpp b/src/libslic3r/GCode.cpp
index c7c2721ed1..78b4300d87 100644
--- a/src/libslic3r/GCode.cpp
+++ b/src/libslic3r/GCode.cpp
@@ -4026,10 +4026,10 @@ void GCode::_do_export(Print& print, GCodeOutputStream &file, ThumbnailsGenerato
                  (int) initial_extruder_id < (int) m_config.physical_extruder_map.values.size())
                     ? m_config.physical_extruder_map.values[(int) initial_extruder_id]
                     : -1;
-            // enable_pressure_advance and pressure_advance are variant-expanded, so their
-            // length is columns, not filament slots. Bound in slot space -- filament_diameter
-            // is one entry per slot -- exactly as the second-layer temperature loop does, then
-            // translate the slot to its column with get_filament_config_index().
+            // enable_pressure_advance and pressure_advance are indexed by COLUMN, not by
+            // filament slot. filament_diameter is one entry per slot and is never
+            // variant-expanded, so it is the slot-space bound; translate the slot to its column
+            // with get_filament_config_index(), as the second-layer temperature loop below does.
             const int num_pa_filament_slots = (int) print.config().filament_diameter.values.size();
             for (int tool_idx : get_imex_active_tools(print)) {
                 // Unlike the second-layer temperature loop, the primary is skipped here:
@@ -6065,11 +6065,11 @@ LayerResult GCode::process_layer(
             // Mutually exclusive with the `else` below, so a head skipped here gets no
             // transition at all. `tool_idx` is physical; the printing head uses this layer's
             // own filament, the parallel carriages resolve through the head map.
-            // nozzle_temperature is variant-expanded, so its length is columns, not slots:
-            // bound in slot space, or an out-of-slot logical reaches get_filament_config_index
-            // and comes back as filament 0.
-            const int num_filament_columns = std::min((int) print.config().nozzle_temperature.values.size(),
-                                                      (int) print.config().filament_diameter.values.size());
+            // nozzle_temperature is indexed by column: bound in slot space, or an out-of-slot
+            // logical reaches get_filament_config_index and comes back as filament 0.
+            // filament_diameter is the slot-space yardstick -- one entry per filament slot,
+            // never variant-expanded -- and is the bound the pressure advance loop uses too.
+            const int num_filament_slots = (int) print.config().filament_diameter.values.size();
             // Bounds-checked, not get_at() -- see IMEXHelpers.hpp. A clamp would hand the
             // "initial" branch below to whichever secondary sits on pem[0], giving it the wrong
             // filament's transition temperature and never its own.
@@ -6083,7 +6083,7 @@ LayerResult GCode::process_layer(
                     ? (int)first_extruder_id
                     : resolve_filament_for_head(
                           m_imex_head_filament_map, m_config.physical_extruder_map, tool_idx);
-                if (logical < 0 || logical >= num_filament_columns) continue;
+                if (logical < 0 || logical >= num_filament_slots) continue;
                 // Variant-expanded printers column each filament; index as the `else` does.
                 int temperature = print.config().nozzle_temperature.get_at(get_filament_config_index(logical));
                 if (temperature > 0)
diff --git a/tests/fff_print/test_imex_mode_gcode.cpp b/tests/fff_print/test_imex_mode_gcode.cpp
index d09feb93c4..04643864e7 100644
--- a/tests/fff_print/test_imex_mode_gcode.cpp
+++ b/tests/fff_print/test_imex_mode_gcode.cpp
@@ -50,6 +50,25 @@ static bool has_line(const std::string &gcode, const std::string &expected)
     return find_exact_line(gcode, expected) != std::string::npos;
 }
 
+// Any command line -- not a comment -- containing `token`. Used where the assertion is that a
+// carriage was addressed at all, rather than with a particular value: the config block the
+// exporter appends restates every setting as "; key = value", so the comment lines have to be
+// excluded or a search for a setting's own name matches itself.
+static bool has_command_containing(const std::string &gcode, const std::string &token)
+{
+    std::size_t pos = 0;
+    while (pos < gcode.size()) {
+        const std::size_t eol = gcode.find('\n', pos);
+        const std::string line = gcode.substr(pos, (eol == std::string::npos ? gcode.size() : eol) - pos);
+        if (!line.empty() && line.front() != ';' && line.find(token) != std::string::npos)
+            return true;
+        if (eol == std::string::npos)
+            break;
+        pos = eol + 1;
+    }
+    return false;
+}
+
 // Mode scripts. Free of placeholders so that a test can tell "the script was emitted" apart
 // from "the script was expanded"; the expansion case below supplies its own template.
 static const char *kCopyScript = "SET_DUAL_CARRIAGE MODE=COPY";
@@ -79,6 +98,12 @@ static void imex_7x4_printer(DynamicPrintConfig &config)
                                        "Direct Drive Standard" },
         { "extruder_printable_height", "0,0,0,0,0,0,0" },
         { "physical_extruder_map",     "0,0,0,0,1,2,3" },
+        // Authored 1..n, as every production path does: PresetBundle writes it in
+        // full_fff_config(), and config load synthesizes it when absent. The all-1s default is
+        // not a neutral placeholder -- get_config_index_base() keys the slot-to-column lookup
+        // on it, and with every entry equal each per-filament vector collapses to filament 1's
+        // value, leaving a per-slot assertion comparing a value against itself.
+        { "filament_self_index",       "1,2,3,4,5,6,7" },
         { "is_imex",                   "1" },
         { "imex_mode_names",           "primary;copy;iq-copy" },
         { "imex_mode_active_tools",    "0:P;0:P,1:C;0:P,1:C,2:C,3:C" },
@@ -315,3 +340,47 @@ TEST_CASE("IMEX mode placeholders are defined on an ordinary single-extruder pri
     CHECK(has_line(gcode, ";IMEX_MODE_INDEX:0"));
     CHECK(has_line(gcode, ";IMEX_MODE_GCODE:"));
 }
+
+// In a parallel mode no tool changes occur, so every active carriage has to be addressed
+// explicitly: once before the print for pressure advance, and again at the second layer for the
+// drop from the initial-layer temperature. Nothing else in the suite covers either emission, and
+// a carriage that falls out of one of those loops emits nothing at all rather than emitting
+// something wrong -- it silently holds the initial-layer temperature, or whatever pressure
+// advance the firmware was last told, for the whole job. So the property worth pinning is that
+// the set of carriages addressed is exactly the set the mode declares active.
+TEST_CASE("Every carriage a parallel mode declares active is addressed, and no other",
+          "[ImexModeGcode][IMEX]")
+{
+    DynamicPrintConfig config = multifilament_config(7);
+    imex_7x4_printer(config);
+    all_regions_on_filament(config, 1);
+    set_mode_gcodes(config, { "", kCopyScript, kQuadScript });
+    config.set_deserialize_strict({
+        { "imex_parallel_mode",               "copy" },
+        { "enable_pressure_advance",          "1,1,1,1,1,1,1" },
+        // Distinct per slot, so each assertion below names one filament and no other. copy
+        // mode's secondary rides physical 1, which physical_extruder_map routes to the fifth
+        // slot -- 0.05 and 244, not the primary's 0.01 and 240.
+        { "pressure_advance",                 "0.010,0.020,0.030,0.040,0.050,0.060,0.070" },
+        { "nozzle_temperature_initial_layer", "235,235,235,235,235,235,235" },
+        { "nozzle_temperature",               "240,241,242,243,244,245,246" },
+    });
+
+    const std::string gcode = slice({ cube(20) }, config);
+
+    // imex_mode_active_tools declares copy as "0:P,1:C", so exactly these two carriages run, and
+    // each transitions to the temperature of the filament its own head is routed to. The other
+    // two heads this printer has must stay untouched.
+    CHECK(has_command_containing(gcode, "M104 S240 T0 ; set IMEX tool temperature"));
+    CHECK(has_command_containing(gcode, "M104 S244 T1 ; set IMEX tool temperature"));
+    CHECK_FALSE(has_command_containing(gcode, "T2 ; set IMEX tool temperature"));
+    CHECK_FALSE(has_command_containing(gcode, "T3 ; set IMEX tool temperature"));
+
+    // The companion loop, which skips the primary because set_extruder() has already emitted it
+    // on the ordinary path. The qualified line is therefore the one that belongs to this loop:
+    // Klipper names the first carriage as the unnumbered "extruder" and the rest by index, so
+    // dropping the secondary removes `extruder1` and leaves `extruder` in place.
+    CHECK(has_command_containing(gcode, "SET_PRESSURE_ADVANCE ADVANCE=0.05 EXTRUDER=extruder1;"));
+    CHECK(has_command_containing(gcode, "SET_PRESSURE_ADVANCE ADVANCE=0.01 EXTRUDER=extruder;"));
+    CHECK_FALSE(has_command_containing(gcode, "EXTRUDER=extruder2;"));
+}