diff --git a/test-linuxcnc-rs274-native.sh b/test-linuxcnc-rs274-native.sh
index ce1bfc2..a83a890 100755
--- a/test-linuxcnc-rs274-native.sh
+++ b/test-linuxcnc-rs274-native.sh
@@ -545,6 +545,10 @@ for case in "${tool_number_error_cases[@]}"; do
     grep -Fq "$expected" "$output_dir/cnc_sim_linuxcnc_$fixture.err"
 done
 CNC_SIM_RS274_VAR="$var_file" "$build_dir/linuxcnc_rs274_dump" tests/gcode/linuxcnc_program_stops.ngc >"$output_dir/cnc_sim_linuxcnc_program_stops.json"
+# Source basis: LinuxCNC src/emc/rs274ngc/interp_check.cc treats exactly
+# P-1.0 as a missing G4 dwell P word, while interp_convert.cc convert_dwell()
+# and interp_queue.cc enqueue_DWELL() pass neighboring values through.
+CNC_SIM_RS274_VAR="$var_file" "$build_dir/linuxcnc_rs274_dump" tests/gcode/linuxcnc_g4_dwell_boundaries.ngc >"$output_dir/cnc_sim_linuxcnc_g4_dwell_boundaries.json"
 CNC_SIM_RS274_VAR="$var_file" "$build_dir/linuxcnc_rs274_dump" tests/gcode/linuxcnc_coolant.ngc >"$output_dir/cnc_sim_linuxcnc_coolant.json"
 CNC_SIM_RS274_VAR="$var_file" "$build_dir/linuxcnc_rs274_dump" tests/gcode/linuxcnc_feed_modes.ngc >"$output_dir/cnc_sim_linuxcnc_feed_modes.json"
 CNC_SIM_RS274_VAR="$var_file" "$build_dir/linuxcnc_rs274_dump" tests/gcode/linuxcnc_motion_modes.ngc >"$output_dir/cnc_sim_linuxcnc_motion_modes.json"
@@ -1366,6 +1370,16 @@ for expected in [(2, 1), (3, 2), (4, 3), (4, 1), (5, 0)]:
     if expected not in stop_states:
         raise SystemExit(f"missing program stop state {expected!r}")
 
+g4_boundaries = load("cnc_sim_linuxcnc_g4_dwell_boundaries.json")
+g4_dwell_states = [
+    (event["line"], event["dwellSeconds"])
+    for event in g4_boundaries
+    if event["type"] == "dwell"
+]
+expected_g4_dwell_states = [(4, -1.001), (5, -0.9999), (6, 3)]
+if g4_dwell_states != expected_g4_dwell_states:
+    raise SystemExit(f"unexpected G4 dwell boundary states: {g4_dwell_states!r}")
+
 coolant = load("cnc_sim_linuxcnc_coolant.json")
 coolant_states = [
     (event["line"], event["reserved"])
diff --git a/test-linuxcnc-source-link.sh b/test-linuxcnc-source-link.sh
index a19d403..0066b4e 100755
--- a/test-linuxcnc-source-link.sh
+++ b/test-linuxcnc-source-link.sh
@@ -298,6 +298,12 @@ done
 CNC_SIM_RS274_VAR="$var_file" \
     "$build_dir/linuxcnc_rs274_source_dump" tests/gcode/linuxcnc_program_stops.ngc \
     >"$output_dir/cnc_sim_linuxcnc_source_program_stops.json"
+# Source basis: LinuxCNC src/emc/rs274ngc/interp_check.cc treats exactly
+# P-1.0 as a missing G4 dwell P word, while interp_convert.cc convert_dwell()
+# and interp_queue.cc enqueue_DWELL() pass neighboring values through.
+CNC_SIM_RS274_VAR="$var_file" \
+    "$build_dir/linuxcnc_rs274_source_dump" tests/gcode/linuxcnc_g4_dwell_boundaries.ngc \
+    >"$output_dir/cnc_sim_linuxcnc_source_g4_dwell_boundaries.json"
 CNC_SIM_RS274_VAR="$var_file" \
     "$build_dir/linuxcnc_rs274_source_dump" tests/gcode/linuxcnc_coolant.ngc \
     >"$output_dir/cnc_sim_linuxcnc_source_coolant.json"
@@ -1916,6 +1922,16 @@ for expected in [(2, 1), (3, 2), (4, 3), (4, 1), (5, 0)]:
     if expected not in stop_states:
         raise SystemExit(f"missing source-linked program stop state {expected!r}")
 
+g4_boundaries = load("cnc_sim_linuxcnc_source_g4_dwell_boundaries.json")
+g4_dwell_states = [
+    (event["line"], event["dwellSeconds"])
+    for event in g4_boundaries
+    if event["type"] == "dwell"
+]
+expected_g4_dwell_states = [(4, -1.001), (5, -0.9999), (6, 3)]
+if g4_dwell_states != expected_g4_dwell_states:
+    raise SystemExit(f"unexpected source-linked G4 dwell boundary states: {g4_dwell_states!r}")
+
 coolant = json.loads((OUTPUT_DIR / "cnc_sim_linuxcnc_source_coolant.json").read_text())
 coolant_states = [
     (event["line"], event["reserved"])
diff --git a/tests/gcode/linuxcnc_g4_dwell_boundaries.ngc b/tests/gcode/linuxcnc_g4_dwell_boundaries.ngc
new file mode 100644
index 0000000..71f70c4
--- /dev/null
+++ b/tests/gcode/linuxcnc_g4_dwell_boundaries.ngc
@@ -0,0 +1,6 @@
+G21 G90
+(Source basis: LinuxCNC src/emc/rs274ngc/interp_check.cc check_g_codes only treats P-1.0 as missing for G4.)
+(Source basis: src/emc/rs274ngc/interp_convert.cc convert_dwell passes other P values to DWELL unchanged.)
+G4 P-1.001
+G4 P-0.9999
+G4 P-2 P3