diff --git a/test-linuxcnc-source-link.sh b/test-linuxcnc-source-link.sh
index f7709a7..9b52da6 100755
--- a/test-linuxcnc-source-link.sh
+++ b/test-linuxcnc-source-link.sh
@@ -641,6 +641,30 @@ for case in "${enhance_block_error_cases[@]}"; do
     fi
     grep -Fq "$expected" "$output_dir/cnc_sim_linuxcnc_source_$fixture.err"
 done
+# Source basis: LinuxCNC src/emc/rs274ngc/interp_check.cc
+# check_g_codes() rejects invalid modal-0 combinations for G4, G10, and G53
+# before convert_g() executes motion or coordinate updates.
+check_g_code_error_cases=(
+    "linuxcnc_g4_missing_p_error|Dwell time missing with g4"
+    "linuxcnc_g4_arc_p_conflict_error|G4 not allowed with G2 or G3 because they both use P"
+    "linuxcnc_g10_invalid_l_error|Line with G10 does not have L0, L1, L10, L11, L2, or L20"
+    "linuxcnc_g10_p_non_integer_error|P value not an integer with G10"
+    "linuxcnc_g10_l2_p_out_of_range_error|P value out of range (0-9) with G10 L2"
+    "linuxcnc_g10_l1_p_out_of_range_error|P value out of range with G10 L1"
+    "linuxcnc_g53_arc_error|Must use g0 or g1 with g53"
+    "linuxcnc_g53_incremental_error|Cannot use g53 incremental"
+)
+for case in "${check_g_code_error_cases[@]}"; do
+    fixture=${case%%|*}
+    expected=${case#*|}
+    if CNC_SIM_RS274_VAR="$var_file" \
+        "$build_dir/linuxcnc_rs274_source_dump" "tests/gcode/$fixture.ngc" \
+        >"$output_dir/cnc_sim_linuxcnc_source_$fixture.json" 2>"$output_dir/cnc_sim_linuxcnc_source_$fixture.err"; then
+        echo "expected $fixture.ngc to fail" >&2
+        exit 1
+    fi
+    grep -Fq "$expected" "$output_dir/cnc_sim_linuxcnc_source_$fixture.err"
+done
 CNC_SIM_RS274_VAR="$var_file" \
     "$build_dir/linuxcnc_rs274_source_dump" tests/gcode/linuxcnc_predefined_positions.ngc \
     >"$output_dir/cnc_sim_linuxcnc_source_predefined_positions.json"
diff --git a/test-native.sh b/test-native.sh
index f63bb8f..9b89e49 100755
--- a/test-native.sh
+++ b/test-native.sh
@@ -153,6 +153,8 @@ grep -F 'tests/gcode/linuxcnc_polar_coordinates.ngc' test-linuxcnc-source-link.s
 grep -F 'tests/gcode/linuxcnc_polar_coordinate_variants.ngc' test-linuxcnc-source-link.sh >/dev/null
 grep -F 'enhance_block() assigns motion_to_be' test-linuxcnc-source-link.sh >/dev/null
 grep -F 'linuxcnc_g80_axis_error|Cannot use axis values with g80' test-linuxcnc-source-link.sh >/dev/null
+grep -F 'check_g_codes() rejects invalid modal-0 combinations' test-linuxcnc-source-link.sh >/dev/null
+grep -F 'linuxcnc_g53_incremental_error|Cannot use g53 incremental' test-linuxcnc-source-link.sh >/dev/null
 grep -F 'lookup_named_param()' test-linuxcnc-rs274-native.sh >/dev/null
 grep -F 'tests/gcode/linuxcnc_readonly_named_parameters.ngc' docs/linuxcnc-porting.md >/dev/null
 grep -F 'rs274ngc_pre.cc ini_load() consumes INI_FILE_NAME' test-web-wasm-node-smoke.cjs >/dev/null
diff --git a/tests/gcode/linuxcnc_g10_invalid_l_error.ngc b/tests/gcode/linuxcnc_g10_invalid_l_error.ngc
new file mode 100644
index 0000000..de4e2a4
--- /dev/null
+++ b/tests/gcode/linuxcnc_g10_invalid_l_error.ngc
@@ -0,0 +1,3 @@
+G21 G90 G17
+G10 L3 P1 X1
+M30
diff --git a/tests/gcode/linuxcnc_g10_l1_p_out_of_range_error.ngc b/tests/gcode/linuxcnc_g10_l1_p_out_of_range_error.ngc
new file mode 100644
index 0000000..2762284
--- /dev/null
+++ b/tests/gcode/linuxcnc_g10_l1_p_out_of_range_error.ngc
@@ -0,0 +1,3 @@
+G21 G90 G17
+G10 L1 P0 X1
+M30
diff --git a/tests/gcode/linuxcnc_g10_l2_p_out_of_range_error.ngc b/tests/gcode/linuxcnc_g10_l2_p_out_of_range_error.ngc
new file mode 100644
index 0000000..8610be3
--- /dev/null
+++ b/tests/gcode/linuxcnc_g10_l2_p_out_of_range_error.ngc
@@ -0,0 +1,3 @@
+G21 G90 G17
+G10 L2 P10 X1
+M30
diff --git a/tests/gcode/linuxcnc_g10_p_non_integer_error.ngc b/tests/gcode/linuxcnc_g10_p_non_integer_error.ngc
new file mode 100644
index 0000000..1250ac2
--- /dev/null
+++ b/tests/gcode/linuxcnc_g10_p_non_integer_error.ngc
@@ -0,0 +1,3 @@
+G21 G90 G17
+G10 L2 P1.2 X1
+M30
diff --git a/tests/gcode/linuxcnc_g4_arc_p_conflict_error.ngc b/tests/gcode/linuxcnc_g4_arc_p_conflict_error.ngc
new file mode 100644
index 0000000..ad5b3d9
--- /dev/null
+++ b/tests/gcode/linuxcnc_g4_arc_p_conflict_error.ngc
@@ -0,0 +1,3 @@
+G21 G90 G17
+G2 G4 X1 Y0 I0 J1 P1
+M30
diff --git a/tests/gcode/linuxcnc_g4_missing_p_error.ngc b/tests/gcode/linuxcnc_g4_missing_p_error.ngc
new file mode 100644
index 0000000..c0332e6
--- /dev/null
+++ b/tests/gcode/linuxcnc_g4_missing_p_error.ngc
@@ -0,0 +1,3 @@
+G21 G90 G17
+G4
+M30
diff --git a/tests/gcode/linuxcnc_g53_arc_error.ngc b/tests/gcode/linuxcnc_g53_arc_error.ngc
new file mode 100644
index 0000000..e1edfd1
--- /dev/null
+++ b/tests/gcode/linuxcnc_g53_arc_error.ngc
@@ -0,0 +1,3 @@
+G21 G90 G17
+G53 G2 X1 Y0 I0 J1
+M30
diff --git a/tests/gcode/linuxcnc_g53_incremental_error.ngc b/tests/gcode/linuxcnc_g53_incremental_error.ngc
new file mode 100644
index 0000000..b4f0651
--- /dev/null
+++ b/tests/gcode/linuxcnc_g53_incremental_error.ngc
@@ -0,0 +1,3 @@
+G21 G91 G17
+G53 G0 X1
+M30