diff --git a/definitions/grib1/scanning_mode.def b/definitions/grib1/scanning_mode.def
index 31a56e27d..19a7f8167 100644
--- a/definitions/grib1/scanning_mode.def
+++ b/definitions/grib1/scanning_mode.def
@@ -9,8 +9,8 @@ flagbit jScansPositively(scanningMode,6)  : dump;
 flagbit jPointsAreConsecutive(scanningMode,5)  : dump;
 constant alternativeRowScanning=0 : dump;
 
-transient iScansPositively = !iScansNegatively : constraint;
-transient jScansNegatively = !jScansPositively : constraint;
+transient iScansPositively = !iScansNegatively : constraint, read_only;
+transient jScansNegatively = !jScansPositively : constraint, read_only;
 
 alias geography.iScansNegatively=iScansNegatively;
 alias geography.jScansPositively=jScansPositively;
diff --git a/definitions/grib2/template.3.healpix.def b/definitions/grib2/template.3.healpix.def
index bf0429a5b..136904194 100644
--- a/definitions/grib2/template.3.healpix.def
+++ b/definitions/grib2/template.3.healpix.def
@@ -32,8 +32,8 @@ alias geography.orderingConvention = orderingConvention;
 flags[1] scanningMode 'grib2/tables/[tablesVersion]/3.13.table';
 flagbit iScansNegatively(scanningMode,7) : dump; # WMO bit 1
 flagbit jScansPositively(scanningMode,6) : dump; # WMO bit 2
-transient iScansPositively = !iScansNegatively : constraint;
-transient jScansNegatively = !jScansPositively : constraint;
+transient iScansPositively = !iScansNegatively : constraint, read_only;
+transient jScansNegatively = !jScansPositively : constraint, read_only;
 
 iterator healpix(numberOfPoints,missingValue,values,Nside,orderingConvention);
 nearest healpix(values,radius,Nx,Ny);
diff --git a/definitions/grib2/template.3.scanning_mode.def b/definitions/grib2/template.3.scanning_mode.def
index 2eb2ca71e..1be608f41 100644
--- a/definitions/grib2/template.3.scanning_mode.def
+++ b/definitions/grib2/template.3.scanning_mode.def
@@ -20,8 +20,8 @@ alias geography.iScansNegatively=iScansNegatively;
 alias geography.jScansPositively=jScansPositively;
 alias geography.jPointsAreConsecutive=jPointsAreConsecutive;
 
-transient iScansPositively = !iScansNegatively : constraint;
-transient jScansNegatively = !jScansPositively : constraint;
+transient iScansPositively = !iScansNegatively : constraint, read_only;
+transient jScansNegatively = !jScansPositively : constraint, read_only;
 
 flagbit scanningMode5(scanningMode,3) = 0: read_only;
 flagbit scanningMode6(scanningMode,2) = 0: read_only;
diff --git a/definitions/grib3/template.4.scanning_mode.def b/definitions/grib3/template.4.scanning_mode.def
index 4fcd2fabb..31414321f 100644
--- a/definitions/grib3/template.4.scanning_mode.def
+++ b/definitions/grib3/template.4.scanning_mode.def
@@ -20,7 +20,7 @@ alias geography.iScansNegatively=iScansNegatively;
 alias geography.jScansPositively=jScansPositively;
 alias geography.jPointsAreConsecutive=jPointsAreConsecutive;
 
-transient iScansPositively = !iScansNegatively : constraint;
+transient iScansPositively = !iScansNegatively : constraint, read_only;
 
 flagbit scanningMode5(scanningMode,3) = 0: read_only;
 flagbit scanningMode6(scanningMode,2) = 0: read_only;
diff --git a/src/grib_accessor.cc b/src/grib_accessor.cc
index a08f5348e..a684b9e1d 100644
--- a/src/grib_accessor.cc
+++ b/src/grib_accessor.cc
@@ -112,7 +112,9 @@ int grib_pack_float(grib_accessor* a, const float* v, size_t* len)
 int grib_pack_expression(grib_accessor* a, grib_expression* e)
 {
     grib_accessor_class* c = a->cclass;
-    //grib_context_log(a->context, GRIB_LOG_DEBUG, "(%s)%s is packing (double) %g",(a->parent->owner)?(a->parent->owner->name):"root", a->name ,v?(*v):0);
+    // grib_context_log(a->context, GRIB_LOG_INFO, "....(%s)%s is packing",
+    //                 (a->parent->owner)?(a->parent->owner->name):"root",
+    //                 a->name);
     while (c) {
         if (c->pack_expression) {
             return c->pack_expression(a, e);
diff --git a/tests/grib_change_scanning.sh b/tests/grib_change_scanning.sh
index 25c5e005b..5f1d46596 100755
--- a/tests/grib_change_scanning.sh
+++ b/tests/grib_change_scanning.sh
@@ -11,6 +11,12 @@
 . ./include.ctest.sh
 
 label="grib_change_scanning_test"
+tempFilt=temp.$label.filt
+tempGribA=temp.$label.A.grib
+tempGribB=temp.$label.B.grib
+tempText=temp.$label.txt
+tempRef=temp.$label.ref
+
 
 editions="1 2"
 gridTypes="regular_ll rotated_ll" 
@@ -77,12 +83,6 @@ done
 
 # alternativeRowScanning
 # -----------------------
-tempFilt=temp.$label.filt
-tempGribA=temp.$label.A.grib
-tempGribB=temp.$label.B.grib
-tempText=temp.$label.txt
-tempRef=temp.$label.ref
-
 cat > $tempFilt <<EOF
   set Nj = 2;
   set latitudeOfFirstGridPointInDegrees = 60;
@@ -141,6 +141,34 @@ ${tools_dir}/grib_set -s swapScanningX:s=1 $ECCODES_SAMPLES_PATH/GRIB2.tmpl $tem
 ${tools_dir}/grib_set -s swapScanningX:i=1 $ECCODES_SAMPLES_PATH/GRIB2.tmpl $tempGribB 2>/dev/null
 cmp $tempGribA $tempGribB
 
+
+# Check constraint keys follow (observe) their expressions
+# --------------------------------------------------------
+cat > $tempFilt <<EOF
+  set iScansNegatively = 1;       # Change from 0 to 1
+  assert (0 == iScansPositively); # should change too
+  set jScansPositively = 1;       # Same for j
+  assert (0 == jScansNegatively);
+EOF
+grib_check_key_equals $file 'iScansNegatively,jScansPositively' '0 0'
+grib_check_key_equals $file 'iScansPositively,jScansNegatively' '1 1'
+${tools_dir}/grib_filter $tempFilt $ECCODES_SAMPLES_PATH/GRIB1.tmpl
+${tools_dir}/grib_filter $tempFilt $ECCODES_SAMPLES_PATH/GRIB2.tmpl
+
+
+# ECC-1819: Keys iScansPositively and jScansNegatively should be read-only
+for s in GRIB1.tmpl GRIB2.tmpl; do
+  for k in iScansPositively jScansNegatively; do
+    set +e
+    ${tools_dir}/grib_set -s $k=0 $ECCODES_SAMPLES_PATH/$s $tempGribA 2>$tempText
+    status=$?
+    set -e
+    [ $status -ne 0 ]
+    grep -q "Value is read only" $tempText
+    rm $tempText
+  done
+done
+
 # Clean up
 rm -f $tempFilt $tempGribA $tempGribB $tempRef $tempText
 rm -f ${data_dir}/scan1.grib ${data_dir}/scan.grib