diff --git a/CHANGELOG.md b/CHANGELOG.md
index 066d81d63..51b38d399 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,7 @@
 
 ## Bug fixes
 
+-   Fixed a bug where boundary conditions were sometimes handled incorrectly in 1+1D models ([#713](https://github.com/pybamm-team/PyBaMM/pull/713))
 -   Corrected a sign error in Dirichlet boundary conditions in the Finite Element Method ([#706](https://github.com/pybamm-team/PyBaMM/pull/706))
 -   Passed the correct dimensional temperature to open circuit potential ([#702](https://github.com/pybamm-team/PyBaMM/pull/702))
 -   Added missing temperature dependence in electrolyte and interface submodels ([#698](https://github.com/pybamm-team/PyBaMM/pull/698))
diff --git a/pybamm/discretisations/discretisation.py b/pybamm/discretisations/discretisation.py
index 0c1ab0c6a..1e1653a8a 100644
--- a/pybamm/discretisations/discretisation.py
+++ b/pybamm/discretisations/discretisation.py
@@ -381,19 +381,26 @@ class Discretisation(object):
                     symbol, domain
                 )
             )
-
         # Replace keys with "left" and "right" as appropriate for 1D meshes
         if isinstance(mesh, pybamm.SubMesh1D):
-            # replace negative and/or positive tab
+            # send boundary conditions applied on the tabs to "left" or "right"
+            # depending on the tab location stored in the mesh
             for tab in ["negative tab", "positive tab"]:
                 if any(tab in side for side in list(bcs.keys())):
                     bcs[mesh.tabs[tab]] = bcs.pop(tab)
-            # replace no tab
-            if any("no tab" in side for side in list(bcs.keys())):
-                if "left" in list(bcs.keys()):
-                    bcs["right"] = bcs.pop("no tab")  # tab at bottom
-                else:
-                    bcs["left"] = bcs.pop("no tab")  # tab at top
+            # if there was a tab at either end, then the boundary conditions
+            # have now been set on "left" and "right" as required by the spatial
+            # method, so there is no need to further modify the bcs dict
+            if all(side in list(bcs.keys()) for side in ["left", "right"]):
+                pass
+            # if both tabs are located at z=0 then the "right" boundary condition
+            # (at z=1) is the condition for "no tab"
+            elif "left" in list(bcs.keys()):
+                bcs["right"] = bcs.pop("no tab")
+            # else if both tabs are located at z=1, the "left" boundary condition
+            # (at z=0) is the condition for "no tab"
+            else:
+                bcs["left"] = bcs.pop("no tab")
 
         return bcs
 

thermal boundary condition in 1plus1D 
when setting boundary conditions by tab in the 1plus1D models if there is a tab at either end, then the right boundary condition gets overridden by the "no tab" condition

diff --git a/tests/unit/test_spatial_methods/test_finite_volume.py b/tests/unit/test_spatial_methods/test_finite_volume.py
index cdee8c22e..99e4cec57 100644
--- a/tests/unit/test_spatial_methods/test_finite_volume.py
+++ b/tests/unit/test_spatial_methods/test_finite_volume.py
@@ -1459,6 +1459,41 @@ class TestFiniteVolume(unittest.TestCase):
         div_eqn_disc = disc.process_symbol(div_eqn)
         div_eqn_disc.evaluate(None, y_test)
 
+    def test_neg_pos_bcs(self):
+        # 2d macroscale
+        mesh = get_1p1d_mesh_for_testing()
+        spatial_methods = {
+            "macroscale": pybamm.FiniteVolume,
+            "negative particle": pybamm.FiniteVolume,
+            "positive particle": pybamm.FiniteVolume,
+            "current collector": pybamm.FiniteVolume,
+        }
+        disc = pybamm.Discretisation(mesh, spatial_methods)
+
+        # var
+        var = pybamm.Variable("var", domain="current collector")
+        disc.set_variable_slices([var])
+        # grad
+        grad_eqn = pybamm.grad(var)
+
+        # bcs (on each tab)
+        boundary_conditions = {
+            var.id: {
+                "negative tab": (pybamm.Scalar(1), "Dirichlet"),
+                "positive tab": (pybamm.Scalar(0), "Neumann"),
+                "no tab": (pybamm.Scalar(8), "Dirichlet"),
+            }
+        }
+        disc.bcs = boundary_conditions
+
+        # check after disc that negative tab goes to left and positive tab goes
+        # to right
+        disc.process_symbol(grad_eqn)
+        self.assertEqual(disc.bcs[var.id]["left"][0].id, pybamm.Scalar(1).id)
+        self.assertEqual(disc.bcs[var.id]["left"][1], "Dirichlet")
+        self.assertEqual(disc.bcs[var.id]["right"][0].id, pybamm.Scalar(0).id)
+        self.assertEqual(disc.bcs[var.id]["right"][1], "Neumann")
+
 
 if __name__ == "__main__":
     print("Add -v for more debug output")

pip install -e .[dev,docs]
parse_log_pytest
pytest
flake8 black pytest
apt-get update apt-get install -y gcc gfortran libopenblas-dev
3.9
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
alabaster==0.7.16
anyio==4.9.0
anytree==2.12.1
argon2-cffi==23.1.0
argon2-cffi-bindings==21.2.0
arrow==1.3.0
asttokens==3.0.0
async-lru==2.0.5
attrs==25.3.0
autograd==1.7.0
babel==2.17.0
beautifulsoup4==4.13.3
black==25.1.0
bleach==6.2.0
casadi==3.7.0
certifi==2025.1.31
cffi==1.17.1
charset-normalizer==3.4.1
click==8.1.8
comm==0.2.2
contourpy==1.3.0
cycler==0.12.1
debugpy==1.8.13
decorator==5.2.1
defusedxml==0.7.1
docutils==0.21.2
exceptiongroup @ file:///croot/exceptiongroup_1706031385326/work
executing==2.2.0
fastjsonschema==2.21.1
flake8==7.2.0
fonttools==4.56.0
fqdn==1.5.1
guzzle_sphinx_theme==0.7.11
h11==0.14.0
httpcore==1.0.7
httpx==0.28.1
idna==3.10
imagesize==1.4.1
importlib_metadata==8.6.1
importlib_resources==6.5.2
iniconfig @ file:///home/linux1/recipes/ci/iniconfig_1610983019677/work
ipykernel==6.29.5
ipython==8.18.1
ipywidgets==8.1.5
isoduration==20.11.0
jedi==0.19.2
Jinja2==3.1.6
json5==0.10.0
jsonpointer==3.0.0
jsonschema==4.23.0
jsonschema-specifications==2024.10.1
jupyter==1.1.1
jupyter-console==6.6.3
jupyter-events==0.12.0
jupyter-lsp==2.2.5
jupyter_client==8.6.3
jupyter_core==5.7.2
jupyter_server==2.15.0
jupyter_server_terminals==0.5.3
jupyterlab==4.3.6
jupyterlab_pygments==0.3.0
jupyterlab_server==2.27.3
jupyterlab_widgets==3.0.13
kiwisolver==1.4.7
MarkupSafe==3.0.2
matplotlib==3.9.4
matplotlib-inline==0.1.7
mccabe==0.7.0
mistune==3.1.3
mypy-extensions==1.0.0
nbclient==0.10.2
nbconvert==7.16.6
nbformat==5.10.4
nest-asyncio==1.6.0
notebook==7.3.3
notebook_shim==0.2.4
numpy==2.0.2
overrides==7.7.0
packaging @ file:///croot/packaging_1734472117206/work
pandas==2.2.3
pandocfilters==1.5.1
parso==0.8.4
pathspec==0.12.1
pexpect==4.9.0
pillow==11.1.0
platformdirs==4.3.7
pluggy @ file:///croot/pluggy_1733169602837/work
prometheus_client==0.21.1
prompt_toolkit==3.0.50
psutil==7.0.0
ptyprocess==0.7.0
pure_eval==0.2.3
-e git+https://github.com/pybamm-team/PyBaMM.git@7d31c680fd19ebf8d7a4fe3e01508fafec58ecc0#egg=pybamm
pycodestyle==2.13.0
pycparser==2.22
pyflakes==3.3.1
Pygments==2.19.1
pyparsing==3.2.3
pytest @ file:///croot/pytest_1738938843180/work
python-dateutil==2.9.0.post0
python-json-logger==3.3.0
pytz==2025.2
PyYAML==6.0.2
pyzmq==26.3.0
referencing==0.36.2
requests==2.32.3
rfc3339-validator==0.1.4
rfc3986-validator==0.1.1
rpds-py==0.24.0
scikit-fem==10.0.2
scipy==1.13.1
Send2Trash==1.8.3
six==1.17.0
sniffio==1.3.1
snowballstemmer==2.2.0
soupsieve==2.6
Sphinx==7.4.7
sphinxcontrib-applehelp==2.0.0
sphinxcontrib-devhelp==2.0.0
sphinxcontrib-htmlhelp==2.1.0
sphinxcontrib-jsmath==1.0.1
sphinxcontrib-qthelp==2.0.0
sphinxcontrib-serializinghtml==2.0.0
stack-data==0.6.3
terminado==0.18.1
tinycss2==1.4.0
tomli @ file:///opt/conda/conda-bld/tomli_1657175507142/work
tornado==6.4.2
traitlets==5.14.3
types-python-dateutil==2.9.0.20241206
typing_extensions==4.13.0
tzdata==2025.2
uri-template==1.3.0
urllib3==2.3.0
wcwidth==0.2.13
webcolors==24.11.1
webencodings==0.5.1
websocket-client==1.8.0
widgetsnbextension==4.0.13
zipp==3.21.0

name: PyBaMM
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - ca-certificates=2025.2.25=h06a4308_0
  - exceptiongroup=1.2.0=py39h06a4308_0
  - iniconfig=1.1.1=pyhd3eb1b0_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.4.4=h6a678d5_1
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - ncurses=6.4=h6a678d5_0
  - openssl=3.0.16=h5eee18b_0
  - packaging=24.2=py39h06a4308_0
  - pip=25.0=py39h06a4308_0
  - pluggy=1.5.0=py39h06a4308_0
  - pytest=8.3.4=py39h06a4308_0
  - python=3.9.21=he870216_1
  - readline=8.2=h5eee18b_0
  - setuptools=75.8.0=py39h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - tomli=2.0.1=py39h06a4308_0
  - wheel=0.45.1=py39h06a4308_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - alabaster==0.7.16
      - anyio==4.9.0
      - anytree==2.12.1
      - argon2-cffi==23.1.0
      - argon2-cffi-bindings==21.2.0
      - arrow==1.3.0
      - asttokens==3.0.0
      - async-lru==2.0.5
      - attrs==25.3.0
      - autograd==1.7.0
      - babel==2.17.0
      - beautifulsoup4==4.13.3
      - black==25.1.0
      - bleach==6.2.0
      - casadi==3.7.0
      - certifi==2025.1.31
      - cffi==1.17.1
      - charset-normalizer==3.4.1
      - click==8.1.8
      - comm==0.2.2
      - contourpy==1.3.0
      - cycler==0.12.1
      - debugpy==1.8.13
      - decorator==5.2.1
      - defusedxml==0.7.1
      - docutils==0.21.2
      - executing==2.2.0
      - fastjsonschema==2.21.1
      - flake8==7.2.0
      - fonttools==4.56.0
      - fqdn==1.5.1
      - guzzle-sphinx-theme==0.7.11
      - h11==0.14.0
      - httpcore==1.0.7
      - httpx==0.28.1
      - idna==3.10
      - imagesize==1.4.1
      - importlib-metadata==8.6.1
      - importlib-resources==6.5.2
      - ipykernel==6.29.5
      - ipython==8.18.1
      - ipywidgets==8.1.5
      - isoduration==20.11.0
      - jedi==0.19.2
      - jinja2==3.1.6
      - json5==0.10.0
      - jsonpointer==3.0.0
      - jsonschema==4.23.0
      - jsonschema-specifications==2024.10.1
      - jupyter==1.1.1
      - jupyter-client==8.6.3
      - jupyter-console==6.6.3
      - jupyter-core==5.7.2
      - jupyter-events==0.12.0
      - jupyter-lsp==2.2.5
      - jupyter-server==2.15.0
      - jupyter-server-terminals==0.5.3
      - jupyterlab==4.3.6
      - jupyterlab-pygments==0.3.0
      - jupyterlab-server==2.27.3
      - jupyterlab-widgets==3.0.13
      - kiwisolver==1.4.7
      - markupsafe==3.0.2
      - matplotlib==3.9.4
      - matplotlib-inline==0.1.7
      - mccabe==0.7.0
      - mistune==3.1.3
      - mypy-extensions==1.0.0
      - nbclient==0.10.2
      - nbconvert==7.16.6
      - nbformat==5.10.4
      - nest-asyncio==1.6.0
      - notebook==7.3.3
      - notebook-shim==0.2.4
      - numpy==2.0.2
      - overrides==7.7.0
      - pandas==2.2.3
      - pandocfilters==1.5.1
      - parso==0.8.4
      - pathspec==0.12.1
      - pexpect==4.9.0
      - pillow==11.1.0
      - platformdirs==4.3.7
      - prometheus-client==0.21.1
      - prompt-toolkit==3.0.50
      - psutil==7.0.0
      - ptyprocess==0.7.0
      - pure-eval==0.2.3
      - pycodestyle==2.13.0
      - pycparser==2.22
      - pyflakes==3.3.1
      - pygments==2.19.1
      - pyparsing==3.2.3
      - python-dateutil==2.9.0.post0
      - python-json-logger==3.3.0
      - pytz==2025.2
      - pyyaml==6.0.2
      - pyzmq==26.3.0
      - referencing==0.36.2
      - requests==2.32.3
      - rfc3339-validator==0.1.4
      - rfc3986-validator==0.1.1
      - rpds-py==0.24.0
      - scikit-fem==10.0.2
      - scipy==1.13.1
      - send2trash==1.8.3
      - six==1.17.0
      - sniffio==1.3.1
      - snowballstemmer==2.2.0
      - soupsieve==2.6
      - sphinx==7.4.7
      - sphinxcontrib-applehelp==2.0.0
      - sphinxcontrib-devhelp==2.0.0
      - sphinxcontrib-htmlhelp==2.1.0
      - sphinxcontrib-jsmath==1.0.1
      - sphinxcontrib-qthelp==2.0.0
      - sphinxcontrib-serializinghtml==2.0.0
      - stack-data==0.6.3
      - terminado==0.18.1
      - tinycss2==1.4.0
      - tornado==6.4.2
      - traitlets==5.14.3
      - types-python-dateutil==2.9.0.20241206
      - typing-extensions==4.13.0
      - tzdata==2025.2
      - uri-template==1.3.0
      - urllib3==2.3.0
      - wcwidth==0.2.13
      - webcolors==24.11.1
      - webencodings==0.5.1
      - websocket-client==1.8.0
      - widgetsnbextension==4.0.13
      - zipp==3.21.0
prefix: /opt/conda/envs/PyBaMM

tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_neg_pos_bcs
tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_add_ghost_nodes tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_add_ghost_nodes_concatenation tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_boundary_value_domain tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_concatenation tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_definite_integral tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_definite_integral_vector tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_delta_function tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_discretise_diffusivity_times_spatial_operator tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_discretise_spatial_variable tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_extrapolate_2d_models tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_extrapolate_left_right tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_grad_1plus1d tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_grad_div_shapes_Dirichlet_and_Neumann_bcs tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_grad_div_shapes_Dirichlet_bcs tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_grad_div_shapes_Neumann_bcs tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_grad_div_shapes_mixed_domain tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_grad_div_with_bcs_on_tab tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_indefinite_integral tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_indefinite_integral_on_nodes tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_jacobian tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_mass_matrix_shape tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_node_to_edge_to_node tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_p2d_add_ghost_nodes tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_p2d_mass_matrix_shape tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_p2d_spherical_grad_div_shapes_Dirichlet_bcs tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_p2d_spherical_grad_div_shapes_Neumann_bcs tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_spherical_grad_div_shapes_Dirichlet_bcs tests/unit/test_spatial_methods/test_finite_volume.py::TestFiniteVolume::test_spherical_grad_div_shapes_Neumann_bcs
pybamm/discretisations/discretisation.py CHANGELOG.md
pybamm/discretisations/discretisation.py CHANGELOG.md