diff --git a/python-stdlib/errno/errno.py b/python-stdlib/errno/errno.py
index c513a7f14..91d844a80 100644
--- a/python-stdlib/errno/errno.py
+++ b/python-stdlib/errno/errno.py
@@ -32,8 +32,15 @@
 EPIPE = 32  # Broken pipe
 EDOM = 33  # Math argument out of domain of func
 ERANGE = 34  # Math result not representable
-EAFNOSUPPORT = 97  # Address family not supported by protocol
-ECONNRESET = 104  # Connection timed out
-ENOTCONN = 107 # Not connected
+ECONNRESET = 104  # Connection reset by peer
+ENOTCONN = 107  # Transport endpoint is not connected
 ETIMEDOUT = 110  # Connection timed out
 EINPROGRESS = 115  # Operation now in progress
+
+# Override the table above with the built-in module's values, which come from
+# this port's system headers.  The u-prefix reaches the built-in module.
+try:
+    from uerrno import *
+except ImportError:
+    # Built without MICROPY_PY_ERRNO.
+    pass
diff --git a/python-stdlib/errno/manifest.py b/python-stdlib/errno/manifest.py
index 075d3403d..3a71c2d82 100644
--- a/python-stdlib/errno/manifest.py
+++ b/python-stdlib/errno/manifest.py
@@ -1,3 +1,3 @@
-metadata(version="0.2.0")
+metadata(version="0.3.0")
 
 module("errno.py")
diff --git a/python-stdlib/errno/test_errno.py b/python-stdlib/errno/test_errno.py
new file mode 100644
index 000000000..2b39257e5
--- /dev/null
+++ b/python-stdlib/errno/test_errno.py
@@ -0,0 +1,38 @@
+# The values the built-in errno module provides come from the system headers
+# the port was built against, and they differ between operating systems and
+# between architectures: ETIMEDOUT is 110 on Linux/x86, 145 on Linux/mips and
+# 60 on macOS.  This module must not override them with a table of its own.
+
+import errno
+
+try:
+    import uerrno
+except ImportError:
+    # Built without MICROPY_PY_ERRNO, so there is nothing to compare against.
+    print("SKIP")
+    raise SystemExit
+
+
+# Every name the built-in module provides has to come through unchanged.
+names = [name for name in dir(uerrno) if name.startswith("E")]
+assert names, dir(uerrno)
+for name in names:
+    theirs = getattr(uerrno, name)
+    ours = getattr(errno, name, None)
+    assert ours is not None, "errno.%s is missing, uerrno.%s is %d" % (name, name, theirs)
+    assert ours == theirs, "errno.%s is %d, uerrno.%s is %d" % (name, ours, name, theirs)
+
+# errorcode has to survive too: mp_errno_to_str() looks names up in it, and it
+# is what makes str(OSError(errno.ENOENT)) readable.
+if hasattr(uerrno, "errorcode"):
+    assert isinstance(errno.errorcode, dict), errno.errorcode
+    for code, name in errno.errorcode.items():
+        assert getattr(errno, name) == code, "errorcode[%d] is %s" % (code, name)
+
+# The names the built-in module does not provide are filled in by this module.
+# Codes 1 to 34 are the same on every platform MicroPython runs on.
+assert errno.EPERM == 1, errno.EPERM
+assert errno.ESRCH == 3, errno.ESRCH
+assert errno.ENOTBLK == 15, errno.ENOTBLK
+assert errno.EDOM == 33, errno.EDOM
+assert errno.ERANGE == 34, errno.ERANGE
diff --git a/tools/ci.sh b/tools/ci.sh
index 7ee7eb4d1..10f16b6ae 100755
--- a/tools/ci.sh
+++ b/tools/ci.sh
@@ -65,6 +65,7 @@ function ci_package_tests_run {
         python-stdlib/base64/test_base64.py \
         python-stdlib/binascii/test_binascii.py \
         python-stdlib/collections-defaultdict/test_defaultdict.py \
+        python-stdlib/errno/test_errno.py \
         python-stdlib/functools/test_partial.py \
         python-stdlib/functools/test_reduce.py \
         python-stdlib/heapq/test_heapq.py \