diff --git a/Makefile.am b/Makefile.am
index 12decd3..a9c7dfe 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -74,11 +74,16 @@ ecutuned_SOURCES = $(SRC_FILES) src/ecutuned.c
 ecutuned_CFLAGS = -DUSESSL -DTHREADED $(INCLUDE_ALL_DIRS) $(COMPILER_FLAGS) $(LOG_FLAGS)
 ecutuned_LDFLAGS = $(LD_FLAG) $(EXTERNAL_LIBS)
 
-TESTS = check_j2534
-check_PROGRAMS = check_j2534
+TESTS = check_j2534 check_passthru_open
+check_PROGRAMS = check_j2534 check_passthru_open
 check_j2534_SOURCES = $(ECUTOOLS_TEST_FILES)
 check_j2534_LDFLAGS = $(LD_FLAG) -lcheck -lj2534
 
+check_passthru_open_SOURCES = tests/check_passthru_open.c src/j2534.c
+check_passthru_open_CFLAGS = $(AM_CFLAGS) -ffunction-sections -fdata-sections
+check_passthru_open_LDFLAGS = -Wl,--gc-sections
+check_passthru_open_LDADD = -lcheck -lsubunit
+
 bundle-install:
 	cd bindings/ruby && bundle install && cd -
 	cd cli && bundle install && cd -
diff --git a/src/j2534.c b/src/j2534.c
index 7cc7979..3a2d34e 100755
--- a/src/j2534.c
+++ b/src/j2534.c
@@ -427,10 +427,13 @@ long PassThruGetNextDevice(SDEVICE *psDevice) {
  *     STATUS_NOERROR                 Function call was successful
  */
 long PassThruOpen(const char *pName, unsigned long *pDeviceID) {
+  j2534_current_api_call = J2534_PassThruOpen;
 
-  syslog(LOG_ERR, "PassThruOpen: pName=%s, pDeviceID=%d", pName, *pDeviceID);
+  if(pName == NULL || pDeviceID == NULL) {
+    return unless_concurrent_call(ERR_NULL_PARAMETER, J2534_PassThruOpen);
+  }
 
-  j2534_current_api_call = J2534_PassThruOpen;
+  syslog(LOG_ERR, "PassThruOpen: pName=%s, pDeviceID=%d", pName, *pDeviceID);
 
   unsigned int shadow_update_topic_len = PASSTHRU_SHADOW_UPDATE_TOPIC + strlen(pName) + 1;
   unsigned int shadow_update_accepted_topic_len = PASSTHRU_SHADOW_UPDATE_ACCEPTED_TOPIC + strlen(pName) + 1;
@@ -438,10 +441,6 @@ long PassThruOpen(const char *pName, unsigned long *pDeviceID) {
   unsigned int msg_rx_topic_len = J2534_MSG_RX_TOPIC + strlen(pName) + 1;
   unsigned int msg_tx_topic_len = J2534_MSG_TX_TOPIC + strlen(pName) + 1;
 
-  if(pName == NULL || pDeviceID == NULL) {
-    return unless_concurrent_call(ERR_NULL_PARAMETER, J2534_PassThruOpen);
-  }
-
   if(!j2534_initialized) {
     vector_init(&j2534_client_vector);
     vector_init(&j2534_selected_channels);
diff --git a/tests/check_passthru_open.c b/tests/check_passthru_open.c
new file mode 100644
index 0000000..f2084b8
--- /dev/null
+++ b/tests/check_passthru_open.c
@@ -0,0 +1,97 @@
+#include <stdlib.h>
+#include <check.h>
+
+#include "j2534.h"
+
+/* PassThruOpen dependencies that are not exercised by NULL validation. */
+void vector_init(vector *items) { (void)items; }
+void vector_add(vector *items, void *item) { (void)items; (void)item; }
+void *vector_get(vector *items, int index) { (void)items; (void)index; return NULL; }
+
+unsigned int awsiot_client_connect(awsiot_client *client)
+{
+  (void)client;
+  return 0;
+}
+
+unsigned int awsiot_client_subscribe(awsiot_client *client,
+                                     const char *topic,
+                                     void *handler,
+                                     void *data)
+{
+  (void)client;
+  (void)topic;
+  (void)handler;
+  (void)data;
+  return 0;
+}
+
+unsigned int awsiot_client_publish(awsiot_client *client,
+                                   const char *topic,
+                                   char *payload)
+{
+  (void)client;
+  (void)topic;
+  (void)payload;
+  return 0;
+}
+
+IoT_Error_t aws_iot_mqtt_yield(AWS_IoT_Client *client, uint32_t timeout_ms)
+{
+  (void)client;
+  (void)timeout_ms;
+  return SUCCESS;
+}
+
+int MYINT_LEN(int *num)
+{
+  (void)num;
+  return 1;
+}
+
+shadow_message *passthru_shadow_parser_parse_state(const char *json)
+{
+  (void)json;
+  return NULL;
+}
+
+void passthru_shadow_parser_free_message(shadow_message *message)
+{
+  (void)message;
+}
+
+START_TEST(test_passthru_open_rejects_null_name)
+{
+  unsigned long device_id = 1;
+
+  ck_assert_int_eq(PassThruOpen(NULL, &device_id), ERR_NULL_PARAMETER);
+}
+END_TEST
+
+START_TEST(test_passthru_open_rejects_null_device_id)
+{
+  ck_assert_int_eq(
+    PassThruOpen("J2534-1:ecutools", NULL),
+    ERR_NULL_PARAMETER
+  );
+}
+END_TEST
+
+int main(void)
+{
+  int failed;
+  Suite *suite = suite_create("PassThruOpen");
+  TCase *null_parameters = tcase_create("null_parameters");
+  SRunner *runner;
+
+  tcase_add_test(null_parameters, test_passthru_open_rejects_null_name);
+  tcase_add_test(null_parameters, test_passthru_open_rejects_null_device_id);
+  suite_add_tcase(suite, null_parameters);
+
+  runner = srunner_create(suite);
+  srunner_run_all(runner, CK_NORMAL);
+  failed = srunner_ntests_failed(runner);
+  srunner_free(runner);
+
+  return failed == 0 ? EXIT_SUCCESS : EXIT_FAILURE;
+}