diff --git a/system/qcomgpsd/modemdiag.py b/system/qcomgpsd/modemdiag.py
index 5d72aeba9e..dc869bfc87 100644
--- a/system/qcomgpsd/modemdiag.py
+++ b/system/qcomgpsd/modemdiag.py
@@ -3,6 +3,8 @@ from serial import Serial
 from crcmod import mkCrcFun
 from struct import pack, unpack_from, calcsize
 
+from openpilot.common.swaglog import cloudlog
+
 class ModemDiag:
   def __init__(self):
     self.serial = self.open_serial()
@@ -77,18 +79,37 @@ def setup_logs(diag, types_to_log):
 
   log_masks = unpack_from('<16I', payload, calcsize(header_spec))
 
+  # Track which requested log ids the modem actually accepted. A partial failure here is the
+  # difference between measurements streaming with no position reports and a working GPS fix.
+  registered = set()
+
   for log_type, log_mask_bitsize in enumerate(log_masks):
     if log_mask_bitsize:
       log_mask = [0] * ((log_mask_bitsize+7)//8)
+      requested = set()
       for i in range(log_mask_bitsize):
-        if ((log_type<<12)|i) in types_to_log:
+        log_id = (log_type<<12)|i
+        if log_id in types_to_log:
           log_mask[i//8] |= 1 << (i%8)
+          requested.add(log_id)
       opcode, payload = send_recv(diag, DIAG_LOG_CONFIG_F, pack('<3xIII',
           LOG_CONFIG_SET_MASK_OP,
           log_type,
           log_mask_bitsize
       ) + bytes(log_mask))
-      assert opcode == DIAG_LOG_CONFIG_F
       operation, status = unpack_from(header_spec, payload)
-      assert operation == LOG_CONFIG_SET_MASK_OP
-      assert status == LOG_CONFIG_SUCCESS_S
+      if opcode != DIAG_LOG_CONFIG_F or operation != LOG_CONFIG_SET_MASK_OP or status != LOG_CONFIG_SUCCESS_S:
+        # Previously bare asserts: the mask write silently failed and the affected log ids
+        # never arrived, with no indication of which ones were lost.
+        lost = sorted(hex(i) for i in requested)
+        raise RuntimeError(f"log mask set failed for group {log_type}: opcode={opcode} operation={operation} status={status} lost={lost}")
+      registered |= requested
+
+  missing = set(types_to_log) - registered
+  if missing:
+    # The modem reported no mask range covering these ids, so they will never be delivered.
+    raise RuntimeError(f"log ids not registered (no mask range): {sorted(hex(i) for i in missing)}")
+
+  if registered:
+    cloudlog.warning(f"diag log registration complete: {sorted(hex(i) for i in registered)}")
+  return registered
diff --git a/system/qcomgpsd/qcomgpsd.py b/system/qcomgpsd/qcomgpsd.py
index d2c97d56c0..b762cd74b3 100755
--- a/system/qcomgpsd/qcomgpsd.py
+++ b/system/qcomgpsd/qcomgpsd.py
@@ -36,6 +36,9 @@ ASSIST_DATA_FILE = '/tmp/xtra3grc.bin'
 ASSIST_DATA_FILE_DOWNLOAD = ASSIST_DATA_FILE + '.download'
 ASSISTANCE_URL = 'http://xtrapath3.izatcloud.net/xtra3grc.bin'
 
+# How often to report that the modem is delivering GNSS logs but no position reports.
+POSITION_REPORT_WARN_INTERVAL = 30.0
+
 LOG_TYPES = [
   LOG_GNSS_GPS_MEASUREMENT_REPORT,
   LOG_GNSS_GLONASS_MEASUREMENT_REPORT,
@@ -294,11 +297,25 @@ def main() -> NoReturn:
 
   pm = messaging.PubMaster(['qcomGnss', 'gpsLocation'])
 
+  # Diagnostics for the two silent paths that yield streaming measurements but no gpsLocation:
+  # the modem never sending position reports at all, and position reports being dropped for
+  # their source field. Without these the failure leaves no trace in the logs.
+  position_reports_seen = 0
+  gnss_logs_seen = 0
+  last_logged_pos_source = -1
+  last_position_warn = time.monotonic()
+
   while 1:
     if os.path.exists(ASSIST_DATA_FILE) and want_assistance:
       setup_quectel(diag)
       want_assistance = False
 
+    now = time.monotonic()
+    if now - last_position_warn >= POSITION_REPORT_WARN_INTERVAL:
+      last_position_warn = now
+      if position_reports_seen == 0 and gnss_logs_seen > 0:
+        cloudlog.error(f"no position reports after {gnss_logs_seen} GNSS logs: {hex(LOG_GNSS_POSITION_REPORT)} is not being delivered by the modem")
+
     opcode, payload = diag.recv()
     if opcode != DIAG_LOG_F:
       cloudlog.error(f"Unhandled opcode: {opcode}")
@@ -315,6 +332,8 @@ def main() -> NoReturn:
     if log_type not in LOG_TYPES:
       continue
 
+    gnss_logs_seen += 1
+
     if DEBUG:
       print(f"{time.time():.4f}: got log: {log_type} len {len(log_payload)}")  # noqa: TID251
 
@@ -362,9 +381,18 @@ def main() -> NoReturn:
             setattr(sv, k, v)
       pm.send('qcomGnss', msg)
     elif log_type == LOG_GNSS_POSITION_REPORT:
+      position_reports_seen += 1
       report = unpack_position(log_payload)
       if report["u_PosSource"] != 2:
+        # 0: none, 1: weighted least-squares, 3: externally injected, 4: internal database.
+        # Only the Kalman filter source (2) is published, so a modem stuck on another source
+        # yields measurement reports with no gpsLocation and no other trace.
+        if report["u_PosSource"] != last_logged_pos_source:
+          last_logged_pos_source = report["u_PosSource"]
+          failure_code, fix_events = report.get("u_FailureCode"), report.get("w_FixEvents")
+          cloudlog.warning(f"dropping position report: u_PosSource={last_logged_pos_source} (need 2), failureCode={failure_code}, fixEvents={fix_events}")
         continue
+      last_logged_pos_source = 2
       vNED = [report["q_FltVelEnuMps[1]"], report["q_FltVelEnuMps[0]"], -report["q_FltVelEnuMps[2]"]]
       vNEDsigma = [report["q_FltVelSigmaMps[1]"], report["q_FltVelSigmaMps[0]"], -report["q_FltVelSigmaMps[2]"]]