diff --git a/tools/replay/can_replay.py b/tools/replay/can_replay.py
index 18fb79461c..98d83d2f21 100755
--- a/tools/replay/can_replay.py
+++ b/tools/replay/can_replay.py
@@ -1,7 +1,7 @@
 #!/usr/bin/env python3
 import os
 import time
-from multiprocessing import Process
+import threading
 from tqdm import tqdm
 
 os.environ['FILEREADER_CACHE'] = '1'
@@ -15,16 +15,17 @@ try:
 except Exception:
   PandaJungle = None  # type: ignore
 
-ROUTE = "77611a1fac303767/2020-03-24--09-50-38"
-NUM_SEGS = 2 # route has 82 segments available
 
 print("Loading log...")
+ROUTE = "77611a1fac303767/2020-03-24--09-50-38"
+NUM_SEGS = 2 # route has 82 segments available
 CAN_MSGS = []
 for i in tqdm(list(range(1, NUM_SEGS+1))):
   log_url = f"https://commadataci.blob.core.windows.net/openpilotci/{ROUTE}/{i}/rlog.bz2"
   lr = LogReader(log_url)
   CAN_MSGS += [can_capnp_to_can_list(m.can) for m in lr if m.which() == 'can']
 
+
 # set both to cycle ignition
 IGN_ON = int(os.getenv("ON", "0"))
 IGN_OFF = int(os.getenv("OFF", "0"))
@@ -33,10 +34,11 @@ if ENABLE_IGN:
   print(f"Cycling ignition: on for {IGN_ON}s, off for {IGN_OFF}s")
 
 
-def send_thread(s):
+def send_thread(s, flock):
   if "Jungle" in str(type(s)):
     if "FLASH" in os.environ:
-      s.flash()
+      with flock:
+        s.flash()
 
     for i in [0, 1, 2, 3, 0xFFFF]:
       s.can_clear(i)
@@ -48,7 +50,6 @@ def send_thread(s):
     s.set_safety_mode(Panda.SAFETY_ALLOUTPUT)
   s.set_can_loopback(False)
 
-
   idx = 0
   ign = True
   rk = Ratekeeper(1 / DT_CTRL, print_delay_threshold=None)
@@ -74,6 +75,7 @@ def connect():
   config_realtime_process(3, 55)
 
   serials = {}
+  flashing_lock = threading.Lock()
   while True:
     # look for new devices
     for p in [Panda, PandaJungle]:
@@ -83,14 +85,14 @@ def connect():
       for s in p.list():
         if s not in serials:
           print("starting send thread for", s)
-          serials[s] = Process(target=send_thread, args=(p(s), ))
+          serials[s] = threading.Thread(target=send_thread, args=(p(s), flashing_lock))
           serials[s].start()
 
-    # try to join all send procs
+    # try to join all send threads
     cur_serials = serials.copy()
-    for s, p in cur_serials.items():
-      p.join(0.01)
-      if p.exitcode is not None:
+    for s, t in cur_serials.items():
+      t.join(0.01)
+      if not t.is_alive():
         del serials[s]
 
     time.sleep(1)