I was successfully running two external monitors (not Apple Thunderbolt) from my 2015 MBP 13". They were both connected to the left side thunderbolt ports via HDMI to Thunderbolt cables. No problems.

I replaced one of the monitors and now both are the exact same monitor (ACER R240HY). The new one is not recognized. Cables are fine, when I swap thunderbolt ports the new monitor is recognized.

Cleared NVRAM, PRAM, and SMC. Didn't work.

Tried unplugging monitors and booting up with cables in first then plugging in monitor. Didn't work.

Searched and read threads for over an hour so asking here.

Thanks for any help!

Update per request:

  • Old/replaced monitor (Viewsonic) works in either port by itself
  • New replacement monitor works in only 1 port either by itself or with the original.
  • Original monitor works in only 1 port either by itself or with the replacement.

In other words, both ports work, all monitors work, all cables work, but the replacement monitor is just not recognized in one of the ports (but it is recognized in the other.)

SPECS Original Two Monitors that worked ACER R240HY - using HDMI to Thunderbolt Cable Viewsonic VA2455 - using HDMI to DVI/Thunderbolt adapter

Replacement monitor ACER R240HY - using HDMI to Thunderbolt Cable

  • Can you expand your explanation of when you "swap TB ports the new monitor is recognized"? Does that mean it starts to function while the other just stops? Can you confirm that the new monitor works? Can you confirm that both TB ports are functional by connecting a single monitor to one then moving it to another verifying that it works in the second port as well?
    – Allan
    Commented Mar 28, 2017 at 16:40
  • Could you add the make and model of all three displays you're attempting to connect AND which adapters you're using on which displays? Commented Mar 28, 2017 at 16:55
  • both edits made
    – justinae
    Commented Mar 28, 2017 at 17:02
  • I'm not sure I understand. Let's say the new monitor works in port A but not in port B. When you attach the old monitor to port B and the new monitor to port A, do they both work? Does the old monitor not work, or does the new monitor not work?
    – NoahL
    Commented Mar 28, 2017 at 17:49

2 Answers 2


So you're not connecting them exactly the same. You're using two adapters on the Viewsonic (an HDMI-DVI cable and an HDMI-Mini DisplayPort cable daisy-chained together) and one on the old Acer. Now you're trying to connect two via HDMI-Mini DisplayPort cables.

I would suggest trying to connect one of your Acer monitors using a normal HDMI cable to the HDMI port on the right side of the MacBook and the other to the left Thunderbolt port using the HDMI-Mini DisplayPort adapter.

  • I agree with the suggestion here - connecting each monitor on opposite sides of the MBP. That's what I'd do next.
    – Monomeeth
    Commented Mar 28, 2017 at 23:36
  • I connected it to the HDMI on the other side and it worked. I would like to use the two thunderbolt ports on the same side. So I went back to the DVI to DVI cable with a DVI to Thunderbolt adapter and now both monitors work using the two Thunderbolt ports. It does seem strangle that I can't just use two HDMI to Thunderbolt cables with the exact same monitor.
    – justinae
    Commented Apr 3, 2017 at 17:24
  • Excellent! Glad to hear it. Commented Apr 3, 2017 at 17:25

There were two solutions. One utilized the HDMI port on the opposite side of the laptop with an DVI to HDMI cable. But I wanted to use the same two Thunderbolt ports so I used a DVI to DVI cable and a DVI to Thunderbolt adapter.


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