You must post code or details needed to reproduce the problem.Please readthe posting guidelines.Just a few minutes to compose a detailed question usually results in much better help.

The Teensy Quick Reference: Code Examples, Tips and Tricks list has a list useful resources and solutions for many common projects.

The Most Common Problems

#2: No COM Port or Serial Device Seen: Teensy uses HID protocol for uploading,not serial. Brand new Teensy boards are shipped with the LED blink example compiledto appear as RawHID. You must program Teensy at least once from Arduino. The COMport (Windows) or Serial Device (Mac, Linux) appears only after Teensy beginsrunning your program. Regular Arduino boards are always serial. Teensy uses HIDand supports many protocols. To use serial, make sure the Tools > USB Type menuis set to 'Serial', and understand Teensy only becomes a serial device when it runsyour program built with this setting.

#3: Dead (usually overheating) Main Chip: More than 4 volts applied to the3.3V power pin instantly kills a Teensy LC or 3.2. Be extremely careful if connectingcircuits to Teensy using both VIN(5V) & 3.3V pins, or when using any external powersupply. Loose wires between Teensy and other electonics accidentally touching arethe most common way Teensy fails. Just a few extra minutes to cover exposed leadsand mechanically secure loose wires, especially if they can easily unplug from asolderless breadboard, can save you from an unpleasant and costly 'learning experience'.

Unreliable Communication

Teensy 3.0 Not Recognized by Teensy Loader

Brand New Teensy Not Recognized

1. First, reboot your computer and remove any other circuitryconnected to the Teensy.
2. The LED should blink when the USB cable is plugged in. EveryTeensy is pre-programmed with a LED blink program during producttesting. No drivers or software on your computer are needed forthe LED blink. Only power from the USB cable is necessary. Ifthe LED does not blink, check the voltage on the board using avoltmeter, or try another cable or USB port, and avoid any USB hubs.If the LED never blinks, do not worry about drivers or software.No LED blink indicates the board is not receiving power!
3. When the pushbutton is pressed, the LED should stop blinking.No software or drivers on your computer are necessary for thebutton to stop the LED blink. When the LED blinks and the pushbuttoncauses the blinking to stop, this is a very good indication yourTeensy hardware is functioning properly.
4. When the LED stops blinking, if the Teensy Loader programis running, it shoulddetect the Teensy board. The 'Press Button To Activate' messagewill disappear and the image will show which board is present.No drivers need to be installed on Windows or Mac for this towork. On Linux, the udev rule file must be installed. Ifthe LED blinks, and the pushbutton stops the blinking, but TeensyLoader never detects your board, and you've followed the stepsabove, the problem is almost certainly a faulty USB cable. ManyUSB cables made for charging devices have only power wires butno data lines.
5. On Windows, the driver installation is only necessary forusing the Arduino Serial Monitor when Teensy implements a Serialdevice (from the Tools > USB Type menu), and to automaticallyreprogram using 'Upload' when Teensy has been programmed as Serial.Without the driver, 5 seconds after you click Upload, a message willappear asking you to press the pushbutton on your Teensy.Windows may give you misleading errors indicating drivers must be installed,but NO DRIVER IS NECESSARY on Windows to simply get Teensy Loaderto recognize your Teensy and program new code. Follow steps 1-3above before worrying about drivers!

Teensy Not Recognized by Teensy Loader

1. Physically disconnect the Teensy.
2. Reboot your computer (complete power off is best)
3. Make sure the Teensy Loader application is running.
4. Hold the reset button down, before plugging in the USB cable.
5. Plug the USB cable in while continuing to hold the button.
6. After the cable is fully inserted, release the button.

On Windows systems, 'strange' problems are occasionally reported, whereWindows will not detect new devices. It does not seem to be unique toTeensy. Creating a new user account sometimes solves the problem. Pluggingthe cable into different USB ports can also help. Rebooting is always a goodidea. The best approach is to try on a Mac or Linux machine, or a differentWindows machine.

No Serial Port While Programming

Arduino boards appear as a serial device for all programmingand communication. Arduino remains connected as a serial device whileit reboots. Teensy is a native USB device. Experience troubleshootingArduino does not apply to Teensy!

During programming, Teensy appears as a HID device.When Teensy reboots, electrically your computer sees Teensy disconnect, asif the cable were physically unplugged. When Teensy begins running yourprogram, the USB is disconnected.

The fast and slow LED blink examples do not enable the USB port.

When programs do use USB, type of device Teensy becomes depends on the codeyou have loaded. Your computer will see a new USB device connect when yourprogram begin using the USB port.In Arduino, the device type is controlled by the Tools > USB Type menu.In C language, the USB code you use determine the type of device. The hid_listenprogram only responds to specific C programs. While troubleshooting, you mustlook for the type of device which your program implements.

TODO: list of C code and Teensyduino device types, VID/PID numbers for each.

Pushbutton Does Not Reset Application

Every Teensy is made with a physical pushbutton to allow recovery from anot-responding program.

The pushbutton does NOT reset Teensy to restart your application. However, ifTeensy is connected to your computer, andTeensy Loader is running and configured for automatic mode (the default whenused with Arduino), pressing the button will enter programming mode.Teensy Loader will quickly reprogram and restart your Teensy. This restart ofyour application is due to Teensy Loader's 'auto' mode, not only the pushbutton.

LED Blinks Green or Blue

Large Programs Mysteriously Crash

In C language, usually PSTR() is used, and special functions which accessflash memory must be called. Often they have a '_P' suffix.

Large data tables or arrays can also cause trouble. If data is constant,PROGMEM and pgm_read_byte() can be used. See theavr-libc manualfor details.

Teensy LC, 3.0, 3.1 do not have this problem. Variables defined with 'const'are placed only in flash memory, but can be accessed normally. Only the older8 bit AVR-based boards require these special steps to prevent strings andread-only variables from consuming limited RAM.

Windows: Serial Driver Installed But Not Used

These actions usually can cause Windows to re-detect the device and begin usingthe correct driver.

1. Run the serial installer program again, or run the Teensyduino installer and use the update driver option on the first step.
2. Click that 'Update Driver' button in the device manager. If the driver installer has run, Windows should be able to find the driver.
3. Plug the cable into a different USB port. The Windows Registry handles each physical port with separate registry entries.
4. Do and/all of the above with or without the device connected. Sometimes Windows will do things differently if the device is present or absent.

If there is no 'USB Serial' at all, please rememober Teensy only becomes a Serial device when programmed to do so. See 'No Serial Port While Programming' above.

Windows 7: Unknown Device (Code 43)

This code 43 error is a mystery. It might be related to laptops enteringsuspend mode? If you have any insight, please contact us!

Pieter Rautenbach found a case where programming the wrong HEX file(compiled for a different chip) causes error 43.

Windows: Teensy Loader Window Does Not Appear

1. Windows 7: Hold the shift key and right click on the Teensy Loader in the Task Bar.
   Windows Vista & XP: Right click on the Teensy Loader in the Task Bar.
2. Select 'Move'.
3. Use your keyboard arrow keys to move the window until it appears.

Additional info from Gilbert Hersschens:

'Since you can't see the missing window, you won't know for sure if it's visible or hidden. But you can see this in the context menu. When the window is shown on the missing display, the menu item 'Move' will be active and the item 'Restore' will be greyed out. When it's hidden, it will be just the opposite. You can only move a window in its visible state.To hide or unhide a window, you can click on the icon in the taskbar to toggle its state or use the corresponding menu items in the context menu.

Linux: Gentoo & Arch AVR Toolchain

If running avr-gcc with a makefile, download Arduino 1.0.1and set your $PATH to access the toolchain inarduino-1.0.1/hardware/tools/avr/bin directory.

Linux: Long Delay Before USB Serial Detected

Edit '/lib/udev/rules.d/77-nm-probe-modem-capabilities.rules',adding this line:

Linux: Many Duplicate Device Names with Ubuntu 9.10

This is caused by the Gnome Modem Manager, which holds the port in (as far as the kerel isconcerned) use even after you have reset the Teensy and the port no longer exists.The simplest solution is to uninstall Modem Manager, and restart the network manageror simply reboot.

You could also try deleting /usr/lib/ModemManager/libmm-plugin-generic.so andof course reboot. This might leave Modem Manager able to work with most modems,but not interfere with Teensy.

Modem Manager's troublesome probing and inability to configure exclusionsfor specific device is a known bug.Hopefully it will be fixed in future releases.

Windows: Teensyuino Installer Unable To Write