How do I back up and restore ArduPilot parameters?

In Mission Planner's full parameter list, the buttons on the right do all three jobs. Save to file writes the aircraft's current parameters to a file on your computer. Load from file reads one back in — note that it only fills the table, so you still have to press Write Params to send it to the board. Write Params is what actually commits any edit. Save a copy the day an aircraft flies well; it turns a bad afternoon of changes into a two-minute recovery.

What does PreArm: Motors: Check frame class and type mean?

FRAME_CLASS is still 0, undefined, or the class and type combination is not one the firmware can build a motor mixer for. Set FRAME_CLASS to match the aircraft — 1 for a quad — set FRAME_TYPE to the layout, and reboot. This particular check is one ArduPilot always runs, so relaxing the optional arming checks will not get past it.

What does PreArm: Need Position Estimate mean?

You are trying to arm in a mode that holds position — Loiter or PosHold — and the estimator has no usable position. Either a GPS has not got a fix yet, or the EKF source parameters name a sensor the aircraft does not actually have, or the source set has not been switched to the one you are using. Arm in Stabilize to confirm everything else is healthy, then work out which source is missing.

What does PreArm: AHRS: EK3 sources require GPS mean?

The EKF source parameters say the estimator should use GPS, and no GPS is connected. Either connect one, or change the EK3_SRC parameters so the estimator uses what the aircraft really has — optical flow, for example. The same message appears with Baro, Compass, Beacon or OpticalFlow in place of GPS, and it always means the same thing: the source configuration names a sensor that is not there.

My flight controller is armed but the motors do not spin

The ESCs are not understanding the signal. A modern 4-in-1 ESC running Bluejay, AM32 or BLHeli_32 usually needs DShot, and ArduPilot's default output protocol is plain PWM — so nothing is wrong except that the two are speaking different languages. Set MOT_PWM_TYPE to 5, DShot300, and reboot. If instead you are running older analogue ESCs, the opposite applies: they need MOT_PWM_TYPE at 0, Normal PWM, and an ESC calibration.

How do I get an ArduPilot OSD on a DJI O3, O4 or Vista air unit?

Use MSP DisplayPort. Set OSD_TYPE to 5, set the serial port the air unit is wired to, to protocol 42 with a baud rate of 115200, and set MSP_OPTIONS to 4 so ArduPilot uses Betaflight font indexes — which is what the DJI goggles expect. Reboot. The port number is whichever UART the air unit is plugged into, so on a board whose video connector is Serial 2, the parameters are SERIAL2_PROTOCOL and SERIAL2_BAUD.

How do I enable bidirectional DShot on ArduPilot?

Set MOT_PWM_TYPE to a DShot rate — DShot300 is the sensible choice — then set SERVO_BLH_BDMASK to the outputs your motors are on, which is usually the first four, a value of 15. Reboot. The ESC has to support it too: AM32, Bluejay and BLHeli_32 do, BLHeli_S and most closed-source ESCs do not. Once it is working, the ESCs report their real RPM back to the flight controller, which is what the RPM-driven harmonic notch filter needs.

How do I connect a GPS or sensor configuration tool through the flight controller?

ArduPilot's serial passthrough turns the flight controller into a transparent bridge, so a tool on your computer can talk directly to a device on one of its UARTs. Set SERIAL_PASS2 to the serial port number the device is on, write parameters, disconnect the ground station but leave the USB cable in, then open your tool on the flight controller's COM port. Two things to know: SERIAL_PASS1 defaults to 0, the USB console, which is the side you want; and passthrough ends by itself after SERIAL_PASSTIMO seconds of silence, 15 by default, and SERIAL_PASS2 resets on reboot.

How do I relax ArduPilot's arming checks?

On 4.7 and later the parameter is ARMING_SKIPCHK and each bit you set is a check that gets skipped, with a default of 0 meaning nothing is skipped. On 4.6 and earlier it is ARMING_CHECK and each bit is a check that gets performed, with a default of 1, the All bit. The meanings are inverted, so numbers from an older tutorial applied to newer firmware disable the wrong things. Use the tick-box editor rather than typing a number, and put the checks back before you fly.

ArduPilot FAQ

Q: How do I configure my ESCs through the flight controller?

ESC passthrough. Set SERVO_BLH_AUTO to 1 and SERVO_BLH_MASK to the outputs the motors are on — 15 for a quad on outputs 1 to 4 — make sure the ESC protocol is a DShot rate, and reboot. Then close the ground station and open an ESC configurator on the flight controller's USB port. Set both parameters back to their defaults when you have finished; leaving passthrough on can stop the ESCs being driven properly.

The problems that come up most often on an ArduPilot build, with the actual parameter names and the actual messages the firmware prints.

If you are setting an aircraft up for the first time and nothing has gone wrong yet, start with ArduPilot Basic Configuration instead — this page assumes something already has.

How do I back up and restore my parameters?

Everything an aircraft knows about itself is in its parameter list, and the list is a file you can keep.

In Mission Planner, Config → Full Parameter List. The buttons down the right-hand side:

Button	What it does
Save to file	writes the aircraft's current parameters to a file on your computer
Load from file	reads a file into the table — it does not send it to the aircraft
Write Params	sends everything in the table to the aircraft and saves it there

Editing a parameter is not the same as writing it — the change lives on your computer until you write it

Loading from a file is a two-step operation and the second step is easy to forget. The file fills the table; Write Params is what commits it.

Do this on the day it flies well

Save a copy of the parameters when an aircraft is flying the way you want. Every later session of experimenting then has a known-good state to return to.

Why will my flight controller not arm?

First, check the aircraft is actually being asked to arm. ArduPilot prints its refusal in the messages area and on the HUD, always beginning with PreArm:. If no such message appears when you try, the arming command never reached the flight controller — check the stick gesture, the arming switch, and that the radio link is alive.

If you do see a message, it is telling you precisely what is wrong. These are the common ones, with ArduPilot's exact wording:

Message	What it means	What to do
`PreArm: Motors: Check frame class and type`	`FRAME_CLASS` is still 0, or the class and type do not make a valid mixer	Set `FRAME_CLASS` and `FRAME_TYPE`, then reboot
`PreArm: GPS 1: Bad fix`	the GPS is connected but has not got a usable fix	Go outside with a clear sky view and wait, or relax the GPS arming check for indoor bench work
`PreArm: Need Position Estimate`	arming in Loiter or PosHold with no usable position source	Check the position source is present and working. Arm in Stabilize to isolate the problem
`PreArm: Need Alt Estimate`	the same, for height	Usually the barometer, or an EKF that has not settled yet
`PreArm: AHRS: EK3 sources require GPS`	the EKF source parameters name GPS and there is no GPS	Connect one, or change `EK3_SRC` to the sensors the aircraft actually has
`PreArm: GPS glitching`	the position is jumping around	Wait. Usually poor sky view, reflections off buildings, or interference
`PreArm: Check EK3_SRC1_POSXY` (or VELXY, POSZ, VELZ, YAW)	that source parameter is set to something invalid	Read the value — it is naming a sensor type the firmware cannot use there
`PreArm: GCS failsafe on`	the ground station link has dropped	Reconnect, or clear the failsafe

The same words appear with Baro, Compass, Beacon or OpticalFlow in place of GPS in the EKF message, and they always mean the same thing: the source configuration names a sensor that is not present.

"Motors: Check frame class and type" cannot be skipped

Most pre-arm checks can be switched off. That one is not on the list — ArduPilot runs it unconditionally, because without a valid mixer it has no way to drive the motors at all. If you see it, relaxing the arming checks will not help.

How do I relax the arming checks?

For bench testing, sometimes reasonably. The parameter changed in ArduPilot 4.7, and the two are inverted:

Firmware	Parameter	The bitmask lists	Default
4.7 and later	`ARMING_SKIPCHK`	checks to skip	`0` — skip nothing
4.6 and earlier	`ARMING_CHECK`	checks to perform	`1` — the "All" bit

Upgrading converts your old value automatically. The trap is following an older guide on newer firmware — its numbers will disable precisely the checks it was telling you to keep.

Use the tick-box editor rather than typing a number, and put them back before you fly. A check that will not pass is describing a real problem with the aircraft.

I am armed, but the motors do not spin

This is an ESC protocol mismatch about nine times in ten, and nothing reports an error because nothing is broken — the two ends are simply speaking different languages.

Your ESCs	Set `MOT_PWM_TYPE` to	Also
Bluejay, AM32, BLHeli_32, BLHeli_S	5 (DShot300)	reboot; no ESC calibration needed
Older analogue ESCs	0 (Normal PWM)	reboot, then calibrate the ESCs

Then check the ordinary things: the flight battery is connected (USB powers the flight controller but not the ESCs), and the safety switch, if the board has one, is released.

How do I put an ArduPilot OSD on a DJI air unit?

DJI's digital systems — O3, O4 and the older Vista — accept an overlay over MSP DisplayPort, so ArduPilot can draw its OSD into the goggles.

OSD_TYPE       = 5      # MSP_DISPLAYPORT
SERIALn_PROTOCOL = 42   # DisplayPort
SERIALn_BAUD   = 115200
MSP_OPTIONS    = 4      # EnableBTFLFonts

Replace n with the serial port the air unit is wired to. On a board whose video connector is Serial 2, that means SERIAL2_PROTOCOL and SERIAL2_BAUD. Reboot afterwards.

MSP_OPTIONS = 4 is bit 2 of a bitmask, EnableBTFLFonts: it tells ArduPilot to use Betaflight's character indexes rather than its own. DJI goggles ship with Betaflight fonts, so without it the OSD appears as the wrong characters rather than not at all — which is a confusing symptom if you do not know to expect it.

How do I enable bidirectional DShot?

Bidirectional DShot lets the ESCs send their measured RPM back to the flight controller. That is what feeds the RPM-driven harmonic notch filter, which is the single most effective vibration filter ArduPilot has, because it tracks the motors instead of guessing at a fixed frequency.

Set MOT_PWM_TYPE to a DShot rate — DShot300 is the sensible choice.
Set SERVO_BLH_BDMASK to the outputs the motors are on. It is a bitmask, one bit per channel starting at bit 0 for channel 1, so outputs 1–4 is 15.
Reboot, then power the aircraft normally.

With the aircraft armed and the throttle just off idle, live RPM values appear in the ground station.

The ESC has to support it. AM32, Bluejay and BLHeli_32 do. BLHeli_S and most closed-source ESCs do not, and on those the mask simply produces no telemetry.

How do I talk to a GPS or sensor through the flight controller?

Normally the flight controller parses everything arriving on its serial ports. Serial passthrough switches that off for one port and bridges it straight to USB, so a configuration tool on your computer can talk directly to a GPS, optical flow sensor or rangefinder without unplugging it from the aircraft.

Connect the flight controller over USB and open the full parameter list.
Set SERIAL_PASS2 to the serial port number the device is on — 4 for a sensor on Serial 4.
Write Params, then disconnect the ground station — but leave the USB cable plugged in.
Open your configuration tool, select the flight controller's COM port, set the right baud rate, and connect.

Three details that are not obvious:

SERIAL_PASS1 defaults to 0, the USB console side of the bridge. That is the value you want, so only SERIAL_PASS2 normally needs setting.
SERIAL_PASS2 resets to −1 on reboot, disabling passthrough, unless SERIAL_PASSTIMO is set to −1.
SERIAL_PASSTIMO defaults to 15 seconds. Passthrough ends when no data has arrived from the first port for that long, and the port reverts to its normal job. If a bridge seems to work and then quietly stops, this is why.

How do I configure my ESCs through the flight controller?

ESC passthrough does the same trick for the motor outputs, so an ESC configurator can read and write ESC settings without you unsoldering anything.

Set SERVO_BLH_AUTO to 1.
Set SERVO_BLH_MASK to the outputs the motors are on — 15 for a quad on outputs 1 to 4.
Make sure the ESC protocol is a DShot rate. Passthrough does not work over PWM.
Write parameters and reboot.
Close the ground station, connect to the flight controller's USB port from an ESC configurator, and connect.

Most ESCs need the flight battery connected for this; an all-in-one board with the ESC on the same PCB will usually manage on USB power alone.

Put the parameters back afterwards

SERVO_BLH_AUTO and SERVO_BLH_MASK are for the duration of a configuration session. Leaving passthrough enabled can stop the ESCs being driven properly in flight.

Configure ESC Parameters covers what to change once you are connected.

Written and maintained by the Robofusion engineering team.

How do I back up and restore my parameters?​

Why will my flight controller not arm?​

How do I relax the arming checks?​

I am armed, but the motors do not spin​

How do I put an ArduPilot OSD on a DJI air unit?​

How do I enable bidirectional DShot?​

How do I talk to a GPS or sensor through the flight controller?​

How do I configure my ESCs through the flight controller?​

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