Troubleshooting
When a robot doesn't work, the fault is usually something simple: a flat battery, a plug in the wrong port, a setting that didn't save. This page is organised by symptom. For each one, the likely causes are listed roughly from most to least common, followed by quick checks. For background on each part, see Radios, ESCs, Failsafes, Radios/Mixing, Battery eliminator circuit (BEC) and Wiring and Soldering.
Safety first: test with the wheels off the ground and the weapon disconnected (belt off, or weapon locked) until you know the robot behaves. Spin weapons up only in a test box. See Safety and LiPo Battery Safety.
A general method
- Charge the battery and check its voltage. A low battery causes all sorts of confusing faults.[1]
- Work along the signal path: power, then radio, then ESC, then motor. Is the power light on? Is the receiver powered and bound (check its LED)? Does the ESC show a ready signal (LED or beeps)? Does the motor turn when the ESC tells it to?
- Swap in known-good parts. Plug a servo into the receiver port to see if the radio is sending a signal. Swap the left and right motors or ESC channels to see whether the fault moves with the part. A spare receiver, ESC and motor in your pit kit make this quick (see Pit Kit and Event Checklist).
- Change one thing at a time, and note what you changed.
- Re-test the failsafe after any fix that touched the radio, trims, mixing, ESCs or wiring (see Failsafes).
- If you're still stuck, ask with details: exact models of radio, receiver, ESCs, motors and battery, and exactly what the robot does. Team Run Amok's Ask Aaron radio guides cover many of these problems.
Won't power on
Likely causes:
- Flat or dead battery, or a battery that has been over-discharged.
- Switch or removable link not making contact, or a switch that has come unsoldered (see Switches).
- A broken wire or cold solder joint at the battery connector.
- Reversed polarity has damaged the ESC (see Wiring and Soldering).
Quick checks: measure the battery voltage at the robot's connector, not just at the pack. Bypass the switch with a known-good link. Wiggle wires gently while watching the power light. If something gets hot, disconnect immediately: you may have a short.
Receiver won't bind
Likely causes:
- Following the wrong bind procedure for that receiver, or the receiver isn't in bind mode.
- Wrong protocol, or a receiver that isn't truly compatible with the transmitter, even on the "same" protocol (see Radios).
- The receiver isn't getting power. Some ESCs have no BEC, so the receiver needs its own supply.[2]
- ExpressLRS: firmware versions don't match. The first number of the version must be the same on the transmitter module and the receiver (3.x with 3.x, for example; a 3.x receiver won't bind to a 4.x transmitter). The binding phrase and regulatory domain must also match.[3][4]
Quick checks: read the receiver's LED. On ExpressLRS, a double blink means bind mode, a slow blink means it's waiting for a transmitter, and solid means connected.[5] Check the right model memory is selected. See ExpressLRS and EdgeTX Setup for ELRS details.
Bound, but no response
Likely causes:
- ESCs plugged into the wrong receiver ports, or a servo plug in backwards or offset by one pin.
- The ESC needs calibrating to your radio, or is waiting for its arming signal. For example, the Repeat Robotics Dominion ESC gives a single blink about five seconds after power-up when it needs calibrating.[6]
- ExpressLRS model match: the receiver is connected but won't output anything if the model-match settings disagree. Its LED shows three blinks then a pause.[7][8]
- Dead ESC or BEC.
Quick checks: use your transmitter's channel monitor to confirm the sticks move the channels. Plug a servo into the drive port. Look up your ESC's LED meanings: for example, a FingerTech tinyESC slow-blinks red when it has no R/C signal.[9]
Drives backwards, spins in circles, or controls are swapped
Likely causes:
- Mixing setup: channel reversing, ESCs swapped between ports, or motor wires reversed.
- Both transmitter mixing and ESC mixing turned on at once.
Quick checks: use the Team Run Amok Mixer Fixer. Answer three questions about what the robot does and it gives you the exact fix. To reverse a single motor, swap the two leads on a brushed motor, or any two of the three leads on a brushless motor.[10] See Radios/Mixing.
Only one motor works
Likely causes:
- Mixing is off (each stick axis drives one side), or mixing is on twice (transmitter and ESC). If "forward" drives one side and "right" drives the other, it's one of these.[11] Some dual ESCs ship with their mixing disabled.[12][13]
- ESC plugged into the wrong receiver channel.
- A broken motor wire or solder joint, a dead motor, or a dead ESC channel.
Quick checks: if the same motor responds to both forward and right, check the wiring and receiver ports.[14] Swap the two motors' leads at the ESC: if the fault follows the motor, it's the motor; if it stays, it's the ESC or signal.
Brushless motor stutters, cogs or won't start
Likely causes:
- Cogging: a sensorless ESC can't tell where the rotor is at low speed and under heavy load, so it fires the windings at the wrong time. Heavy weapons and brushless drive are the usual victims.[15]
- Low or weak battery sagging under the startup load.[16]
- ESC settings not suited to the motor or load.
- A broken or poorly soldered motor phase wire.
- Mechanical drag: a tight belt, rubbing weapon, or bent shaft.
Quick checks: charge the battery and spin the motor by hand to feel for drag. Then try the ESC settings: on BLHeli_S, higher commutation timing and demag compensation help with stutter on fast throttle changes.[17] On Bluejay, raise the minimum startup power if the motor won't start at low throttle.[18] AM32 has a sinusoidal startup mode for getting larger motors up to speed.[19] See ESC Firmware.
Weapon won't start or arm
Likely causes:
- The ESC didn't see zero throttle at power-up. Most weapon ESCs won't arm until they see a low throttle signal. BLHeli_S beeps three times at power-on, a low tone when it sees a signal, then a high tone once throttle is at zero; it won't run before that.[20]
- The weapon switch, knob or trim isn't at its true "off" position. On some radios a switch only enables a knob, so a bumped knob breaks the arming sequence.[21]
- ESC throttle range not calibrated to your radio, or set to bidirectional when you meant one-way.
- Cogging or drag on startup (see above).
Quick checks: power on with the weapon control at zero and listen for the arming beeps. Check the channel monitor shows the weapon channel at its minimum. Caution: on BLHeli_S, powering up at full throttle starts throttle calibration, so only calibrate with the weapon removed or the belt off.[22]
The ESC beeps
Beep and LED codes are specific to each ESC and firmware, so check the maker's documentation rather than guessing. Common patterns:
- Startup beeps (often a tune or a count of cells) just mean it's powered.
- Arming beeps mean it saw a signal and zero throttle (see above).
- Repeated beeps while idle are often a "beacon" for finding lost aircraft. On BLHeli_S and Bluejay they start after a set time at zero throttle; a high beacon strength can heat up motors and ESCs, so turn it down or off.[23][24]
- Continuous or unusual beeps often mean no signal, throttle not at zero, or a fault.
Documentation: BLHeli_S manual, Bluejay wiki, AM32, and your ESC vendor's product page (for example the FingerTech tinyESC LED table).
Brownouts: receiver resets when the weapon spins up or the robot is hit
A brownout is when the receiver's or ESC's supply voltage drops too low to keep working. The receiver drops its signal and the ESCs restart their startup sequence, leaving you a sitting duck.[25]
Likely causes:
- Battery sag: a small, old or under-charged battery can't supply the weapon's startup current, so the whole system's voltage dips.[26]
- BEC overloaded: by servos or a power-hungry receiver. Some ESC BECs supply less current at higher battery voltages.[27]
- Some ESCs reset when they take a hard hit, which looks like a brownout.[28]
- More than one BEC feeding the receiver.
Quick checks and fixes: use a fresh, well-rated battery. Ramp the weapon up instead of slamming full throttle. A capacitor across the power pins of a spare receiver port can hold the receiver voltage up during short dips.[29] Power the receiver from one stand-alone BEC and disconnect the others, unless your ESCs are designed to share (see Battery eliminator circuit (BEC)).[30]
Twitches at neutral, hums, or creeps
Likely causes:
- Transmitter trim not matching the ESC's neutral point.
- ESC not calibrated to the radio's range (see your ESC's manual; the tinyESC has a calibrate routine, for example).[31]
- Unequal motor start-up, so one side moves first.
- Electrical noise or a loose signal plug.
Quick checks: center the trims, then calibrate the ESCs. For unequal start-up, prop the wheels up and use subtrim on the slower side until both start together.[32] The Radios page covers humming and creeping in more detail. Re-set and re-test your failsafe afterwards.
Failsafe doesn't work
Don't fight with a robot that fails this test. The usual causes are channels left on "hold", the ExpressLRS default putting a one-way weapon ESC at half throttle, or a failsafe stored with sticks or trims away from "stopped". See Failsafes for the full list and test procedure.
Motors or ESCs overheating
Likely causes:
- Too much load: a heavy weapon, too little reduction, or too high a voltage for the motor.[33]
- Stalls: pushing matches, a jammed weapon, or binding gearboxes. ESCs without current limiting can be damaged by long stalls.[34]
- Brushless drive motors run at low RPM for long periods overheat quickly.[35]
- ESC too small for the motor, or mounted with no airflow (see ESCs).
- High beacon beep strength (see above).
Quick checks: feel the motor and ESC after a timed run. Spin everything by hand for drag. Reduce load by lowering voltage or increasing the reduction.[36] If the robot slowly loses power mid-match, it may be ESC thermal protection kicking in.[37] See Speed Reduction and Insect Class Motors.
Battery puffing or voltage sag
Likely causes:
- Drawing more current than the pack can deliver, over-discharging it, or heat (including leaving it in the sun).[38]
- A pack that's too small. Safe current is roughly the C rating times the capacity in amp-hours: 30C × 0.3 Ah = 9 A.[39]
- An aging pack that can no longer deliver its rated current.
What to do: a puffy LiPo is damaged. Don't use it or charge it – retire it safely as described in LiPo Battery Safety.[40] For sag, use a bigger or higher-rated pack (see Batteries). Most builders disable the ESC's low-voltage cutoff so the robot doesn't shut down mid-fight, which makes it more important to size the battery properly.[41]
Intermittent loss of signal
Likely causes:
- Antenna blocked or damaged: metal and carbon fiber block radio signals. Antennas laid flat against metal, cut, or crushed in a hit receive poorly.[42]
- Brownouts (see above).
- Loose plugs: servo plugs that work loose in hits.
- Electrical noise from motors and ESCs.[43]
Quick checks: get the antenna out of the metal box, or behind a plastic window, and don't change its length. Keep the receiver away from ESCs and motors, keep power wires short, and add 0.1 µF capacitors across brushed motor terminals.[44] Secure plugs with hot glue or tape (see Wiring and Soldering). If your radio has telemetry, watch the signal and receiver voltage while driving.
Wheels slip or the robot is slow
Likely causes:
- Radio limits: end points, dual rates or mix rates below 100%. Some transmitters' built-in mixes give each channel only part of its travel.[45]
- ESC not calibrated to the radio's full stick range.
- Low battery or voltage sag.
- Not enough weight on the drive wheels. Pushing force depends on the weight on the driven wheels and the tire grip, not motor power. A rule of thumb for two-wheel robots is around 65% of the weight on the wheels.[46][47]
- Four-wheel scrub: four-wheel-drive robots drag their wheels sideways in turns, which makes them sluggish.[48]
- Hard, worn or dusty tires, or the wrong gearing (see Drive Systems).
Quick checks: look at the channel monitor at full stick to check for full travel. Check the tires and keep them clean. Work out your expected top speed using the formula on Drive Systems.