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The Toolbox — Troubleshooting

Aviation Electronics

Symptom-based diagnostic guides for transponders and general avionics systems.

Aviation Transponder

ATC reports no transponder reply

Possible Causes

  • •Failed transponder
  • •Antenna fault
  • •Incorrect mode/code
  • •Low transmit power

Diagnostic Steps

  1. 1.Verify transponder is in ALT or ON mode
  2. 2.Check transponder antenna connection at belly of aircraft
  3. 3.Confirm correct squawk code
  4. 4.Have transponder output power tested — minimum 125W for Mode C

Altitude encoding error

Possible Causes

  • •Faulty encoding altimeter
  • •Incorrect wiring to transponder
  • •Gray code error
  • •Pressure altimeter out of calibration

Diagnostic Steps

  1. 1.Compare transponder altitude readout to calibrated altimeter
  2. 2.Have encoding altimeter tested and calibrated
  3. 3.Check connector and wiring between altimeter encoder and transponder
  4. 4.Verify transponder reports correct altitude to ATC during flight check

Transponder passes test but ATC cannot see aircraft

Possible Causes

  • •Antenna shielded by airframe
  • •Antenna damaged or corroded
  • •Cable loss excessive
  • •Wrong antenna location

Diagnostic Steps

  1. 1.Inspect antenna for cracks, corrosion, or physical damage
  2. 2.Check coax from transponder to antenna for continuity and low loss
  3. 3.Verify antenna has clear view of sky — should be on belly of aircraft
  4. 4.Have antenna VSWR measured

Aircraft Avionics (General)

Avionics bus failure / multiple units going offline

Possible Causes

  • •Avionics bus fuse or circuit breaker tripped
  • •Bus voltage low
  • •Faulty avionics master switch
  • •Short circuit on bus

Diagnostic Steps

  1. 1.Check avionics master switch and circuit breaker
  2. 2.Measure bus voltage with avionics master on — should be 27–29V (28V aircraft)
  3. 3.Remove units one at a time to find shorted unit
  4. 4.Inspect bus bar and wiring for signs of heat damage

Unit powers on but display not readable

Possible Causes

  • •Display backlight failure
  • •Contrast setting wrong
  • •Temperature extreme
  • •Display hardware fault

Diagnostic Steps

  1. 1.Check contrast and brightness settings in unit menu
  2. 2.Allow unit to warm up in cold conditions
  3. 3.Inspect display for physical damage
  4. 4.Check for moisture behind display bezel

Persistent circuit breaker trips

Possible Causes

  • •Short circuit in wiring
  • •Overloaded circuit
  • •Failing unit drawing excess current
  • •Undersized circuit breaker

Diagnostic Steps

  1. 1.Measure current draw of circuit — should not exceed 80% of breaker rating
  2. 2.Disconnect units one at a time to find faulty unit
  3. 3.Inspect wiring for chafing against airframe
  4. 4.Check connector pins for pushed-back or shorted contacts