- HVAC
- Air conditioner
42
Fujitsu 42 on an air conditioner: indoor heat exchanger sensor
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
On Fujitsu wall-mounted splits, 42 (E:42) means an error in the indoor unit heat exchanger temperature sensor. Write down the code and the lamp flashes and consult authorised service personnel, as Fujitsu asks.
Safety first
Stop and check this first
- This Fujitsu uses a flammable refrigerant and Fujitsu requires a valid certificate to work on the circuit. It also warns of electric shock: after switching off, wait 5 minutes before touching electrical components. Do not open the unit.
Safe checks you can perform
- Code and lamp flashes
- Calling authorised service
- Connector, thermistor and board voltage
Leave these tasks to a qualified professional
- Do not attempt this professional task: Follow Fujitsu's three check points
- Do not attempt this professional task: Power reset and test after the repair
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Follow Fujitsu's three check points
What they doThe technician checks the connector, measures the thermistor and checks the board voltage before replacing anything.
They must show youthe measured resistance against Fujitsu's table and the terminal voltage.
In writingwhich part was replaced and why.
Power reset and test after the repair
What they doAfter reconnecting or replacing, the technician resets the power as Fujitsu says and checks the lamps stop flashing.
They must show youthe unit running without error flashes.
In writingdate and result of the test.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Code and lamp flashes
- Who:
- you.
- How:
- read the code on the wired remote, if you have one, or count the flashes of the indoor unit lamps.
- Result that matters:
- E:42, or 4 flashes of the operation lamp and 2 of the timer lamp with the economy lamp flashing continuously.
- If it matches:
- it is the indoor heat exchanger sensor; carry on with this page.
- If it is 4 and 1 flashes:
- it is E:41, the room temperature sensor; look up that page.
- Evidence to keep:
- a short video of the lamps or a photo of the remote, with the date.
Evidence: Fujitsu General air conditioner wall mounted type service manual, ASEH07/09/12/14KMCG and AOEH07/09/12/14KMCG
Calling authorised service
- Who:
- you.
- How:
- Fujitsu says to consult authorised service personnel when an error code is displayed. If there is also a burning smell, stop the unit and disconnect the power at the breaker.
- Result that matters:
- that the technician has the code and there are no risk symptoms.
- If there is a burning smell or smoke:
- switch off at the breaker and report that symptom.
- If there is only the code:
- switch off with the remote and ask for a check.
- Evidence to keep:
- a note of the date and the mode it was in.
Evidence: Fujitsu General operation manual, wall-mounted air conditioner ASYG12KGTF
Connector, thermistor and board voltage
- Who:
- an air conditioning technician.
- How:
- with the power isolated checks the connector, measures the thermistor resistance against Fujitsu's table and, with power on, checks for 5 V DC at the thermistor terminal on the board.
- Result that matters:
- where the chain of sensor, connector and board fails.
- If the thermistor is open or shorted:
- it is replaced and the power reset.
- If the voltage does not appear on the board:
- Fujitsu says to replace the main board.
- Evidence to keep:
- a job sheet with the measured resistance and voltage.
Fujitsu warns of electric shock: after switching off the power, wait 5 minutes before touching electrical components. Technician only.
Evidence: Fujitsu General air conditioner wall mounted type service manual, ASEH07/09/12/14KMCG and AOEH07/09/12/14KMCG
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- HVAC
- Brand
- Fujitsu
- Product type
- Air conditioner
Known code variants
- E42
- E:42
- Fujitsu 42
- Fujitsu E42
What it means
Fujitsu shows E:42 when it detects the indoor heat exchanger temperature thermistor as open or short-circuited. The lamps show it with 4 flashes of the operation lamp, 2 of the timer lamp and continuous flashing of the economy lamp. Fujitsu gives as causes a connector connection failure, a failed thermistor or the main board. The code does not say which: the technician checks connector, thermistor and voltage on the board, in that order.
Warnings and stop conditions
- Highsource verified
This Fujitsu uses a flammable refrigerant and Fujitsu requires a valid certificate to work on the circuit. It also warns of electric shock: after switching off, wait 5 minutes before touching electrical components. Do not open the unit.
Evidence: Fujitsu General air conditioner wall mounted type service manual, ASEH07/09/12/14KMCG and AOEH07/09/12/14KMCG
Symptoms and causes
Probable causes · in order
- Thermistor connector loose or misconnectedPossible
Fujitsu's first point: connector loose, removed, wrongly connected or the cable open. It is fixed by reconnecting and resetting the power.
Evidence: Fujitsu General air conditioner wall mounted type service manual, ASEH07/09/12/14KMCG and AOEH07/09/12/14KMCG
- Failed heat exchanger thermistorPossible
Confirmed by measuring its resistance against Fujitsu's thermistor value table; if it is open or shorted, it is replaced.
Evidence: Fujitsu General air conditioner wall mounted type service manual, ASEH07/09/12/14KMCG and AOEH07/09/12/14KMCG
- Indoor main boardPossible
Only if connector and thermistor are fine: Fujitsu says to replace the board if the 5 V DC does not appear at the thermistor terminal.
Evidence: Fujitsu General air conditioner wall mounted type service manual, ASEH07/09/12/14KMCG and AOEH07/09/12/14KMCG
Virtual technician
Does 42 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Fujitsu General · 2024 · manual
- 2
Fujitsu General operation manual, wall-mounted air conditioner ASYG12KGTF
Fujitsu General · manual
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