H8 • Daikin
Fault code H8 Daikin FTXJ: compressor input fault — diagnosis
On documented FTXJ wall splits (installer guide 4PFR769827), H8 signals a compressor input system fault on the outdoor unit — not an indoor regulation sensor fault.
Proof status : Manufacturer reference listed — check model coverage
Source: Daikin documentation (installer/service manual – according to product reference) · Last verified 9 September 2026
Founder, AskMarcel.app
Founder of AskMarcel.app. Sheets are written from manufacturer documentation; the reference is shown when it has been verified.
In brief
Fault code H8 on Daikin means: On documented FTXJ wall splits (installer guide 4PFR769827), H8 signals a compressor input system fault on the outdoor unit — not an indoor regulation sensor fault. Most common cause: Failed compressor supply, loose or oxidised inverter connections, or compressor/inverter fault. The OEM procedure targets compressor input first. First action: Isolate the power supply before any work on the wiring.. Call a registered engineer if the fault returns after basic checks or involves refrigerant work (F-Gas registered engineer required).
Per FTXJ installer guide (4PFR769827, p.55), H8 signals a compressor input system fault on the outdoor unit.
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What does fault code H8 on Daikin mean?
On documented FTXJ wall splits (installer guide 4PFR769827), H8 signals a compressor input system fault on the outdoor unit — not an indoor regulation sensor fault.
Failed compressor supply, loose or oxidised inverter connections, or compressor/inverter fault. The OEM procedure targets compressor input first.
Differences between product ranges
| Range | Meaning | Exclusion |
|---|---|---|
| FTXJ / FTXJ-AW9 / FTXJ-AB9 (wall splits) | Compressor input system fault (installer guide p.55) | Do not extrapolate to Altherma air-to-water without that model's manual |
| Altherma 4 (air-to-water) | Not documented in the cited FTXJ source — open the Altherma 4 manual for your reference | Historical « Altherma 4 » URL kept; technical meaning here is FTXJ only |
Symptoms
- Heat pump shutdown or degraded operation
- Intermittent restarts
- Fault code H8 displayed on the user interface
- Reduced heating/DHW performance
Safety notice
Isolate power before working on inverter wiring or the outdoor board. Power-circuit and compressor tests are for registered engineers only.
Diagnosis
1. Secure and Record the Context
- Isolate the power supply before any work on the wiring.
- Note the exact model, configuration (heating/DHW), and the context of occurrence (start-up, defrost, DHW).
- Check if other fault codes are present in the history.
2. Check Connections and Wiring
- Visually inspect connectors (oxidation, poor contact, bent pins).
- Check terminal block tightness and cable routing (abrasion, pinching).
- If possible, re-clip connectors and test stability on restart.
3. Isolate Sensor / Board / Communication
- Compare available measurements (temperatures, pressures) with actual behaviour.
- Test by substitution if a sensor is suspected (if you have the part).
- If the fault persists, focus on the relevant board/module and the manufacturer's procedure.
Repair and return to service — Daikin fault H8
Per FTXJ installer guide (p.55): verify compressor supply, inverter wiring continuity and compressor condition before declaring the board faulty. This is not a remote reset procedure.
When should you call an F-Gas engineer?
Call an F-Gas engineer if the fault persists after connector checks, after a recent surge, or before replacing compressor/inverter board.
Affected models
- Altherma 4
- FTXJ-AB9
- FTXJ-AW9
- FTXJ-AS9
Field context
May appear after a storm (surge), long period of inactivity, or attempted start-up with the service valve closed.
Technical source data
The H8 documented here is the FTXJ installer-guide meaning (4PFR769827 p.55): compressor input system on the outdoor unit. The historical « Altherma 4 » URL must not be generalised to air-to-water PAC without that model's manual.
Sources
- ftxj-ab9-installer-reference-guide-4pfr769827-6c-french.pdf — 4PFR769827-6C (p. 55)
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Exact match
H8 — Daikin
On documented FTXJ wall splits (installer guide 4PFR769827), H8 signals a compressor input system fault on the outdoor unit — not an indoor regulation sensor fault.
Source : Daikin documentation (installer/service manual – according to product reference)
Frequently asked questions
Is H8 on Altherma 4 the same as on FTXJ?
This article documents H8 as defined in the FTXJ installer guide (p.55): compressor input. On an Altherma 4 heat pump, open that model's manual — the meaning of H8 may differ.
Is the Daikin H8 always a compressor fault?
No: the installer manual first points to the compressor input system (power supply, wiring, inverter board) — not the compressor alone.
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