Summary

Bosch's domestic tumble dryer range spans vented, condenser, and heat pump technologies, with the heat pump (WTW-series) models now the most common new sale in the UK due to their significantly lower running costs and A+++ energy ratings. Heat pump dryers work by recirculating and dehumidifying air through a sealed refrigerant-based heat pump circuit rather than venting hot air outside or condensing moisture with mains-cold water, which introduces a distinct fault family (heat pump-specific codes) not present on simpler condenser or vented models.

Bosch's fault code scheme uses an "E" prefix followed by two digits, shown on the dryer's digital display. Codes broadly fall into families covering condensation/drainage faults, heating faults, moisture sensor faults, and — on heat pump models specifically — heat pump circuit faults. As with other appliance brands in this guide, exact code meanings can vary between model generations, and this guide should be treated as a starting point rather than a definitive manufacturer table.

This guide covers the error codes with the most consistent independent trade documentation, plus the diagnostic approach for the two highest-frequency domestic callouts: a condensation/drainage fault (the single most common tumble dryer callout across all technologies) and a heat pump fault specific to WTW-series models.

Key Facts

Quick Reference Table

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Code Likely meaning Most likely cause DIY/competent-engineer checkable?
E03 Condensation drainage error Blocked/kinked drain hose, tank not seated correctly Yes — check hose and tank first
E04 Heat pump fault (WTW-series only) Blocked heat exchanger/filter, failed impeller, refrigerant fault Partial — clean filters/exchanger first; refrigerant work needs F-Gas
E08 Condensate tank full Tank needs emptying, or float switch fault if recurring immediately Yes — empty tank, check float switch
E42 Water tank fault (condenser models) Tank full, improperly seated, float switch fault Yes — check tank seating and float
E41 Heating element relay fault Relay not switching correctly No — needs engineer-level electrical diagnosis
E56 Moisture sensor fault Fouled or failed sensor strips in drum Yes — clean sensor strips first
E63 Heating element failure Failed element No — needs engineer-level replacement
Any code not listed Unconfirmed for this specific model/generation **** —

Detailed Guidance

Diagnosing E03/E04 — the condenser/heat exchanger cleaning check

E03 or E04 displayed
        |
        v
Clean the condenser unit and both filters
(fluff filter + condenser filter, if fitted)
per the manufacturer's cleaning procedure
        |
   ----FAULT CLEARS----> Advise customer on regular
        |                 condenser cleaning schedule
        |
     FAULT PERSISTS
        |
        v
Check condensation drain hose/tank for
kinks, blockages, or incorrect seating
        |
   ----BLOCKAGE FOUND----> Clear, re-test
        |
     NO BLOCKAGE FOUND
        |
        v
(WTW/heat pump models only)
Check heat exchanger and impeller for
fluff buildup beyond the standard filter
        |
   ----BLOCKED----> Clear per manufacturer procedure, re-test
        |
     CLEAR
        |
        v
Suspect heat pump circuit fault
(refrigerant, compressor, or sensor) —
refer to F-Gas certified engineer for
heat pump models

Condenser and filter cleaning resolves the substantial majority of E03/E04 callouts on Bosch dryers, and this should always be the first check regardless of which specific code is shown — fluff and lint accumulation in the condenser unit is the dominant real-world cause across the whole fault-code range, not just the drainage-specific codes.

Diagnosing E08/E42 — condensate tank and float switch faults

Most condenser dryers without a permanent drain connection rely on a removable water tank with a float switch that triggers when full. A genuine callout distinction to make on-site:

Heating faults (E41, E63) — relay vs element distinction

These two codes point to different failure points that require different repairs:

Frequently Asked Questions

Why does my Bosch heat pump dryer keep showing E04 even after I clean the filters?

If cleaning the standard fluff filter doesn't clear the fault, check the heat exchanger itself (a separate, less frequently cleaned component on most WTW-series models) for fluff buildup — this is a very common cause of recurring E04 faults that isn't resolved by filter cleaning alone. If the heat exchanger is also clear, the fault likely points to the heat pump's refrigerant circuit or impeller, which needs specialist diagnosis.

Is E04 always a refrigerant leak on a heat pump dryer?

No — airflow blockage (fluff in the heat exchanger, a failed impeller not circulating air correctly) is a far more common cause than a genuine refrigerant fault. Refrigerant-circuit diagnosis and repair requires F-Gas certification and should only be pursued once airflow-related causes have been ruled out.

How often should the condenser be cleaned to avoid these fault codes?

Bosch recommends cleaning the condenser unit and filters regularly — as a rule of thumb, every few months for average household use, more frequently for heavy use or households with pets (pet hair accelerates fluff buildup significantly). Advising customers on a regular cleaning schedule during a callout can prevent repeat E03/E04 visits.

My dryer shows no code but clothes aren't drying properly — what should I check?

Before assuming an electrical or heating fault, check the same things that trigger E03/E04 codes — a blocked condenser, filter, or heat exchanger reduces drying performance well before it triggers a hard fault code. This is the most common cause of a "not drying properly" complaint with no displayed error.

Regulations & Standards