What E:04 Means
Bosch fits a second, independent relay whose only job is to cut the heating circuit if the main relay misbehaves, and E:04 is logged when that safety relay itself does not test correctly. The chart also lists a ground fault at either the heat pump heater or the zeolite heater as a cause, because leakage on those circuits is what the safety relay is watching for in the first place.
Common Causes of E:04
- Safety heater relay in the power module has failed
- Leakage to ground in the heat pump heater
- Leakage to ground in the zeolite drying heater
🔧 One Safe Check You Can Try
Nothing here is a home check. This code concerns the backup device that protects the heating circuit, so the sensible response is to stop running cycles and book a diagnosis rather than to reset and see what happens.
⚠ Why You Shouldn't Go Further Than That
- This is the layer of protection that is supposed to act when the first layer fails, so running the machine while it is faulted removes a safeguard you cannot see working.
- Distinguishing a relay fault from heater leakage needs insulation testing, which is a different instrument from the multimeter most households own.
- Both heaters named here are replaced as sealed assemblies, so an exploratory strip-down rarely ends with anything reusable.
- Opening the base pan on a machine still inside its coverage period may affect what the manufacturer will honour, so check your terms first.
Opening a panel to inspect or repair something yourself doesn't automatically cancel your manufacturer's warranty — but it can risk accidental damage, and some manufacturers do have specific service conditions. Check your product's warranty terms, or ask us, before deciding.