Bosch washers are built to be efficient and reliable, but like any smart appliance they use error codes to flag problems. If your display shows Er:04, the machine is telling you something’s off in the control module—the electronic “brain” that coordinates wash cycles, water levels, motor speed, and safety checks.

What Er:04 usually means

In plain terms, the washer’s control electronics detected a faulty signal or a condition it can’t safely manage. That can be a brief software hiccup (easy) or a persistent hardware issue (needs attention). Power blips, moisture on connectors, or a loose harness can all trigger it; so can a failing board.

First thing to try — a proper reset

Power the washer off and unplug it. Give it a solid 3 minutes so the control capacitors discharge and the logic clears. Plug it back in and run a short cycle (rinse/spin is fine). If the code doesn’t come back, you likely had a momentary glitch—keep using the machine and just keep an eye on it.

If Er:04 returns

Work through these simple, safe checks before calling a technician:

• Power quality and outlet: Make sure the plug is seated firmly and the outlet isn’t shared with heavy loads that cause voltage dips. If you’re using a surge strip, try a direct wall outlet for the test cycle.
• Moisture and condensation: Look for any signs of water under or behind the washer. Humid laundry rooms and minor leaks can leave moisture on connectors, which confuses the control board. Dry any damp areas, improve ventilation, and try the reset again.
• Overloading and cycle abuse: Very heavy loads, chronic overdosing of detergent (excess suds), or consistently selecting the wrong cycle can push sensors out of range and provoke a fault. Run a small, balanced load and see if the message clears.
• Door and latch behavior: If the door wasn’t fully latched or the latch hesitated, the control can throw a general fault. Close the door firmly and retry.

If the code reappears immediately or mid-cycle after these checks, you’re likely looking at a wiring or control issue—for example, an oxidized connector on the main board, a damaged wire harness, or a failing control module.

When to call a pro (and what they’ll do)

At this point, it’s reasonable to get a qualified Bosch technician involved. They’ll put the washer into service mode to read stored faults, inspect the main PCB and connectors for heat marks or corrosion, test supply voltage and ground continuity, and verify sensor inputs (pressure switch, door lock signal, motor feedback). If needed, they’ll re-seat or replace the harness, update or replace the control board, and run a full diagnostic cycle.

Prevention going forward

Keep the washer on a stable, dedicated outlet; use the right amount of HE detergent; avoid chronic overloading; and keep the area dry with decent airflow. Those small habits go a long way toward preventing control-module complaints like Er:04 from popping up again.

Er:04 points to the washer’s control system detecting something it can’t reconcile. A careful unplug–wait–replug reset often clears a one-off glitch. If the message returns, rule out power, moisture, loading, and latch issues. Persistent Er:04 after that typically needs a technician to check wiring and the control board and, if required, replace the module.