Error .04

Safety precautions:

- Do not attempt internal electrical or gas repairs unless you are a qualified Gas Safe engineer. Working on gas or mains electrical components is dangerous.

- Before any visual checks, switch the boiler off at the isolator and, if you are opening covers, isolate electrical supply. If you are uncomfortable doing any of the checks below, stop and call a Gas Safe engineer.

Initial homeowner checks you can do safely:

1. Reset the boiler once using the reset button and note whether the code returns immediately or after a period of use. Repeated resets without addressing the cause are not recommended.

2. Check boiler status lights, gas supply to the property and that other gas appliances work to rule out a supply issue.

3. Check system pressure on the boiler gauge — if pressure is well below 1 bar, top up to the recommended level (around 1–1.5 bar when cold) following your boiler manual. Low pressure can cause faults.

4. Look for obvious physical issues: visible water leaks, disconnected or very loose external wiring to the boiler, or signs of rodent damage to cables. Don’t open the boiler casing for electrical work — only visual inspection from outside is recommended.

5. Note operating conditions when the fault appears (e.g., immediately on demand, after long run, only for heating or hot water). Record the exact fault code and sequence for the engineer.

Specific diagnostic and fix steps for a qualified engineer (do not attempt unless competent and Gas Safe registered):

1. Isolate gas and electrical supplies and remove the boiler cover following manufacturer instructions.

2. Visually inspect NTC connectors, wiring harness and PCB plugs for corrosion, water ingress, damaged insulation or pin damage. Re-seat all NTC plugs securely.

3. Measure resistance of each NTC at the plug with a digital multimeter and compare to the manufacturer specification (check the service manual). Many Vaillant NTCs are approx. 10 kΩ at 25°C but confirm with the boiler’s documentation. Check for open circuit, near-zero (short) or implausible steady values.

4. Check wiring harness continuity between the sensor plug and the PCB. Test for short to earth/ground and between conductors.

5. If any sensor or cable fails tests, replace the defective NTC or harness. Vaillant guidance often advises replacing all NTCs as an initial diagnostic step to eliminate sensor variability and speed up fault-finding.

6. After replacement, re-check connections, restore supplies and clear fault codes. Run the boiler and monitor sensor readings and operation to confirm the fault is resolved.

7. If sensors and wiring check good but the fault persists, test the PCB input circuits for the NTC channels — a PCB fault or blown component can present as repeated NTC errors and may require PCB replacement.

8. Investigate secondary causes if appropriate: pump operation, flow/return temperatures, blockages or airlocks which can lead to temperature limit trips and confuse diagnostics.

Important notes and when to call a professional:

- Any work involving opening the boiler, testing live circuits, replacing sensors on the primary circuit, or working with the gas supply must be carried out by a Gas Safe registered engineer.

- Do not bypass safety devices, override interlocks or jury-rig sensor connections to clear the code.

- If the simple homeowner checks (reset, pressure, visual inspection) don’t clear the fault, contact a Gas Safe engineer. Provide them the exact model (Vaillant Ecomax Pro 828E), the error code (.04 / F.04), details of when it appears, and any steps you’ve already taken. This will speed diagnosis and repair.