The E420 error on your Bosch Dishwasher means the appliance has detected a fault with the turbidity sensor circuit, which monitors how clean or dirty the water is during a wash cycle. This usually stops the cycle partway through or prevents it from progressing to the rinse stage. In many cases this is caused by a build-up of debris on the sensor lens or a wiring fault, and it can often be cleared without calling an engineer.
| Error code | E420 |
|---|---|
| Meaning | Turbidity (soil) sensor circuit fault or blockage |
| Severity | Medium — the dishwasher may still run but with reduced wash performance |
| Time to fix | 15–40 minutes |
| Tools needed | Screwdriver, soft cloth, multimeter (optional) |
What the E420 error means
Bosch dishwashers use a turbidity sensor to measure how dirty the wash water is, which helps the machine decide how long to run the cycle and whether an extra rinse is needed. The E420 code appears when this sensor sends an inconsistent or missing signal to the control board. It's similar in concept to sensor faults found on other Bosch appliances, including the sensor and circuit issues that can trigger problems like a Bosch Drill Battery Not Connecting fault, where a poor connection rather than a broken part is often to blame.
Common causes
- Food debris, grease, or limescale coating the turbidity sensor lens
- Loose or corroded wiring connector at the sensor
- Faulty turbidity sensor unit itself
- Water quality issues (very hard water or excessive detergent residue)
- Main control board fault misreading the sensor signal
- Blocked filter or spray arm causing abnormal water turbidity readings
How to fix the E420 error
- Power down and reset: Turn off the dishwasher at the wall, wait two minutes, then switch it back on. This clears many temporary sensor glitches.
- Clean the filters and spray arms: Remove and rinse the lower filter assembly and check the spray arm holes for blockages, as debris here can affect turbidity readings.
- Locate and clean the turbidity sensor: This is usually mounted inside the tub near the base. Wipe the lens gently with a soft, damp cloth to remove grease or limescale film.
- Check the wiring connection: With the power off, access the sensor from behind the panel and confirm the connector is firmly seated and free from corrosion, much like checking a loose terminal when troubleshooting a Bosch Drill Won'T Start issue.
- Run a test cycle: Start a normal wash and watch whether the error returns partway through, which confirms whether the fault is intermittent or constant.
- Replace the sensor if needed: If cleaning and reconnecting doesn't resolve it, the turbidity sensor itself may need replacing by a qualified appliance engineer.
FAQ
Can I still use my dishwasher with an E420 error showing?
You may be able to run a cycle, but wash performance and cycle timing could be affected until the fault is fixed.
Is E420 dangerous or a safety risk?
No, it's a sensor communication fault rather than an electrical or water safety issue, so it's safe to investigate yourself with the power off.
How do I know if it's the sensor or the control board?
If cleaning the sensor and checking wiring doesn't clear the code, and a multimeter shows the sensor is working correctly, the control board is the more likely cause.
