Bosch WTH70 Repair Manual for Error Code 5X

Overview of Error Code 5X

The Bosch WTH70 is a high‑efficiency heat‑pump air‑conditioning unit. When it encounters a fault, the display will show a two‑digit error code. Code 5X specifically points to a problem with the indoor temperature sensor or its wiring. A misreading or disconnection will trigger the protection mode to prevent damage to the compressor.

Below is a detailed, step‑by‑step manual to diagnose, repair, and verify that the WTH70 runs correctly after the error has been cleared.

What You’ll Need

All work must be performed with the unit powered off and the mains disconnected. If you are not comfortable working with electrical components, contact a qualified HVAC technician.

Understanding the Error Code

The display reads “5X” where X is a single digit that indicates the particular sensor that has failed:

The unit keeps a log of the last three codes; if the error persists after a reset, it means the fault has not been fixed.

Safety First

1. Disconnect the Power – Move the circuit breaker to the off position. Verify that the unit is dead by touching the terminal block with the multimeter set to continuity mode. No beep should sound.
2. Allow the Unit to Cool – Heat‑pump compressors can remain hot for up to 30 minutes after power is removed. Wait before touching the compressor or moving the unit.
3. Work on a Dry Surface – Avoid working in damp or wet environments to reduce the risk of electric shock.
4. Wear Protective Gear – Safety goggles and gloves are recommended, especially when using tools near electrical components.

Inspecting the Sensor Housing

The indoor temperature sensors are housed in a small metal panel on the rear of the indoor unit. To access them:

1. Remove the front panel cover with a Phillips screwdriver.
2. Locate the sensor housing – it is a rectangular box marked “TEMP SENS”.
3. Unscrew the housing and set it aside on a clean surface.

Step 1 – Test the Wiring Continuity

1. Identify the four wires connected to each sensor. Usually, there are two wires: a signal wire and a ground or power wire.
2. With the multimeter in continuity mode, touch one probe to each wire pair. A short beep indicates good continuity.
3. Record any breaks, shorts, or open circuits.

If continuity is broken, the wire must be repaired or replaced. Use electrical tape to secure the connection or replace the entire cable if it is visibly damaged.

Step 2 – Verify Sensor Output

With the unit still off, test the sensor output voltage:

1. Reconnect the sensor to the control board.
2. Re‑install the sensor housing.
3. Power on the system only for the purpose of testing; keep the unit in standby mode.
4. Using the multimeter set to 10 V, measure the voltage at the sensor connector.
5. Compare the reading to the manufacturer’s spec sheet (typically 2.5 V for each sensor).

A voltage far below or above the spec indicates a faulty sensor that must be replaced.

Step 3 – Replace the Faulty Sensor

1. Remove the defective sensor by unscrewing it from the sensor housing.
2. Install the new Bosch sensor (part number WTH70‑INS‑SEN‑01) into the same connector.
3. Tighten all screws to the manufacturer’s torque setting (usually 0.5 Nm).
4. Re‑install the sensor housing.

If the sensor was not the issue, the problem could lie in the wiring or control board. In that case, proceed to the next step.

Step 4 – Inspect the Control Board

The control board is located behind the front panel. To examine it:

1. Remove the front panel cover.
2. Look for any burn marks, loose solder joints, or damaged components.
3. Inspect the sensor input pads for corrosion or debris.

If the board shows visible damage, it must be sent to Bosch or replaced with an OEM part. A technician can re‑solder any loose connections.

Step 5 – Clear the Error Code

Once the sensor and wiring are verified, the error must be cleared:

1. Turn the unit off and wait 30 seconds.
2. Press and hold the MODE button for 5 seconds.
3. Release the button; the display should flash and then return to normal mode.

If the code reappears, double‑check the sensor wiring and the new sensor installation.

Step 6 – Verify System Operation

1. Power the unit back on.
2. Let it run for 10 minutes in cooling mode.
3. Observe the display: it should show the current indoor temperature and no error code.
4. Use an external thermometer to confirm the unit is cooling the room to the set point.

If the error returns or the unit fails to start, you may have a deeper electrical fault. At that point, professional service is recommended.

Common Pitfalls and How to Avoid Them

Preventive Maintenance Checklist

Following this schedule helps keep the WTH70 in optimal condition and reduces the likelihood of error codes.

Final Thoughts

Error code 5X in the Bosch WTH70 is a clear indicator of an indoor temperature sensor problem. By methodically testing wiring, verifying sensor output, replacing the defective sensor, and clearing the error code, you can restore normal operation without professional assistance. Always prioritize safety, use proper tools, and refer to the official service manual for specific part numbers and torque values.

With the unit repaired and running smoothly, you’ll enjoy consistent indoor comfort and improved energy efficiency. If you encounter any unexpected behavior or the error recurs, contact Bosch support or an HVAC professional to ensure your system remains reliable.