The Bosch refrigerator compressor is the heart of the cooling system, pumping refrigerant through the sealed circuit to remove heat from the interior. When it fails, the refrigerator stops cooling entirely. However, many symptoms that appear to be compressor failure are actually caused by supporting components that are far less expensive to replace. This guide walks through a systematic diagnosis starting with the simplest and cheapest possibilities before moving to the compressor itself.
Bosch refrigerator compressor failure symptoms
Before diagnosing, confirm you are seeing genuine compressor-related symptoms. A failing compressor typically presents as the refrigerator not cooling in either compartment, the compressor not running at all (no humming sound from the back), the compressor starting briefly then shutting off with a click, or the compressor running continuously without achieving target temperatures. Each symptom points to different components in the compressor circuit.
Step 1: Check the power supply and condenser coils
Verify the refrigerator has power and the outlet is functioning. Then clean the condenser coils. This is not busywork. Dust-clogged condenser coils are the number one reason a compressor overheats and shuts down on thermal protection. The coils are located behind the kick plate at the bottom or on the rear of the unit. Vacuum them with a brush attachment and use a coil cleaning brush to reach between the fins. After cleaning, allow the compressor 30 minutes to cool down, then plug the unit back in and listen for the compressor to start. If it runs normally after coil cleaning, the compressor itself is fine and overheating was the issue.
Step 2: Test the start relay
The start relay provides the initial power surge that gets the compressor motor spinning. It is a small device plugged directly into the compressor terminals, accessible from the rear of the refrigerator behind the lower access panel. Unplug the refrigerator, pull the start relay off the compressor pins, and shake it. If it rattles, the internal contacts have broken and the relay has failed. This is one of the most common and least expensive fixes for a non-starting compressor, costing $20 to $40 for the part. Replace the relay and test.
You can also test the relay with a multimeter. Place the probes on the relay terminals (between the Start and Run connections). You should see continuity in one orientation. If the relay shows no continuity in either direction, it has failed.
Step 3: Check the condenser fan motor
The condenser fan pulls air across the condenser coils to dissipate heat. If the fan motor has failed, the condenser cannot release heat effectively, and the compressor overheats and shuts down. With the refrigerator running (or attempting to run), listen for the condenser fan at the rear bottom of the unit. If the compressor is humming but you do not hear the fan, the fan motor may have seized. Spin the fan blade by hand. It should turn freely. If it is stiff or seized, replace the fan motor. This is a $30 to $60 part and a 30-minute repair that resolves many apparent compressor failures.
Step 4: Test temperature controls and thermistor
The control board uses thermistor (NTC sensor) readings to decide when to run the compressor. If a thermistor reads incorrectly, the board may not signal the compressor to start. Locate the thermistors, which are small probes clipped to the evaporator or mounted inside the compartment walls. Disconnect a thermistor and measure its resistance with a multimeter. At room temperature (approximately 77 degrees Fahrenheit), a typical Bosch NTC sensor reads 4,000 to 6,000 ohms. At freezer temperatures (0 degrees Fahrenheit), it reads approximately 15,000 to 20,000 ohms. A reading far outside these ranges indicates a failed sensor. Replacing a thermistor costs $10 to $25 and takes minutes. Check all refrigerator error codes on the display for sensor-specific faults.
Step 5: Compressor winding resistance test
If the start relay, condenser fan, and thermistors are all functional but the compressor still will not run, test the compressor windings directly. Unplug the refrigerator. Remove the start relay and overload protector from the compressor pins, exposing three terminals: Common (C), Start (S), and Run (R). Using a multimeter set to ohms, measure resistance between each pair of terminals:
- C to S — Start winding resistance (typically 5 to 15 ohms)
- C to R — Run winding resistance (typically 3 to 10 ohms)
- S to R — Should equal the sum of C-to-S plus C-to-R
If any pair shows no continuity (infinite resistance), the winding is open and the compressor has failed. If any pair shows near-zero resistance, a winding is shorted. Also test from each terminal to the compressor housing (ground). Any continuity to ground indicates a grounded winding, which is a safety hazard and confirms compressor failure.
Compressor noise diagnosis
A compressor that runs but makes abnormal noises is in its final stages. A loud buzzing followed by clicking indicates the motor is drawing excessive current and the overload protector is tripping repeatedly. A knocking or clanking sound suggests internal mechanical damage to the piston or valve plate. A high-pitched whine may indicate a refrigerant leak, as the compressor struggles against abnormal pressures. All of these require sealed-system service. For model-specific compressor specifications and warranty information, visit bosch-home.com.
When to call a technician
Condenser coil cleaning, start relay replacement, condenser fan motor replacement, and thermistor testing are all manageable DIY tasks. Compressor winding resistance testing requires a multimeter and some electrical knowledge but is safe on an unplugged unit. However, compressor replacement is a sealed-system repair that requires EPA Section 608 certification for refrigerant recovery and recharge. Do not attempt compressor replacement yourself. Schedule Bosch refrigerator repair with a certified technician for compressor diagnosis and replacement.
