how to diagnose and fix a common problem with the B2/7

In this blog post, I will explain how to diagnose and fix a common problem with the B2/7 (Right hot film mass air flow sensor) on Mercedes-Benz vehicles. This sensor measures the amount of air entering the engine and sends a signal to the engine control unit (ECU) to adjust the fuel injection and ignition timing. If the sensor is faulty or has a loose contact, it can cause various symptoms such as poor performance, rough idle, hesitation, stalling, increased fuel consumption and emissions.

One of the possible fault codes that can be triggered by a faulty B2/7 sensor is P0104, which means that the sensor circuit has an intermittent or erratic signal. This can indicate a loose contact with low frequency between the sensor and the ECU. To confirm this diagnosis, you will need a scan tool that can read live data from the sensor and display its frequency and voltage values. You will also need a multimeter and a wiring diagram to check the continuity and resistance of the sensor circuit.

The first step is to locate the B2/7 sensor on your vehicle. It is usually mounted on the air intake hose near the air filter box on the right side of the engine. You will see a five-pin electrical connector attached to it. Disconnect the connector and inspect it for any signs of corrosion, damage or bent pins. If you find any problems, clean or replace the connector as needed.

Next, you will need to check the voltage supply and ground of the sensor circuit using your multimeter. Set your multimeter to DC volts and connect the positive lead to pin 1 of the connector (red wire) and the negative lead to pin 2 (brown wire). You should see a reading of about 12 volts with the ignition on. If not, check for a blown fuse or a broken wire in the circuit.

Then, you will need to check the signal and reference voltage of the sensor circuit using your scan tool and multimeter. Connect your scan tool to the OBD-II port under the dashboard and select live data from the B2/7 sensor. You should see a frequency value between 1 and 5 kHz and a voltage value between 0 and 5 volts depending on the air flow. If you see a fixed or zero value, there is a problem with the sensor or its wiring.

To verify this, connect your multimeter to pin 3 of the connector (yellow wire) and pin 4 (green wire). These are the signal and reference voltage wires respectively. You should see a varying voltage between 0 and 5 volts as you rev up the engine. If not, there is a break or short in one of these wires.

Finally, you will need to check the heater circuit of the sensor using your multimeter. The heater circuit helps to warm up the sensor faster and improve its accuracy. It consists of a heating element inside the sensor and two wires that connect it to pin 5 of the connector (white wire) and pin 2 (brown wire). To test it, set your multimeter to ohms and measure the resistance between these two wires. You should see a reading of about 3 ohms with the ignition off. If you see an infinite or very high resistance, there is an open circuit in the heater element or its wiring.

If you have performed all these tests and found no problems with the wiring or connector, then you can conclude that your B2/7 sensor is faulty and needs to be replaced. To do this, you will need to remove the air intake hose from the sensor and unscrew it from its mount. Then, install a new sensor in reverse order and reconnect everything. Clear any fault codes from your scan tool and test drive your vehicle to see if it runs smoothly.

I hope this blog post has been helpful for anyone who has encountered this problem with their B2/7 sensor. If you have any questions or comments, please feel free to leave them below.