ALLPCB
Case overview
A 2021 Mercedes-Benz E260L with an M264 engine and approximately 15,000 km recorded. The owner reported that the multimedia display's air quality option showed "sensor stopped functioning" (Figure 1). Under normal conditions the display should show the PM2.5 index (Figure 2).
Figure 1: Air quality option showing "Sensor stopped functioning"
Figure 2: PM2.5 index on the multimedia display
Initial diagnostics
A fault scanner found a current fault code stored in the automatic climate control module (N22/1): "B1A7004 particulate sensor (B31/4) functional fault, internal fault present".
Component location and wiring
B31/4 is installed in the driver's footwell area (Figure 4). Its wiring connector has three wires (Figure 5): power, ground, and the LIN bus. B31/4 monitors in-cabin particulate concentration and sends particulate data to N22/1 via the LIN bus.
Figure 4: Installation location of B31/4
Figure 5: B31/4 circuit and connector
Signal inspection and decoding
An attempt to read B31/4 data with the fault scanner returned no relevant data items. Multimeter checks of B31/4 power and ground were normal.
Using an automotive oscilloscope to measure the LIN bus signal at B31/4 (Figure 6) showed LIN bus activity with a high level of about 12.9 V and a low level about 1.1 V, with no obvious electrical abnormality. Decoding the signal revealed 15 data frame IDs. After disconnecting the B31/4 wiring connector and measuring the LIN bus again, comparison showed that with the connector attached frame ID 9C produced response data, while after disconnecting the connector frame ID 9C had no response. From this behavior it was inferred that frame ID 9C corresponds to B31/4.
Figure 6: LIN bus waveform at B31/4 on the fault vehicle
Comparison with a normal vehicle
Measuring and decoding the same LIN bus on a normal vehicle's B31/4 showed that frame ID 9C response data had a changing first byte, and the multimedia display's PM2.5 index changed synchronously. Further analysis showed the decimal value of the first byte (interpreted from hexadecimal) matched the PM2.5 index shown on the multimedia display. For example, hex 10 corresponds to decimal 16, hex 0F corresponds to decimal 15, and the display showed 16 or 15 accordingly.
Diagnosis and repair
Analysis shows that although frame ID 9C had response data, the response payload did not change; the first byte remained FF (255 decimal). This behavior is consistent with the B31/4 internal processor having crashed and repeatedly sending the same erroneous data. Based on these findings, B31/4 was confirmed to be defective.
After replacing B31/4 and road testing, the multimedia display's air quality option returned to normal and the fault was cleared.
Fault summary
Although the fault scanner could not read B31/4 data, an oscilloscope with decoding capability allowed data-level analysis of B31/4 signals and enabled precise fault localization.
Reprinted from Automotive Maintenance and Repair, 2025 Jan first half-month issue
