P0137 low voltage O2 sensor bank 1 sensor 2 W205

[ML350_Colin]

The Engine Management Light illuminated last month on our W205 C200 petrol 7G automatic. I have an iCarsoft reader and it reported the error low voltage o2 sensor bank 1 sensor 2 which I cleared and it came back after a few of runs from cold.

I have replaced the oxygen sensor (Bosch component) and erased the mistake. In accordance with the service schedule, I performed an oil and filter change and changed the spark plugs, since the vehicle has reached 38,000 kilometres and is now 8 years old (2016). I believed this issue had been resolved, but after five drives from a perfectly fresh start, it reoccurred today. The MOT is scheduled for next week, and while I believe I could temporarily resolve the issue by resetting it just before to the test, I am want to thoroughly investigate the underlying problem.

I believe 'Star' readers may provide more insights, so I may need to see a local Mercedes expert, since I prefer not to engage with the major dealer due to a negative experience with my previous business vehicles before retirement. I am willing to do more physical or visual inspections prior to reaching that level; thus, I am publishing here to get comments or insights from others who have encountered this issue.

[bvlenci]

The second O2 sensor is located downstream of the catalytic converter. This indicates that the mixture is lean, either due to excessive oxygen, an air leak in the exhaust system, or the ECU reaching the rich limit for closed loop control, while still insufficiently injecting fuel—possibly due to low fuel pressure or malfunctioning injectors. Examine the current O2 sensor 2 measurement; there may be a disruption in the wiring to it. Verify the measurements for O2 sensor 1 (pre-catalytic converter). If this is functioning correctly (0.2 - 0.8 volts every few seconds), with a lambda measurement of about 1.00 once the engine has reached operating temperature.

The post-catalytic converter oxygen sensor typically provides a stable value of around 0.5 volts.

If a visual inspection of the exhaust manifold, pipes, and catalytic converter is OK, there are no observable leaks on the intake manifold, and the fuel pressure is adequate, it may be prudent to consult an expert.

[bvlenci]

https://en.wikipedia.org/wiki/Oxygen_sensor

[ML350_Colin]

Thank you for your response.

I have inspected the wire from the sensor to the best of my ability, as it vanishes into the loom inside the confines of the engine compartment. Additionally, verified the ground continuity to the sensor's body, which measured around 0 ohms, indicating a satisfactory connection. The pre-catalytic converter sensor activates, as you indicated, once it reaches operational temperature, while both the pre- and post-catalytic converter sensors seem to initiate at a constant voltage during open loop mode during warm-up, with typical 'cold' readings of 1.275V for the pre-catalytic converter and 0.200V for the post-catalytic converter.

Infrequently, when starting from a cold state, the post-catalytic converter oxygen sensor displays 0.000V during open loop mode, albeit it is uncertain whether this is a misleading indicator.

I've drove the vehicle while my wife checks the live readings and the two sensors appear to go up and down in sync once warmed up, the pre cat value normally higher and the lambda reading around 1.000 too.

I have thoroughly inspected the exhaust from underneath and seen no visual or auditory signs of a leak. I have seen a high-pitched screech when the engine is shut off, which has been occurring for an extended period prior to the illumination of the EML. The scream ceases when the throttle body undergoes a reset (I can hear a clicking sound from that region after shutting down), but it sometimes reemerges. I have attempted to identify the source of the scream; but, its intermittent nature complicates the determination of whether applying pressure to a pipe or component influences it.

I believe I may need to see an expert about Star. I detest capitulating, and I want to prevent them from indiscriminately tossing components at the vehicle in an attempt to rectify it; thus, I aim to ensure that I have thoroughly examined all possibilities beforehand. I will need to submit it for the MOT to see whether any issues are identified. I have a family visit approaching, so I will have limited time for repairs after the MOT due to poor scheduling. I am reluctant to do a 300-mile journey in our Alfa Mito, therefore I may ultimately resort to renting a vehicle at this pace. 😞

The EML activates earlier when my wife operates the vehicle.I typically drive more cautiously, whereas she accelerates more aggressively between traffic signals. The EML may activate after two drives with her, but it may need over five drives with me; nevertheless, I am uncertain how to interpret this. 😄

[Rashman4u]

If the catalyst is functioning properly, the pre and post sensors should not exhibit correlated readings. The pre-catalyst sensor should approximately exhibit a sine wave pattern. The post-catalyst sensor should register a low value if the catalyst is functioning well.

[ML350_Colin]

That does not seem favourable. It is somewhat regrettable that the vehicle has only endured 38,000 kilometres.

The back O2 sensor sometimes registers around 0.200V with few fluctuations, however the pre-catalytic converter sensor exhibits erratic changes. I have used a cat cleaner in the vehicle, but it seems to have little effect (I generally consider additions to be very desperate, to be frank).

The vehicle may fail the MOT on emissions if the catalytic converter is malfunctioning, irrespective of whether the engine management light is on at that moment.

On the positive side, I can do the replacement myself; nevertheless, it will not be economical if this is really the issue. 😞

[developer]

An illuminated EML will always result in failure.

[ML350_Colin]

I appreciate that; but, I usually clear the EML, and it remains off for many days. I am only trying to complete the MOT with the light out. I may address the matter at my convenience.