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I have a Mercedes B180 (Diesel) that is showing a light on the dashboard: a little engine symbol. I've had a mechanic look at it, and the code said 'oil pressure sensor', so it was changed. The light still comes on and it is the same fault code - the theory is that it is the cable from the sensor that is bad, so perhaps I should try to change that.

However, some cars, I know, have cables that are basically integrated into one, large bundle, in which case you have to replace the whole lot; a Mazda I owned years ago was like that - one of the connectors to the headlight was rusty, and it would cost £300 (+ work, of course) to fix. You can see why I'm hesitating.

So, could anybody give me some idea about whether this is a simple cable or an expensive, integrated thing? If you know a part number too, that would be great, then I could call Mercedes and ask for a price.

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  • Instead of changing parts (ie: sensor, and now wiring), you should prove the problem is the wiring. Has the mechanic checked the connector to the sensor? Has the mechanic checked to ensure the sensor which was installed is actually good/working? Are you sure you actually have oil pressure? There's a ton of things which should have been checked well before parts were replaced. It's always good to know what you're working on, because the alternative of "Remove & Replace" gets expensive really fast. Commented Sep 20, 2021 at 14:12
  • @Pᴀᴜʟsᴛᴇʀ2 I agree, he should, but ... As for myself, I'm not sure of anything except that it costs me money, and there is a balance to be struck between spending money on checking and spending it on trying to change a thing if it isn't too expensive. If I knew about cars, I wouldn't go to a mechanic as much.
    – j4nd3r53n
    Commented Sep 20, 2021 at 14:20
  • I understand what you're saying, however, when it comes to cost, when you "R&R" instead of diagnosing, it will invariably cost you more money. As it stands, you should ask for them to replace the old part and get your money back. Commented Sep 20, 2021 at 14:27
  • Checking that you really have oil pressure is very cheap compared to replacing a blown engine.
    – Solar Mike
    Commented Sep 20, 2021 at 15:11
  • @SolarMike Not sure if I understand - you mean whether I actually have oil in the engine? I know that, at least; the car is serviced regularly, and of course I check the dipstick myself.
    – j4nd3r53n
    Commented Sep 20, 2021 at 15:41

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It is highly unlikely you will find any modern car that will have a totally separate cable just for the oil pressure sensor. The engine will likely have a single harness that has all connections integrated.

That being said, a decent mechanic should be able to diagnose if there is a broken wire or a bad connector. If it is a bad connector, it shouldn’t be difficult to replace the connector. If there is a broken wire an additional wire could be added to the harness to substitute for the broken one.

The wire from the oil pressure sensor will undoubtedly go to the engine ECU, so it should be easy enough to check with a multimeter and replace the single wire if necessary.

As others have commented, it is essential that you know that the oil pump is working and supplying the correct oil pressure. Oil sat in the sump will not lubricate the engine.

The mechanic should be able to screw in a mechanical oil pressure gauge to verify that the oil pump is working correctly.

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