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Discussion Starter · #1 ·
Hello, I have a 2003 325 Ci Convertable (automatic, in UK) which has the bifecta issue (DSC+amber brake light warning). Plugging in a code reader lists "5E21 Pressure sensor 2". I have replaced this but the code was still present. 6 months later I have replaced the sensor again in case the first was a dud along with bleeding the brakes and cleaning the ends to the plug which goes into the sensor, but the error persists and as every guide and post I seem to find online says that replacing the sensor solves the problem (which in this case, it hasnt), I am a bit stuck on where to go next.

To give some extra background, 6 months ago, I also replaced pressure sensor 1 when I first replaced sensor 2 (late August) to be proactive, which then also triggered an error code, 5E20 (Pressure sensor 1) and upgraded the bifecta to a trifecta when the ABS light then came on, along with a handful of front wheel speed sensor faults for both front wheels. However this was solved by changing both front wheel speed sensors - once done and clearing the error codes, 5E20 and the wheel speed sensor codes have never came back and the ABS light has also stayed off. But the bifecta and 5E21 remains.

So to sum it up:
  • Bifecta issue present since early August
  • 5E21 pressure sensor 2 fault is present
  • It reappears as soon as you clear the fault codes
  • The DSC and amber brake light are constantly on (they have not turned themselves back off after driving, nor is it an intermittant issue) and once you start driving, you hear a beep after a few meters
  • Pressure sensor 2 (one nearest to the windscreen) has been changed twice (once in late August and once yesterday) with two different sensors
  • Brakes have been bled. No brake fluid appears to be leaking.
  • Plug connectors have been cleaned.
  • Car itself appears to behave - certainly no brake issues etc and brakes feel ok
  • Trifecta appeared when pressure sensor 1 and 2 were first changed (late August) but was sorted by changing both front wheel sensors in January, also solving the 5e20 pressure sensor 1 problem - none of these codes have appeared since.

Looking around online appears to yield no answers beyond "Change Pressure sensor 2" as every case I find of doing this is able to solve the issue, which in this case has not. Could it be the rear wheel sensors also need changed, even though they are not producing an error code? (Given changing pressure sensor 1 then appeared to trigger the front wheel sensor issues).
Hopefully it is not the DSC unit itself - the fact that the trifecta issue has resolved makes me think it is ok.
If it is not the rear speed sensors, does anybody know what could be causing this issue or any other thoughts and areas to try?
 

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2004 330Ci 115k miles
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16,298 Posts
I suspect it is the DSC module itself that is internally bad. The common failure mode is some internal connection(s) fail, sometimes intermittantly (which can be maddening). Replacing with a good used one from a newer model (2004+) E46 and coding it to your car is the cost-efficient fix.

Have you tested the pressure sensors using a multimeter? Search on here for what/how to measure them. You would need a helper to push the brake pedal to various positions while you take readings. I believe it is best to always test a part before replacing it, to avoid unnecesary guesswork.
 

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Discussion Starter · #3 ·
Thanks BaliDawg foir your response - I will admit I was hoping you were not going to say the DCS unit! Its odd as otherwise it is all good, I even unplugged the right rear sensor from its little black box (Not the end that goes into the wheel) last night to see if the DCS would flag it and it immediately did so. Thankfully used ones do not appear as expensive as I first feared and there a few guides on how to replace.

I thought I would give the pressure sensor I removed (which is only 6ish months old) a test but couldnt find my multimeter, so looks like I will need to purchase another. Until then I will bleed the rear brakes again as I done some more looking around and a couple of cases said they had to bleed the rears 3-4 times before their error code finally dissappeared. Plus its easy and cost free to do.

I may also give the rear wheel sensors a clean and see if that does anything (as the fronts were listed as ok until pressure sensor 1 was replaced - after that, they triggered error codes, pulled them out and one of them looked melted!). Failing that, I already have a K+DCAN cable on its way so I can use the INPA software in case that finds a bit more information than my normal code reader (for £15 its worth a go!). I do know the car had a wheel alignment about a month or two before this all hapened, somewhere I read it could be the steering sensor needs recalibrated - its probably not that, but again, its a job which INPA can do, and one more thing to rule out!

I will keep this post updated, if only in case if somebody else ends up with the same issue then theres some info on it!
 

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2004 330Ci 115k miles
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Set up BMW Standard Tools on a cheap old Windows laptop and use INPA to verify DSC signals under question, such as wheel speed, brake pressure, etc. It is way easier than manually probing at various electrical connectors. Plus you can see the freeze frame data with the trouble code, which gives context to when it was triggered.

The idea here is to verify the sensor is working properly yet the DSC is not seeing its signal. This proves DSC is at fault.

A used MK60 is less than $US 100 and simple to replace. I can remotely code it to your car. Being RHD, I think it is fairily accessible if mounted on left (passenger) side, correct?
 

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Discussion Starter · #5 ·
Set up BMW Standard Tools on a cheap old Windows laptop and use INPA to verify DSC signals under question, such as wheel speed, brake pressure, etc. It is way easier than manually probing at various electrical connectors. Plus you can see the freeze frame data with the trouble code, which gives context to when it was triggered.

The idea here is to verify the sensor is working properly yet the DSC is not seeing its signal. This proves DSC is at fault.

A used MK60 is less than $US 100 and simple to replace. I can remotely code it to your car. Being RHD, I think it is fairily accessible if mounted on left (passenger) side, correct?
Sorry about the delay - I received an INPA cable only to find it needed the 7/8 pin adapter. So had to order it again and wait for it to arrive, which it finally did two hours ago.

So I performed a check on INPA and these were the two readings, the first is with me not touching anything and the second one is when I put my foot on the brakes - sensor 1 correctly picks this up. HOWEVER sensor two did not register anything, although it flips between two readings of its own accord, 194.16 mostly but sometimes flicks to 193.96 for a second or two before snapping back to 194.16. I also checked the steering angle sensor and this looked good. This was all performed when stationary:
923120

IMG_3516.JPG


Interestingly, I decided to look around the INPA a little and check the temperature area. I spotted the Voltage Uref would also alternate between two values of its own accord, 9.9 but flips to 0.0 for a second or two before going back to 9.9. Could this be tied in? Is it going to be something unrelated?

IMG_3522.JPG

IMG_3524.JPG


Regarding the DSC unit location, its located on the right hand side of the car same as the drivers side (RHD) with the brake fluid reservoir but at first glance it doesnt look too hard to remove. Hope the snapshots help!
 

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E30M3 Race F10 535 R1150Rt M Coupe
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You're going to have to dig in there and do some testing!
Conversely you could just throw parts at it (DSC unit) code and if it works, well good for you.
If it doesn't you're back to the testing regimen I've suggested.

I always prefer to diagnose, know what/why, then fix.

Rules to live by when fixing cars; The Three C's: Complaint, Cause, Correction.
With an emphasis on the later two!
 

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Discussion Starter · #7 ·
Did you able to fix it. If so, how did you fix it? Uref reads the same as mine. Put a 2-junkyard pressure sensor and a amazon on cheap one. Same error code 5e21
Sort of. BaliDawg was right in that there was a problem with the DSC unit itself. however there was a bit more to it.

So I ordered a replacement (pre-owned) DSC unit. I replaced the old one with the new one and the 5E21 issue was still there. So I swapped the sensors round. The 5E21 issue dissappears but 5E20 code now appears (pressure sensor 1). I then put the old DSC unit back in, and there was a 5E20 and 5E21 error (Both pressure sensors). With the old unit still in, I swapped the sensors again and 5E20 stops but 5E21 remains. Once more I put the replacement DSC unit in to the car and 5E21 remains. Swap the sensors round once more, 5E20 comes back while 5E21 goes away.

In short, the DSC unit did have an issue where it was constantly spitting out a 5E21 issue but with no other code so there was a definate fault. However this was also masking a dud sensor.

Final thought however was that I changed the dud sensor with the one it replaced a few days prior and this was also dud. Either I was very unlucky with the sensors I bought or the DSC unit was also frying sensors. I suspect the former as the sensors were both cheaper OEMs from ebay.
 

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unless it's mentioned, I didn't see which u considered pressure sensor 1 or 2. there are many conflicting posts on interweb which is which. I think most says #1 is the one closer to the front of car and #2 closer to the driver.

when I had pressure sensor #2 fault, (i believe) i replaced the one closer to the air filter, as my understanding is numbering starts with #1 closer to the brake booster.

with much time and effort consumed, (me assuming our numbering differs) we could only wish I'm wrong.
 
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