Hi Guys:
I just got the code 4340 shortly after switching to LED lights, to include the third brake light on the spoiler and the rear glass. I've ordered a new switch considering that they're affordable and my car is 92K miles already.

My brake lights work just fine, just the codes show even when I reset them. Anybody else running LEDs?

If the switch doesn't fix the issue, next step would be to switch back to incandescent lights.

I'll update with the result.

Incredibly, 12 years later, still a useful awesome post. Thanks mate.

Looking forward for the job tomorrow :-)

Update:

I know is an old post but, just as it did for me, somebody will find it useful.

It was the LED lights. In summary, replaced the brake light switch (although the old one tested fine) and the problem continued. Just to test, I replaced the third brake lights back to the incandescent bulbs, reset the code and it has not come back.

I also found some non-Porsche related posts in another forum about similar problems and describes a fix using a 50W 30A resistor inline with the third stop light or one of the main stop lights. Seems like older car with CAN bus suffer from this. Maybe the ABS computer is expecting a specific draw in power from the stop lights and throws an error code when the draw is lower than expected.

For now, I went back to replacing the third brake light bulbs back to incandescent while leaving the rest on LED. I ordered an inline 50W30A resistor (very cheap on Amazon) and will test the fix when it gets here.

Hope it helps the next guy down the line.

Regards,

The gift that keeps on giving!
Pleased this and the subsequent posts have been so helpful.

Yes, great topic.
I have this problem, ..but I think from another reason : being a lover of *firm* brake pedal feel, I changed the OEM master cylinder (of my 2001 Boxster S, that has PSM) against a TRW (oem supplier) larger one. (used on GT2 and GT3).
The results are amazing, just getting the perfect brakes !
(it is true that in the front, I have 350mm floating rotors and 6 piston monobloc calipers).
Well, since the change of the master cylinder, I have a lot of "4460" with ABS And PSM lights on. Some time I'm free of these for 100 miles or so, hoping for cure, but... no, lights come back !

There are plenty of theory on that, where the big MC do not have the same fluid circuit than the OEM MC.
This may be one of the reason, i.e.: building up too much fluid pressure ..too early.

I want to revisit the switch problem (for sure, I did make that change).
But the one that I got, do not have the plunger "clicks" adjustment.
I still tend to think that yes, the problem is having too much pressure too early (btw, this is what makes the super great feeling of the brakes), but I think that what generates the PSM and ABS lights on (error 4460) is the difference on the timing between the reading of the pressure sensor (on the PSM pump assembly) ..and the Brake stop switch !
I.e.: if in the brake pedal travel, you have a certain level of pressure to close, ..or earlier than the brake on switch, .. then you got it.
I think one way is to have one of those brake pedal switch that has the clicks adjustment, or, may be a kind of Teflon spacer between to top of the switch and the pedal space that touch the switch, this to have the brake switch on a bit earlier.
I did not had time to work on this yet, but I'm going to try.
Some guys on other forums think that the big MC built too much pressure, but frankly, I do not think so. I would imagine that one that brake REAL hard, with the OEM MC of the 986, would also got high pressure, ..but much "later" in the pedal travel distance !

It seems to me that what I'm reading here, tend to keep me think that you really need to have Pedal brake on first, ..and later in the pedal travel distance, pressure is building up. If the reverse happen, I get the feeling that this does trigger PSM/ABS error.
Sure, if you limit the pressure (adding a pressure limiter in the circuit), you delay the threshold of the PSM pressure sensor, ..and solve the problem.
But, I'm for trying to act at the "other end" !!

Or, depending on our brakes, ie.: great brake fluid, very well flushed with good thick pads (and rotors !), I would think that a brake switch that is not well adjusted, and tend to go on a bit late, ..can be the real source of the problem.
More to come !
..and thanks again to all of you for the great topic.

Very useful info. Just replace a faulty switch that was not passing correct info to the PASM/ABS but the Brake lights still worked. Was scary driving round corners off the brakes and the ABS kicking in, not nice at all.

I made a quick vid to show the expected vs unexpected behaviour when testing resistance with a multimeter. I know its my first post but I'm no youTuber and just thought would be useful for some who prefer moving pictures :)

https://youtu.be/jWU_NCgHiY8

https://i.imgur.com/d216D6A.png

What causes the fault is when the car does not see brake line pressure and switch connectivity at the same (relative) time. Very common problem that you all have found the cure for. A scan tool with live data will help confirm the issue before parts replacement.

Cheers

Very interesting.

Recently I noticed the rear brake lights on my 996 C2 Tiptronic were stuck on. I thought this odd because the switch was changed only 2500 miles ago when it was diagnosed by the garage as an issue.

Then I started getting an ABS/PSM error warning on the dash. First off occasionally, them more frequently. Errors disappear when you knock off the car and only reappear after some minutes driving. I got out of the car a few times and found the brake lights stuck on.

So I suspect its brake switch related and order a new one and fit it. Pig if a job btw.

Brake lights now work fine, and I go for a drive. Error messages come up and I let the car stop without using the brakes and find.....brake lights stuck on again!!

On investigation the 'ratchet' in the switch 'piston has settled in a position where the brake pedal has stopped ever so slightly away and so the piston doesn't get compressed. I re-extend it. Happens all over again.

But now driving with these messages on all the time the car now goes into 'limp' mode and just ticks over. Sometimes brake lights on, sometime not. Engine resumes normal service after a minute or two parked, running. Very disconcerting.

So, there being no way to adjust the pedal position or the switch, I make up a 'spacer' on the face if the pedal to ensure it contacts the piston on the switch all the time, to prove a theory. Brake light service resumes as normal and.....5 minutes later ABS/PSM error is back. I've just triple-checked and the brake lights are working as normal, spacer still effective.

Now my local mechanic plugged his tester into the car and got error 4340 Brake Switch A Circuit. What function is A as distinct from B ?

I'm getting really cheesed off with it at this stage and it's annual inspection is due shortly which it won't pass with this light on.

Looking at your video om wondering why your OEM doesn't have the ratchet feature. Is that true of all OEM Porsche switches ?

And another PSM & ABS light alarms......................i bought recently a gt street Techart 996 turbo , ans i have this problem.

I replaced the stop light swith pedal , with a Porsche part , no result,

Durametric says nothing .

ABS sensors ???

I'm going to change the brakes fluid , hoping that could be a solution ...............but there are plenty of possibilities.

I'm a bit lost.:rolleyes:

Load test your battery; when the battery is on the way out, it will trigger ABS and PSM faults.

Ok , thanks , but but my battery seems to be ok , voltage is good , and Ctek charger always connected.

:)

Load test it................

Also check that MAF.

I tore the connector off of my wire recently (long story) and that abs psm light came on straight away.

Computer wants to know what's happening at the intake so it can make good psm decisions.

THX mates for all these advices ,

I'm going to check battery load , and changing MAF in a first time , and will let you know the result. ;)

Some news ,

I have had my battery load tested , all is ok.

I'm going to replace today the mass air sensor , and i will see..............

I don't think it's expensive to buy a new one, but you may try to inspect and clean the old one first.

There is a thin wire drop inside which should be visible. If it is very dirty or broken then that would confirm the problem.

Good luck and please do report back.

Mass air sensor changed , stop light switch changed , and still the psm / abs problem.

I'm a bit fed up with this problem ,

I'm going to be obliged to have a real piwi test to check that , and finding exactly where is located the trouble.

My durametric pro version says NOTHING , this tool is just .............

ABS sensors , hydraulic pump.................i'll find it , but it's really boring at long.

Next step : i'm going to change the abs hydraulic unit sensor , this could be a solution on my own.

Excellent posts. I have a problem with a 2004 996 C4S. When I turn on the parking lights I get an ABS/PSM error. Diametric indicates P0571 DTC. I went through multiple brake sensor switches that would not fix the problem. Finally had a local indie shop diagnose and they replaced the brake switch and no joy. I don’t think they validated their repair because there’s no change on the condition. Would appreciate solutions from anyone with similar experience. At this point will start looking for parking light connector and bulb issues and then possibly the headlamp dash switch. Not sure that a bad fuse is an issue but easy enough to check.

In your case , on my opinion , it could be a light swith problem , simply.