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Clifton B.
Clifton B., Auto Service Technician
Category: Porsche
Satisfied Customers: 3905
Experience:  Master Auto Tech
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2004 Carrera 4s. When driving with headlights off, all is

Customer Question

2004 Carrera 4s. When driving with headlights off, all is well. As soon as I turn on the headlights, both the ABS PSM warning lights go on.
Submitted: 1 year ago.
Category: Porsche
Expert:  rpmdiag replied 1 year ago.

Hi, I’m Ron and I can help you.
I have been in this business since 1975 and also teach automotive repair. I am confident we can resolve your problem together. Please understand that I don't know your skill level. I can't see, smell, hear, or touch the vehicle either, so it may take a few replies to get to satisfactory results. Please be patient!
I also try to give all of the information you may need, expecting a “positive rating” in return. I will also continue assisting you after you have rated me, if necessary. If there is any reason I cannot get a positive rating, please let me know why “in advance”, so I can go another direction for you. I am sorry you’re having this problem; I’ll help you through it.

This is module related and is most likely caused on the BUS line. You need to attach a scan tool to not only read codes, BUT also read data from node to node with bi-directional tests.

Thanks, Ron

Customer: replied 1 year ago.
I am aware of the fact that I can take the C4s in for diagnostic. All it takes is a couple big, I just thought that someone with your experience could listen to the symptom and come up with something to try before needing an appointment with my local Porsche shop. The fact that the warning lights only go on when the headlights are activated indicates that the problem is not related to the ABS and/or the PSM. I just drove the car 1200 miles with the lights off without throwing a code.
Expert:  rpmdiag replied 1 year ago.

My database is down right now so I am letting Frank take over.



Expert:  Pete replied 1 year ago.

Hi my name is ***** ***** I will be happy to assist with this today.In some cases I may need more info to assist better.I apologize for any delay.

This definitely sounds like a low voltage issue here causing this.When the head lights are on this draws a large amount of power from the battery and if there is any issues with a weak alternator or battery or a fault with the battery cables themselves being corroded or loose this causes this here.I would start off with having the cables checked to see if any are loose or corroded at all.If a fault is found it will need repaired.If these are checked and found to be o.k. I would have the alternator and battery tested to see if either are at fault needing replaced.A local parts store will usually test both of these at no charge to you.If these are all found to be o.k. let me know.

If more help is needed, use the reply tab to continue our conversation. If no further assistance is needed, kindly rate my service.You can rate at any time and we can continue to work on your question as this will not close out your question.Keep in mind in some cases it can be difficult to fully diagnose or help repair your vehicle without seeing it and I work on delivering the best possible answer from what I see to work with.

Thanks Pete

Expert:  porscheguru replied 1 year ago.

Hi, I am going to be helping you with this and please reply when you are at the car. Since these cars are getting older, we are starting to see some issues with the wiring harnesses. I can have you take a quick look at something to see if that is the reason you are having this problem. It is Sunday, and I have some errands to run but I will have my phone and I will be alerted when you return to this chat. Disregard the "I'm ready to be rated" because it is selected but I can not deselect it.

Expert:  Clifton B. replied 1 year ago.

hi do you have any water are moisture in any of the tail or head lights >>?

Customer: replied 1 year ago.
I hear you.This problem arose during the wet season. I drive my 4s cab as fast as I can within the limits of inertia and road conditions. I hit the rev limiter in 5th gear often. When lights pop up it is a concern. This car is a ski machine not a garage queen. Checked the headlights, they have moisture. I will take them apart as soon as I have free time. I don't f**k with cars when we have pow days. I change oil at 3000 miles and pay attention to indicators.
Expert:  Clifton B. replied 1 year ago.

ok please rate the service before you go and let me know after you clean and seal the light if you still have the problem .