F355 Check Engine light puzzle (still)

What is the Bosch number of your pre-cat O2 sensors?
Wrong O2 sensors there can cause these symptoms.

 

+1 The 97 part numbers are incorrect. Check the 99 part numbers. Also see this thread:
http://www.ferrarichat.com/forum/showthread.php?t=201140

 

Also, here is a schematic. You'll have to translate from German. I used the Google translator for this. The schematic shows that the sensors all return to pin 46 of the Motronic. Check for this connection. Also, the O2 sensor heater circuits have a 30A fuse somewhere. Check this fuse.
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Motronic is behind the passenger seat.
Pin 46 is NOT connected to ground ... it should not be connected to the ground. That precisely what may happen if you have not the correct sensors and it will affect the reading of the 4 sensors. Check again the reference of the pre-cat sensors.
Try to disconnect the pre-cat sensors and check if the post cat are still faulty.
You can also measure the voltage between this wire (pin 46 at Motronic level or at sensors level) and the chassis. You should see a voltage close to 750mV.

 

You could go through the schematic for all of the O2 sensor connections and verify continuity between the Motronic end and the sensor end. Also, for each connection, verify that it is not shorted to ground. I saw from one of your previous posts that you had electrical problems in other areas.
In my car, I had some issues with bad relays as well. Check the relays in the passenger footwell. Several of them are the same relay part number, so you can swap them around and see if your symptoms change.

 

Please post the Bosch part numbers for the O2 sensors you are currently using.

They should be as follows for a 5.2 motronic 1996 to 1999 F355:

Pre-cat: 13819


Post-cat: 13820

 

I have the same car with the same problem. It has been just over 1 year since the light came on. I have had it in to the local Ferrari Dealer and 2 independant shops with no luck finding the problem. I am going to send it to Pennsylvania, where the highly recommended Dr. Ferrari will keep it till the problem is fixed. I have had enough. Good luck to us both.

 

The harness with a blue mark is for the pre-cat sensors.
Very strange it affects the 4 sensors with the correct type of pre-cat sensors. Have you measured the voltage between the chassis and pin 46 (or the grey wire of any sensor)?

 

andmoz,

from your other posts, it looks like you are located in the Central Valley in CA. If that is so, and you feel like taking a drive over the hill to Gilroy, we could do a point-by-point electrical comparison between your vehicle and my 97 F355 Spider. I also have a spare Motronic ECU that we could try.

My car is going in to Brian Crall today to get some work done, but I expect to have it back once it quits raining long enough for me to go pick it up.

Tom

 

email sent

 

Please keep us posted on what you guys find. Thanks, and good luck!

 

Alright, I have my car back now. Please make sure your OBDII connector is up to date so we can take readings. See the attachment. Let me know when you are available.
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If they determined the headers are the problem why would they recommend replacing the o2 sensors and ECU's too?? Doesn't sound like great "service" to me. Sounds to me like they're not really that sure, they enjoy "throwing" your money at the problem or they have a boat payment due. Perhaps all of the above??

 

Is it low output voltage or low voltage to the heaters? A header leak would cause a leaner reading at the sensor and show higher voltage.

 

Interesting. Most of the time a pre o2 leak will cause such problems but it's been posted on this forum that a failed header cannot be diagnosed electrically on these cars.

 

Nice clean/clear schematic. Do you have any others?

I bought one of the 355 CD's on ebay but the schematics are crap.

 

+1, time for a second opinion, from Brian Crall.

 

What codes are you getting, specifically?

 

My offer still stands if you want to do a side by side comparison with my 97 F355 Spider. I had the same low voltage problem and it was related to the O2 sensors. My headers were bad, but I was able to resolve the low voltage issue prior to changing the headers.

I have a spare Motronic ECU that we could try. I also have real time monitoring software that we can use to monitor the voltages on your O2 sensors. This is how I found my issue. I unplugged the sensors and then plugged them in one at a time while watching the real time data. When the voltage went to zero, I found my bad sensor. If it is not the sensors, we can still unplug each item in the electrical path until we find the issue (relays, fuses, wires, etc.)