355 Few codes, headache

Purple/black for the bypass, purple for the RH cat. Not sure how easy that is to spot in that location. The other two colours are the same. You may have to do continuity checks between the ECU and the connectors. Anyway, if this is a recent development, it won’t be the plugs swapped over. There was no issues straight after the major.

 

Don’t have an ohmmeter? I’m sure the fuse is ok. There’d probably be more messages if the fuse was blown, but it would be good to know if the fuse checks ok with an ohmmeter… or better still, 13~14 volts on both ends of the fuse with the car running. Or even better, check the voltage at the O2 sensor connectors (although that may be difficult with the engine running). Maybe you could compare the resistances across the white wires on each sensor (engine off). It will give you something to do whilst waiting for your cat ECUs.

 

No, with the 355, if the TCU signal is removed, that input to the ECU is pulled up to 5V which is way beyond triggering level. The slow down light will still come on and within 15 minutes the ignition will cut out.

 

Not sure if this is normal. Were the O2 sensors hot? Cold, they should be just over 2 ohms.

 

Interesting. I’ll try and confirm the resistances on my car later today. Were the plugs disconnected when you did the tests?

(EDIT) Sorry, cross posted. Ignition off. You’re doing the test correctly.

 

Definitely sounds wrong. Are you sure you had the sensors/lower plugs connected when doing the resistance check?

I had forgotten how difficult the plugs are to access on these cars. I checked my right bank upper connections, and the sensor wiring with the blue tape on it was 3.5 ohms (upstream sensor) and the downstream sensor was 2.1 ohms. I see there is a small extension harness on each probe. By "upper connections", I mean the highest (most easily accessible) plugs.

In typical fashion, I broke (or found broken bits) on my car. The inlet ducting had fallen off the airbox and the clip on one of the O2 sensor plugs had broken off.

 

I'm wondering if you have standard Bosch O2 sensors.

 

Top sensor? Do you mean top sensor connectors? I've just removed the metal clips on my connectors, so I'm not sure which is top and and which is bottom. Do you have blue tape on the harness anywhere?

Whatever you do, don't mix up the pre and post cat connections.

I'd definitely replace the O2 sensor showing open circuit. I've read somewhere that you should replace the sensors in pairs. Whether they mean post/precat sensors on one side or, say, the precat sensors on both sides, I'm not sure. I guess the latter.

 

Hard to say from the photos. What you need to do is make sure that the blue-taped plug is attached to the forward (precat) O2 sensor

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My car may be different. I have two plugs bolted to an upright.

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On the right hand side of my car, the upper plug is part of the precat O2 sensor cabling (with blue tape around the ECU side harness). The cables shown are extension cables which go to the O2 sensor plugs lower down. You can mix up the harnesses on my car.

Left hand side here:

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It's difficult to tell which is the precat sensor cabling without pulling the car to bits or getting a mirror or boroscope to check. Also in the photo are the plugs for the Technistrada thermocouple ECU.

 

Part of the engine wiring diagram:

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(actually the wire colour change on my car begins at the higher frame-mounted plug and the black (N) ground wire should be on the sensor, not the plug)

I see I added a note some time ago, saying that the pre/post cat resistances are different. Your 3.8 ohm reading is probably the post cat sensor. I don't know why you're getting 0.54 ohms on the other sensors. Seems much too low. And you certainly shouldn't be getting open circuit.

I can't remember if heater failures force the ECUs into open loop mode or illuminate the CEL.

 

Can you confirm that the harnesses with the blue tape on them actually go to the correct sensors (on the cats)? Then create a table with values:

LH precat
LH post cat
RH precat
RH post cat.

"Top" and "bottom" is not giving me any reference points. If the resistances match (left to right), excluding, of course, the open circuit one, then it could be due to the sensor type (i.e. normal on your car) or perhaps it could be an age issue (why they are different from mine).

Do you have blue tape on the sensor and on the harnesses? I think my ECU harness and sensor extension harnesses have blue tape, but not the sensor leads themselves (although it's hard to see that far down).

Once you have confirmed the open circuit sensor location, I would order two of that type. Last time I checked, they were hard to find.
e.g. precat: Bosch 0258003819, Ferrari p/n 169953

If you are able to find them, test the resistance before installation.

I thought I had at least one precat sensor in my spares, but only found rear ones.

 

Here's an interesting thread on O2 heaters.

https://www.ferrarichat.com/forum/posts/142914608/

It seems that the ECU can control the current going to the heater. I just thought the input to the ECU was for some kind of heater powerloss monitoring.