The 360 is fitted with catalytic converter (Cat) temperature control system that can trigger a rather alarming message on the dash that tells the driver to “SLOW DOWN”. This message is sometimes accompanied by a loss of engine power and if the driver doesn’t understand what is happening it can be a worrying moment.
The purpose of the system is to protect the catalytic converters from overheating, most commonly caused by fuel entering the exhaust system and burning in manifold and Cat. If this happens then irreversible and expensive damage can occur to the Cats and the oxygen (lambda) sensors.
The system works by measuring the temperature of the exhaust gases exiting the Cat, if the temperature exceeds approximate 900 Celsius then the SLOW DOWN warning message is displayed on the dash (flashing). If the temperature continues climb above 940 Celsius then SLOW DOWN message is lit continuously and the Motronic shuts down the side of the engine affected (the CEL light is also lit but an error code is not stored).
The exhaust gas temperature is measured by a thermocouple in each Cat – this picture shows how they are positioned in the cat outlet:
Each thermocouple is connected to what are commonly called Cat ECU’s (number 4 in the diagram below). They are in fact a simple signal converter which takes the signal from the thermocouple and converts it into a linear voltage between 0-5 volts (depending on the temperature of the exhaust gas). This voltage is sent to the engine ECU (Motronic) which then performs the operations detailed above.
There are a number of situations that can trigger a slow down message. When trying to diagnose what the problem is the first thing to do is determine if the Cat is actually overheating. If the message is displayed upon first starting and when the engine is still cold then it is highly likely that the Cat is not too hot. If the message appears whilst driving, then pull over shut the engine off and take a look at the Cats – if one of them is glowing red like in the picture below then it is overheating. Note: This is not my picture I have taken it from Club Scuderia.
Another way to check if one of the Cats is too hot is to use an IR temperature reader on both cats – if either is approaching 1000 Celsius then it is overheating. If the Cat is genuinely overheating then do not drive the car as costly damage can be done from the unburnt fuel entering the exhaust system. The most common cause of this is a faulty coil pack on the bank of cylinders that the hot Cat is connected to however a bad spark plug or even a leaking inlet manifold gasket are other common causes. One way to pinpoint which cylinder is the problem is to use an IR gun on each of the ports on the exhaust manifold. If one is significantly cooler than the others then that is your problematic cylinder.
If neither Cat is overheating then the SLOW DOWN message is spurious and caused by a fault with the monitoring system. It is possible that that the thermocouple has gone bad and a first point of call would be to check the wiring between the thermocouple and Cat “ECU” as well as the connections. The thermocouple can be checked for resistance across the two connections – if it is open then it has gone bad.
The integrity of the ground at the Cat ECU should be measured – test for continuity between a known ground and the black (middle) wire on the connector that joins the main wiring harness to the Cat ECU. Here is the wiring diagram for that connector:
If the earth is bad then check the grounding of the harness. The location of the ground on the rear chassis, under the lights and near to the exhaust valves (OEM exhaust) – 11L & 34L in the diagram below (circled in red):
However, the most likely issue is the Cat ECU itself as they are prone to failure. Moisture can enter the unit and cause it to stop working. Ferrari actually updated the unit to try and counter these problems but they still fail – new part number is 179278 (old number is 173533). You can tell if a unit is the revised model if the resin is green as in the photo below:
There is an alternative to replacing the Cat ECU’s with Ferrari items. Aerospace Logic have developed a system that replaces both Cat ECU’s and provides real time exhaust gas temperature monitoring. It’s not cheap but I have no doubt that it is far superior to the OEM system – perhaps worth considering if your car is a keeper?
Aldous Voice 
Great article. Being a 360 owner it’s postings like this that make such a difference in being educated about the car we enjoy. Question about Cat replacements. There are many aftermarket options (sport cats, etc) that are noted to throw a CEL. It seems to me that would be risky if you’d always have a false positive reading knowing other issues could be looming. Any experience with Cat replacement and recommendations are welcomed.
Thank You.
It’s worth taking time to understand what is happening with the cat and the oxygen (lambda) sensors. On the 360, each cat has an oxygen sensor before and after the core. The core itself is made up of two parts, the first reduces NOx emissions and the second reduces Hydrocarbon and CO emissions (oxidation). The former requires a rich mixture to operate properly whereas the letter requires a lean mixture. The first oxygen sensor samples the exhaust gas as it enters the cat and ensures it is rich enough for the NOx reduction process to work. The second sensor samples the exhaust gas as it leaves the cat and ensures that the oxidation process is working.
The amount of fuel injected at any one point in time is determined by the ECU fuel maps. For all the input data, an amount of fuel is injected into the cylinders. As well as this fixed amount from the map, the ECU is also calculating an addition or reduction depending on the signals from the oxygen sensors. This additional amount is called fuel trim. The fuel trim is constantly cycling between rich and lean to satisfy both processes in the cat.
CEL errors occur when the fuel trim reaches a certain threshold (too lean or too rich). These errors can occur with a failing oxygen sensor or MAF (as well as other sensors) and also occur with leaks allowing unmetered air into the engine (such as a leaking inlet manifold gasket).
However, they will also occur when the ECU cannot resolve the correct signals from the oxygen sensors due to the cat processes not occurring properly or not at all. This could be from a failing cat or from a poorly performing aftermarket cat or from using straight through (decat) pipes.
Personally, I have fitted custom made 200 cell sports cats and have not experienced any issues and my Long Term Fuel Trim (LTFT) values are benign. I did take the precaution of fitting four brand new oxygen sensors to ensure the gas sampling was correct. I also know of other people running various brands of sports cats without any issues. With regards to straight through (decat) pipes – the addition of sensor extenders can improve the time before the CEL appears but it will always appear when the fuel trim hits the threshold. As you say, this could cloud other issues. From a personal perspective I don’t think decats are worth the trouble but they do produce awesome flames out of the exhaust!
Can you tell me the cylinder ordering please on a 360 spider 2002? I am assuming as I stand at the back of the car looking forward the right hand side furthest forward is 1, 3, 5 and 7 and the left hand side front first is 2,4,6,8?
Thanks
Hi James,
As you stand at the back of the car looking forward, cylinder 1 is furthest forward on the right hand side. 2, 3 & 4 are also on the right hand side with 4 closest to you.
5, 6, 7 & 8 are on the left hand side with 5 closest to you and 8 furthest forward.
Thanks,
Aldous.
Ah thank you so much. I thought it would be easy to find online but clearly I had no idea where to look. Really appreciate you taking the time to reply.
No problem. There’s a good pic here, it’s of the F430 engine but the numbering is the same.
Hello Aldous
My name is Alex from California, I enjoy your website. By the way, congratulation on your new F430. Anyway, my 2004 360 spider having problem with the “Slow Down” warning light and lost of power. I’ve follow all of your instructions. Replaced both ECUs, check ground on both side, measured the resistant on both of the thermal couples (1.7 Ohms).
I also followed your procedure for reset the ECUs. The car drove OK, but still lack of power for the first 15 minutes. But and then the “Slow Down” light came back. So I decided to put her back in the garage. Please help……
You have to have the fault reset using the Ferrari system. I had a similar problem on a V12 replaced everything one piece at a time. Even the cats. It was a very expensive exercise because I was doing it myself and the slow down fault just hung there until I finally got it on a Ferrari SD2 machine and they cleared to code. It was the sensor BTW which is always my first port of call
ECU Error code P0720 AND P0715 flashing on the error report …… could you assist me please . My name is Sharad and i love my ferrari 360 spider 2004 ….i hail from India and privileged to be on air with all of you…… cheers and god bless
Those codes relate to the input shaft speed sensor. Part number 168729
how do i locate part number 168729 may i ask you to assist me regards this
Try here: http://www.eurospares.co.uk/search?partNumber=168729
This is Great thank. I have changed my cats, and exhaust system, because my cats were already broken, but before the change the slow down was showing up, and after the change still making it. So I started to think this was because of the sensors? it shows up, after driving for about 30 min. what do you think?
The slow down message is not triggered by a faulty cat or exhaust. It is caused by unburnt fuel in the exhaust or a faulty sensing circuit
I have exactly the same symptom just like in the picture of the cat being red hot right at the same place of the cat. I changed all 8 sparks and one coil since obd showed p0308.( looking at the car from behind, far into the engine room on the left side)
But still if i press the gas little harder to pass over 5000rpm, engine light blinks and slowdown blinks. And trigger p0308…
Any guess or help…?
This is Korea and my car is 2004 360 modena
You have a misfire on one or more cylinders on the bank with the red-hot cat. Misfires can be caused by bad coils, bad plugs, intake gasket leaks, broken valve spring, bad injector, etc, etc.
I would start by trying to work out which cylinder is misfiring by measuring the temperature of each cylinder exhaust outlet. Then work from there…
I truely thank you for your reply. This is Korea and you are being such great help. You are already popular among ferrari club members in Korea.
Since I already have 8 coils, I will change all 8 coils and come back to u soon. Again, Thank you very much. Jey.
I have changed plugs and coils and now it flies wow…
No slowdown
No p0308
No codes no matter how hard i floored it
Thank you very much again for your help!
Hello Aldous, I hope you are well! I was hoping for your advice on recurring CEL’s that I’m getting on the 360. I’m getting codes p0300, p0302, p0303 and p0307 pending. Occasionally other cylinders will show up too. Light comes on after cold start after car hasn’t been started for a week or so. All spark plugs and coil packs were done 2 months ago, thermo ecu’s too. Could this be a problem with the 02 sensors or thermo’s? Any advice would be greatly appreciated!
Many many thanks!
Sam.