What is the Ferrari slow down light?
The Ferrari slow down light is a warning system designed to alert the driver to an overheating catalytic converter in the exhaust system. Commonly installed on most Ferrari models, it is a very important warning system however the factory design is failure prone and many owners have experienced a false slow down warning.
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Catalytic slow down ECU link
Why it exists
Like most modern internal combustion engine cars, Ferrari are required to have catalytic converters in the exhaust system to reduce emissions of harmful substances into the atmosphere. In most jurisdictions it is an offence to remove a catalytic converter.
Catalytic converter cores are made from very expensive rare metals (commonly platinum and palladium) making them a high value part. Looking at current Ferrari spare parts costs a modern catalytic converter runs north of $3000 to $5000 and most Ferrari cars have 2 or more. Their operation is based around catalysing exhaust gas at very high temperatures into less harmful gasses. A by-product of this process is a lot of heat. Operation is reliant on an engine operating at correct tune with with fuel being burnt fully as intended before being ejected into the exhaust system. If the engine is in poor tune and excess fuel escapes combustion it will be burnt up in the catalytic converter and result in very high heat (over 1000°C), a red hot glowing cat, increased risk of vehicle fire and internal catalyst damage is not far away. At these temperatures a catalyst can be damaged in a matter of minutes resulting in a very high repair bill.
Ferrari have designed a system which actively monitors the internal exhaust gas temperature inside the catalytic converter core and illuminates a warning light, and if conditions are bad enough cuts the engine output power, based on the measured exhaust temperature. This is a very clever system and protects our cars when working as intended, however rather unfortunately the design is weak and very prone to false alarms. Most 355, 456, 550, 575 and 360 owners have experienced the dreaded slow down light, and those of us unlucky enough have had recurring warnings causing cylinder shutdown and resulting loss of power from the engine.
Ferrari execution
In the diagram below is a diagram of Ferrari's solution for a 360, but this is very similar for all 1980's to 2000's models from 328 up to the 430. Item (2) is the catalytic converter, item (3) is a thermocouple which is a special component that outputs a voltage in relation to temperature (think of it as an electronic thermometer), and item (4) is an ECU (sometimes called cat ECU or CCU - Catalyst Control Unit which is terminology used for the rest of this article) which cleans up the signal from the thermocouple and sends a signal to the main engine control computer and dashboard to warn of problems. This ECU is a problem part and cause of a lot of false alarms, keep reading below for more details....
Typical Ferrari Exhaust System Design
Below is a graph of the exhaust gas temperature and associated alarm behaviour. Again this is for a 360, but the system is very similar for most cars.
Point A - First over temperature warning 960°C, slow down light flashes on the dashboard.
Point B - Second over temperature warning 970°C, slow down light stays on solid.
Point C - Third over temperature warning 980°C, slow down light stays on solid and engine shuts down one cylinder bank to protect the catalytic converter.
So why the common problems?
Firstly, and important to note is that sometimes the warning light is indicating a genuine problem and attention needs to be given. An overheating cat can be dangerous and poses an increased fire risk.
A solution often offered on forums is to simply disconnect the CCU, remove the slow down light bulb or cover it in black tape. Do not do this unless the whole catalytic converter system is removed! If there is a problem it needs to be known immediately because of the risk of damage to the catalytic converter and increased heat in the engine bay.
The most important thing is to have the system working reliably as the factory intended, so A) roadside problems are quickly identified by the driver to keep the car safe, and B) a technician can diagnose and quickly fix the problem.
However, as noted earlier in this article, the system is unreliable and unfortunately now has a bad reputation. Early release models had an unreliably designed CCU and most 355 and 456 owners are familiar with the slow down light problem. These CCU's had a black colour resin potting compound and have a near 100% failure rate. If the black resin parts are still in your car replace them now. More recently released versions had a green potting compound, however these are now also proving to be prone to failure. The old black resin Ferrari part is 160454 with redesigned parts 173533 and 179278 following trying to improve reliability. As these parts live in the engine bay they are subject to very harsh conditions - many extreme temperature cycles, steam, dust, water and age - it's little wonder they eventually break. My recommendation is to treat the CCU as a replaceable service item and renew every few years to ensure peace of mind and trust in the warning system.
Ferrari Factory CCU units - older failure prone black unit on the left
To fix the problem is often straightforward, simply unplug the old CCU and plug in a new unit. This is a step most owners can do themselves. The major problem is that over the last few years Ferrari have increased the cost of this part to a very high level, now retailing for USD$500 per unit in North America, with most cars needing two replacements (and certain models of 355 take three). The cost drives owners to try to swap units from side to side, or buy one good unit and swap until the problem resolves. While this may work, it's expensive and as one unit has failed you can be sure the second is not far away from also going awry. There is a theory that water ingress causes the failures and various people have tried silicone seals around the potting, however this is a bodge at best and there are mixed reports of this approach actually working.
As an electronic design engineer this system has troubled me, and after ownership of both 355 and 360 models I have come to the conclusion that there is a way to improve the design. I also aimed to design a part with much lower than factory costs with the intent to provide them as an affordable service item allowing these beautiful cars to remain on the road in as close to OEM operation as possible (for familiarity to technicians following the factory service manuals) and to keep them operating safely. My alternative and new design is shown below, and is available for sale in the products section on this site.
Our improved design - TS179278
Other causes of the slow down light
There are 2 other causes which can cause similar symptoms.
Firstly is a genuine overheat condition caused by unburnt fuel in the exhaust. This is the system working as intended and the cause is an engine misfire, injector / spark plug problem, air leak in the exhaust, failed O2 sensors, broken coil or rather spirited driving for an extended period of time.
Secondly is a failed thermocouple (see part (3) in top diagram). While uncommon they can fail, usually to an open circuit condition which the CCU will detect as an overheating state. If intermittent warnings continue after replacing the CCU's changing the thermocouples is a recommended second step (assuming the engine has been checked and cleared of misfires).