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?