What is a CAT ECU and why are they needed? Read on for more.

The 355 is fitted with catalytic converters in order to control exhaust emissions. These are fitted in the exhaust system between the manifold (headers) and the rear exhaust silencer. Some owners remove the cats and fit replacement pipes for improved throttle response, but whether they are present or not, a pair of CAT ECU's have to be present otherwise the Motronic engine control unit will signal a fault on the dashboard and probably cut the engine.

The purpose of the CAT ECU is to monitor the temperature of the catalytic converters to ensure they stay within correct operating range. Cats naturally run very hot - around 500 degrees celsius is a normal hot idle temperature, with the temperature increasing with engine revs/load. Cats only operate efficiently at relatively high temperatures.

Monitoring the operating temperature is particularly important on the 355 because of those cars that have ended up with an engine fire. The cause of an engine fire isn't always known and is definitely not always caused by an overheating catalytic converter but having something in that area that can get out of control temperature wise is clearly a risk, so owners should pay attention when a fault is signalled in this area.

The cats may overheat for several reasons; a non-exhaustive list (no pun intended) is below;

  • Manifold / header leaks - air is sucked in through the broken manifold, goes past the lambda sensors, which then report lean running. The ECU then richens the mixture in response and this causes unburnt fuel to burn inside the already hot catalytic converter. This makes it overheat.
  • Faulty lambda sensors giving a rich reading. The ECU leans out the fuel mixture which causes the engine to run hotter.
  • Very long periods of hard driving can make the cats reach their temperature limits.

The 355 has a warning system on the dashboard referred to as the "Slow Down Light". This light is a separate system to the more common MIL (check engine) lamp which signals some sort of engine or sensor malfunction.

The Slow Down light has three states:

  • Off - normal operation
  • Flashing - warning that the cats are beginning to get too hot
  • Solid - the cats are definitely too hot. This mode is accompanied by a cut of the engine bank on the side whose cat is overheating. Your V8 becomes a straight four and obviously performance is drastically reduced. This is also often referred to as "limp home" mode. When the temperature cools the bank will fire up again.

So how are the Catalytic converter temperatures measured?

Within each cat is a thermocouple. The thermocouple sits in the middle of the exhaust gas flow after it comes out of the cat itself. Thus it is actually measuring exhaust gas temperature rather than the surface temperature of the catalytic converter, but one is a reliable approximation of the other anway.

Each CAT ECU has an input and an output. The input is the temperature from the thermocouple, plus a power feed since it's obviously an electronic circuit. The output is a signal to the ECU which tells it the temperature of the cat using a relatively simple set of rules.

Sadly the CAT ECU component is terribly unreliable, and quite expensive to replace at around £180 in the UK. There are two of them in the car and for those who do their own maintenance and have a Motronic 5.2 car, there is only one slow down light so it's hard to know which side has gone wrong if there is a problem, and indeed whether the problem is an ECU problem or a genuine catalytic converter overheat. There are ways to find out which side is affected but that is outside the scope of this article.

Motronic 2.7 cas have a different engine management strategy - there are actually two main ECUs; one for each bank. Consequently on the dashbord there are two slow down lights - 1-4 and 5-8. Thus earlier cars are easier to diagnose. All cars use the same part number regardles of age (179278). This part is used on other vehicles of a similar age too - for example the 456. In fact it seems to be used on a few other models too - 360, 550, and even F50.

The design of the CAT ECU was changed at one point to improve reliability but it didn't seem to work - most owners still have a problem at one time or another. It seems to be more due to the environment they are in as 360's don't seem to suffer quite as much as 355's. The early units have a black potting compound surrounding the electronics, whereas in later units it is gree.