Catalyser - overheat sensors, overheat ECU
& warning light
The cat overheat sensors linked to an associated cat overheat
ECU and a cockpit panel warning light, were an addition to the
RV8s exported to Japan and a number of members with reimported
RV8s appear to have experienced difficulties with them. Our
anonymous contributor explains how to approach this problem.
RV8s manufactured for the Japanese market have a number of small
but significant differences from the UK specification models
with regard to both the 6-light warning panel (situated immediately
below the radio console) and the catalytic converter itself.
Although there are known to be some variations between the two
basic warning light layouts [for the UK and Japan specification
models], the normal arrangement reading left to right across
the panel is as follows:
the UK specification models of both MG RV8s and MGFs, there
is no facility for alerting the driver to an overheating catalyser.
On the other hand, the owners of Japanese specification models
will have no indication of a fault within the main engine ECU
since these models lack a MIL (Malfunction Indicator Light).
From a personal standpoint I would have preferred to see the
Japanese specification arrangement on all RV8s with the added
benefit of the Engine ECU MIL replacing the seat belt warning
Brake fluid level
Engine ECU MIL
Handbrake & brake fluid level
are really only three reasons for the Cat Overheat light to
what is the remedial action? Go to the nearest MG Rover or Land
Rover MOT testing station which is staffed by competent individuals.
Ask for the foreman and do not be fobbed off with some inexperienced
19 year old. (These types seem to abound in many motor trade
establishments. They are not "cool"; they and other
similarly inexperienced people are potentially lethal and must
be kept away from your vehicle at all costs). Ask for a print-out
of your exhaust emissions from their Sun DGA 1800 Engine Analyser.
This is an integral part of the MOT test and, if asked for separately,
should cost no more than £10. If the readings are as set
out in the panel below, then your catalytic converter is working
correctly. The sensors (part number WDN 10001) are not available
in the UK. However, they are obtainable from MG Rover in Japan
through HS Imports (tel: 01278 789024) and cost around £150
catalyser temperature is at or above 900oC with the attendant
risk of component meltdown (literally!).
The sensor(s) is or are faulty.
Cat Overheat ECU and/or its associated wiring are in some
way defective, leading to a short circuit.
The Cat Overheat ECU and/or its associated wiring are
in some way defective, leading to a short circuit.
the sensors may not of course solve the problem. The fault could
lie within the Overheat ECU and/or the associated wiring. The
main difficulty in tracing this kind of fault is the lack of
information relating to these sensors and their ECU. Most sensors
are thermistors of the negative temperature co-efficient (NTC)
type which means their resistance falls with increasing temperature.
What the diagnostician needs to know are the resistance values
at particular points over a given temperature range. Neither
MG Rover in the UK nor their
For a larger copy of these test results - table
appear to have this information which of course is crucial to
the testing of any such component.
In short, if the exhaust gases are correct, then the catalyser
is doing its job properly. After all, owners of UK specification
models do not have the benefit of an overheat warning light
and would only know that they had a problem when some other
physical evidence became apparent for example the smell of very
hot metal, coupled with a marked change or degradation in both
fuel economy and overall performance. As I said earlier, once
the temperature of the catalyser rises beyond 900 C, "meltdown"
will almost certainly occur. This involves components literally
melting and fusing together such that the exhaust flow will
be physically impeded. This condition is frankly so unusual
that I have been unable to find anyone with first-hand evidence
of a genuinely overheating catalyser where the vehicle was in
normal everyday use. A lean mixture or the use of unleaded fuel
of less than 95 RON could well give rise to an overheat condition
in the catalyser and elsewhere - for example a higher than normal
coolant temperature. In this connection it is worth noting that
the relevant handbooks and manual state "95 RON MINIMUM".
This statement implies that fuel with a RON of less than 95
may well be available in countries outside the UK. Personally
I have always used 98 RON super unleaded - cost cutting can
be a dangerous obsession!
essence then, if your exhaust emissions are correct and all
other physical evidence indicates no obvious malfunction, simply
remove the warning light bulb. One can of course go down the
route of changing the sensor(s), changing the Overheat ECU,
changing the associated wiring etc. However, this is the path
of madness. The whole exercise is likely to prove both unrewarding
in terms of time and effort, and very expensive indeed. In the
you will have achieved is the elimination of the fault in a
component that was almost certainly installed with the intention
of protecting the catalyser from possible abuse or damage through
the use of inferior grades of fuel that are in any case absent
from the civilised World. This of course explains why all petrol
fuelled motor vehicles destined for the Hone Market (UK) do
not have Cat Overheat Sensors and their associated engine management
components fitted as standard equipment. Quite simply it would
be an unnecessary, expensive and wholly superfluous.
In preparing this workshop note, I should like to thank both
John Corten-Miller and Phil Cooley of the Corten-Miller Performance
Centre at Boston in Lincolnshire, who gave me a great deal of
help and advice with both this and other issues of a diagnostic
nature relating to the MG RV8. Although their operation will
handle most aspects of motor vehicle engineering and servicing,
their particular area of expertise lies in specialist diagnostics
aimed at improving motor vehicle performance. They are agents
for Weber, Lucas, Bosch, ASNU (specialist in injector diagnostics),
Superchips (based in Buckingham) and deal with both fuel injected
and normally aspirated engines. I am more than happy to recommend
their services. Corten-Miller Garages can be contacted at Eaudykes,
Friskney, Boston, Lincolnshire and on 01754 820341 and at email