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CATALYST MONITOR

The Catalyst Efficiency Monitor is the test sequence used by the EECV to monitor the Catalyst(s) for proper and efficient function.

The operation of the Emissions control in OBD depends on the sensors being at proper operating temperature. Both the catalysts and the HO2S sensors need to be at several hundred degrees C before they are working efficiently, so the latter have heaters built in to get them up to operating temperature as soon as possible. Because the HO2S sensors need to operate at near stoichometry they cannot be used when rich fuel/air mixtures are needed and so:

Now we must consider the configuration of the Sensors and the Catalysts. The HO2S sensors are numbered so that the upstream sensor 1 is always on the Bank which contains Cylinder # One. The down stream sensor is numbered 2. Hence the DOHC engine designs contain the HO2S sensors 11 (forward of the Catalyst) and 12 (downstream of the catalyst). 

On the V6 24V (the 12V is not included here because it uses the EECIV and a Y-pipe arrangement) the driver's side of the engine contains Cylinder 1, so the configuration is shown here:

HO2S (Hot Oxygen) sensors develop a voltage between 0.1 volt and 0.9 volt depending on the amount of Hot Oxygen to which it is exposed in the exhaust stream. In Lean conditions (High in Oxygen, low fuel level) the sensor returns a low voltage, while in Rich conditions, (Low Oxygen, high fuel) it reads high. The change between the two readings occurs suddenly in what is known as a 'switch', but this term does not have any electrical connotation.. This effect is illustrated in the figure below:

Note that the sensor 'switches' suddenly depending on the stoichometry of the exhaust gas. The sensor is not measuring the Lamda of the exhaust, but only whether it is Rich or Lean - and this illustrates that the term Lambda Sensor is a misnomer. It's correct name is Hot Oxygen sensor, abbreviated in OBD to HO2S.

Why go to the bother of changing constantly between Rich and Lean of Stoichometry? Why not stick at Lambda and stay there? The answer is the way in which the modern Fast Light Off 3-way catalysts work. These catalysts are designed to reduce levels of Nitreous Oxide (NOx), Carbon Monoxide (CO) and unburnt fuel hydrocarbons (HC) to the lowest levels possible, using chemical reactions with the active element. This uses an oxidation function to convert CO and HC, and a reduction function to convert NOx. The following figure demonstrates this:

During Lean periods, the Catalyst converts the HC and stores the remaining oxygen, while in Rich periods the catalyst releases the stored oxygen and converts the excess HC. Outside of this narrow band of stoichometry the catalyst efficiency drops off sharply. HO2S sensors cannot detect the exact stoichometry of the exhaust gas, they can only detect if the gas is Rich or Lean  - so fuelling is controlled by the PCM to switch to Lean (short timing on INJectors) when the HO2S detects a Rich mixture, and switch to Rich fuelling when the exhaust gas is shown as Lean. Additional control of NOx is achieved through Exhaust Gas Recirculation (EGR) which has a separate Monitor.

The Catalyst Monitor 
The OBD Catalyst Monitor tests for the HC conversion efficiency of the catalyst(s) by comparing the upstream HO2S sensors with the Downstream and this test is carried out in a steady state vehicle operation.

Before the EXECUTIVE runs the test the following conditions must be met:

There are two methods used by the EECV PCM. In some OBD compliant vehicles built between 1994 and 1996 (This may be the Ford Scorpio)  the PCM carries out a 20 second test, by transferring the closed loop fuel control from the front to the rear sensors. This allows the stored oxygen in the catalyst to expel and after 20 seconds compares their switch-rate with a threshold frequency in a table in memory. If the switch ratio is higher than the threshold in memory then a DTC is stored.

In 1996-on vehicles, the switch rate of the rear sensors is compared to the switch on the front. It follows that if the catalyst is efficient it will be removing HC and HO and the rear sensor will show a slow switch rate compared with the front. By dividing the number of downstream switches with those of the upstream gives a catalyst switch ratio, and a typical threshold is 75%, above which a DTC will be recorded.

DTCs

P0420 Catalyst Efficiency below Threshold Bank 1

P0430 Catalyst Efficiency below Threshold Bank 2

Monitor Execution
Once per Driving Cycle. (On MIL equipped vehicles this test must be failed three times before the MIL light is switched on.)

Monitor Sequence
The following tests must be complete before the Catalyst Monitor can run:

It should be noted that faulty HO2S sensors do not produce the DTCs P0420 and P0430. The sensors must all be tested and shown to be reliable before the Catalyst Monitor can be called by the Executive.

Sensors OK
The following sensors have been tested and shown to be good before this monitor can be run:

DRIVEABILITY CONCERNS
None. With no MIL light if a catalyst has failed the owner will be unaware of it until the MOT emissions test. On the 24V, one good catalyst and one failing one may be borderline on emissions. Depending on the efficiency of the Fuel Trims, the spark plugs and the INJectors it may be possible for the 24V to squeak through the emissions test with two failing catalysts, although performance of the engine will be poor - running smoothly but well down on power. On the 16V DOHC engines with one catalyst this is less likely.

CATALYST RATTLE
Ford were aware of a batch of faulty catalysts on the 24V Cosworth and warned of this in Technical Service Bulletin 111/95 on 31/8/95. Symptoms were a rattling from the affected catalyst. If the catalyst brick had moved, this could cause a restriction in exhaust gas flow. Symptoms for that, in addition to the rattling, was a marked loss in engine power and perhaps a whistling noise. EGR pipes could also be dislodged by the increase in exhaust pressure.

Replacement catalysts have proved to be more durable, but their service life may be perhaps six years.

Diagrams © Ford (Europe)