On-Board Diagnostic Systems

by James Newsom

For years, the primary functions of motor oil have been to lubricate, seal, cool, clean and protect engine parts from corrosion. Modern motor oils, however, are asked to provide additional performance and protection in ways unheard of only a few years ago. Federally mandated vehicle emissions controls and fuel economy requirements have created a completely new dimension in motor oil performance: the protection of emission controls and related equipment.

Engine emissions control is a highly computerized activity, with virtually all of an engine's operational parameters being monitored, controlled and coordinated by an on-board computer. Within the on-board computer is the "On Board Diagnostic System 2" (OBD2), with its various sensors located throughout the vehicle, many either directly or indirectly in contact with motor oil. These sensors send information to the computer, which adjusts engine operations based on everything from the relative humidity of the incoming air to the oxygen content of the exhaust gases. Additionally, the computer stores critical information regarding engine operation that can be retrieved and used to diagnose engine or emission system problems. To give you some perspective on this system, a modern gasoline engine has more raw computing power than the Apollo spacecraft that first took Americans to the moon. OBD2 systems were mandated for all 1996 cars and will be on all future cars until OBD3 is mandated (probably around 2004).

Motor oils are now asked to protect many of these sensors and the equipment they monitor. While many on-board sensors are affected directly or indirectly by motor oil quality, oxygen sensors and catalytic converters are two of the most important parts of the emission control system that are directly affected.

Oxygen Sensors

New cars have oxygen sensors located before and after the catalytic converter. The first measures oxygen content of the combustion gas and sends an electrical signal to the computer, which then adjusts the air/fuel ratio to achieve the ideal mixture for the current driving conditions. In practice, the oxygen sensor sends this information hundreds of times a minute, resulting in continuous air/fuel adjustments. If the sensor fails, your car will lose power, its fuel economy will deteriorate and tail pipe emissions will increase.

Premature failure of this sensor can be prevented if the oil you use is formulated to be compatible with the oxygen sensor. Oil-derived phosphorus and volatile motor oil components can cause oxygen sensors to fail. That's why leading manufacturers like Pennzoil have formulated low-volatility and low-phosphorus oils since 1993 .

The second oxygen sensor provides the computer the information necessary to determine the effectiveness of the catalytic converter. A comparison of oxygen content before and after the catalyst enables the computer to determine catalyst activity. If at any time emissions exceed 1.5 times the federally mandated limit, a Malfunction Indicator Lamp (MIL) is illuminated on the dashboard. In the future, vehicles will not pass inspection if this lamp is lit. The second sensor is affected by the same oil-related factors as the first.

Catalytic Converters

The catalytic converter is similar to the oxygen sensors in that oil-derived phosphorus and volatile oil components can shorten its life. Too much phosphorus in the oil can result in the catalyst's surface being "glazed over" with an inert pyrophosphate that in effect kills the catalyst. Volatile oil components can leave carbon deposits on the catalyst's surface, which in time will also reduce its effectiveness.

In the past it was relatively easy to overlook a failed catalytic converter. Since there was no way of knowing that your catalyst was dead, and most states did not require emissions testing as a part of vehicle inspection, catalytic converter failures went almost unnoticed. With the addition of MIL lamps and the prospect that inspection will be tied to them, however, improperly maintained emission systems will become an expensive repair. The formulation introduced by Pennzoil in 1993 helps protect the catalytic converter in much the same fashion that it helps to protect the oxygen sensors.

To a lesser extent, knock control devices, PCV and EGR valves, coolant temperature sensors, etc., can all be affected by the motor oil you use. Choosing the proper oil can help ensure long and trouble-free service for many of these devices.

A note of caution in choosing the right motor oil: reducing an oil's phosphorus content and volatility is not a simple process. Formulas must be carefully re-balanced by expert formulators to ensure that low temperature startability and protection against engine wear are not compromised in the process. To be sure that your motor oil meets all of the requirements of modern engines, choose a brand that you know and trust.