[MNBoxster]

Hi,

With all the discussion lately about O² Sensors, I thought I'd put together a short Primer on how they work, how to Test them, and when you should replace them.

In the Past, when Cars were Analog and operated using Carburettors and Distributors, the various parameters of the Engine were Static, that is, they were set-up for a very limited range of Operating Conditions. The Car could not adapt to differing conditions or loads it encountered beyond a simple Mechanical Timing Advance (Centrifugal and Vacuum) and an additional Carburettor Circuit to supply additional Fuel when operating at High RPMs.

Driven primarily by concerns over Emissions, about 30 years ago, Engineers began designing Engines which were much more Dynamic, that is, they had the ability to adapt to various conditions which resulted in lower Emissions.

At first, these Engines had a greater, but still limited, range of Operating Parameters. The devices used to sense changing conditions were relatively crude and could not adapt quickly. Through the years, advances in Digital Circuitry and Computing power have increased the range of conditions to which an Engine can adapt. A modern ECU samples Operating Condition some 50 million times each second!

As an Engine encounters different conditions, the composition of it's Exhaust changes in a Measurable and Predictable way, specifically it's Oxygen Content. The Ideal Air/Fuel mixture for a Gasoline Internal Combustion Engine turns out to be 14.7:1 (14.7 parts Air to 1 Part Fuel). This is known as a Stoichiometric Mixture.

If there is less Air (principally O²) than this ideal ratio, then there will be Fuel left over after Combustion. This is called a Rich Mixture. Rich Mixtures are bad because the Unburned Fuel creates excess Emissions.

If there is more Air than this ideal ratio, then there is excess Oxygen after Combustion. This is called a Lean Mixture. A Lean Mixture tends to produce more Nitrogen-Oxide Emissions, and, it can cause poor Performance and even Engine Damage.

The first thing which Engineers needed was a method to detect the changes in the Engine so it could adapt. It was decided to use an Oxygen Concentration detecting device. Engineers began looking to produce a Sensor which would signal these changes to an ECU (Electronic Control Unit) - DME in Porsche Parlance.

Based upon the level of Oxygen in the Exhaust Gasses (More=Lean, Less=Rich), the ECU would alter the Fuel quantity and Engine Timing to achieve, and maintain, a Stoichiometric Mixture.

It was found that a unique group of materials would generate a small, controlled, electrical signal if heated and then exposed to Oxygen, a major component of Exhaust Gasses. An additional requirement was that material would need to react quickly to any changes in order that the Engine adjustments occur in a timely manner - about 300 MS (quite literally the Blink of an Eye!).

The best material suited for this Exhaust Gas Sampling turned out to be a Ceramic made up primarily Cobalt and Oxygen - Cobalt Oxygen Monoxide Ceramic (there are other such Materials as well, principally Zirconia and Titania ). When heated in excess of 650° F, and exposed to Oxygen, such as in an Exhaust stream, this material would generate, as a result of a Chemical Reaction, an Electrical Signal ranging from 0.0 - 1.1 volts. The Value of this Electrical Signal varies inversely to the amount of remnant O² in the Exhaust Gasses - reduced voltage for a Lean Mixture (more O² present), increased voltage for a Rich Mixture (Less O² present).

By creating a set of pre-programmed conditions and responses to those conditions, an ECU could use this electrical signal, compare it to it's set of pre-programmed responses (known as MAPs) and then apply changes to the Engine's Fuel Quantity, Mechanical, and Ignition Timing to allow the Engine to dynamically meet the changing demands and optimize Torque, Power, Fuel Economy, and Emissions.

A Sensor was produced (Patented by Ford Motor Co. (US Patent # 3.993.028)) and subsequently Licensed to Outside Suppliers to manufacture, principally Robert Bosch Gmbh, but also Denso and Beru (the Boxster uses Bosch Sensors)). These would 'Live' in the Exhaust Stream and sense and signal changes in the Exhaust composition (O² content) to the ECU. These are called O² Sensors (also known as lambda sensors, or EGO sensors).

In it, a pellet of the Reactive Material (Cobalt Monoxide, or Zirconia and Titania ) was mated to Electrical Wires on either end and encased in another Ceramic (usually Alumina) to withstand the Heat Extremes of this environment (700°F - 950°F). A Sampling Hole was made in the Alumina Case so the Sensor could come into direct contact with the Exhaust Gasses.

When the remnant Oxygen in the Exhaust contacted the Reactive Pellet, an electrical signal (proportionate to the quantity of O² present) would be generated and sent to the ECU. Initially, these Sensors used the Heat of the Exhaust itself to heat the Sensor to the point where it would react (650°F). But, this process usually took in excess of 3 min. and during this time, the Engine would run rough and produce excess Emissions. So, later, a simple Heater Coil was introduced, surrounding the Reactive Material which would heat it to 900°F in under 10 sec.

When you start your Car, the O² Sensor's Heater Coil warms up the Sensor and it begins to generate an Electrical Signal. In normal Cruising, this signal will be approx. 0.040 volts and gradually rise to approx. 0.5 volts. Although its readings start to be valid within a 10-20 sec., the Sensor doesn't get utilized fully by the ECU until the Engine is fully warmed. Until then, it's running in an Open Loop and generates around 0.6volts to 0.8 volts, meaning a fairly Rich Mixture (15:1-22:1) is being directed to the Engine by the ECU which aids Starting and prevents Stalling. Once the ECU switches over to a Closed Loop, the O² Sensor rapidly changes between 0.40 volts and 0.65 volts when Cruising. At higher Acceleration (Rich Mixture), it is between 0.6 volts and 0.8 volts. At WOT (Wide Open Throttle), it generates between 0.92 volts and 0.96 volts. If you let off the Accelerator (Lean Mixture), the Voltage drops off to 0.0 volts, then rises to 0.2 volts, and finally returns to between 0.40 volts and 0.65 volts.

On Modern, OBDII compliant ECUs, two O² Sensors are used, both before and after the Catalytic Convertors, to better increase the Sensitivity of the System. The ECU averages these Signals to more accurately change the Operating Parameters of the Engine and to maximize the efficiency of the Catalytic Converters.

Because of the ever-increasing functions of a Modern Digital ECU, O² Sensors' impact on the operation of the Engine have increased dramatically, as have the problems arising from a malfunctioning O² Sensor.

These 'live' in an extremely Harsh environment and are subject to Fouling (where combustion products deposit on the Reactive Material, denying it direct contact with the O² in the Exhaust), broken Signal Wires and broken Heat Coils. The Sensor, can also be damaged by Lead, Oil, Silicon Sealants, some Gasoline Additives, and Antifreeze. Other Engine Malfunctions such as a Misfiring or Overheating will also damage the Sensor. Some of these failures can be progressive, resulting in diminished Fuel Economy and poor Operation and Response by the Engine, while others are more severe and will destroy the Sensor.

Robert Bosch (the Worlds largest O² Sensor Manufacturer) reports the usable lifespan of their O² Sensors as 30k mi. for an Unheated Sensor and 60k mi. for their Heated O² Sensors (which the Boxster uses). The difference is because Combustion Products condense much more readily on an Unheated Sensor ,until it reaches Operating Temperatures, and so it Fouls more quickly.

The advantages to having a properly operating O² Sensor are increased Drivability and a 10%-15% increase in Fuel Economy (according to Robert Bosch, this will save the Avg. Driver $100 a year in excess Fuel Consumption - probably more at today's Fuel Prices). That's a pretty good Offset to the cost of replacement.

There are a variety of ways to Test your O² Sensors. But, due to the Complexity of the Modern ECU, and that fact that On-Car Testing requires working around very Hot Exhaust Components, it is recommended that a simple Bench Test is conducted instead.

Simple Bench Testing of O² Sensors:

Use a high impedance DC voltmeter.

Clamp the Sensor in a vice.

Clamp your Negative(-) Voltmeter Lead to the Sensor Case, and the Positive(+) Lead to the Output Wire.

Use a Propane Torch set to High and use the inner blue flame tip to heat the Fluted or Perforated area of the Sensor.

You should see a DC voltage of at least 0.6volts within 20 seconds.

If not, most likely cause is an Open Circuit internally or fouling.

If OK, remove from flame. You should see a drop to under 0.1 volt within 4 seconds.

If not, the Sensor is likely Silicone fouled.

If OK, heat for two full minutes and watch for drops in voltage (you don't want to see any). Sometimes, the Internal Connections will open up under heat. This is the same as a Loose Wire and is a failure.

If the sensor is OK at this point, and will switch from High to Low quickly in response to the flame, the sensor is good.

Realize that Good or Bad is relative. O² Sensors can have diminished operation.

Any O² Sensor that will generate 0.9 volts or more when heated, show 0.1 volts or less within one second of flame removal, and pass the two-minute heat test is good regardless of age.

There are several model O² Sensors used on Boxsters. These are all made by Robert Bosch. An OEM and a Universal Sensor are available for each. The Universal Sensors are the same for all Models. Source: Robert Bosch, Gmbh

On the '97-'99 2.5L Cars, both the Upstream and Downstream Sensors are the same and the Part # is:

OEM - 13806

Universal - 15738

For '00-'06 2.7L eGas Cars, the OEM is different Upstream/Downstream, while the Universal is the same.

OEM Upstream - 15182

OEM Downstream - 15183

Universal Up/Down - 15738

For '00-'06 3.2L eGas Cars, the OEM is different Upstream/Downstream, while the Universal is the same.

OEM Upstream - 15182

OEM Downstream - 15183

Universal Up/Down - 15738

I know this is probably waay more than most of you ever wanted to know about O² Sensors. But, hopefully some will find it useful, or copy it for future reference. It appears that there is Real Benefit to changing these Sensors as a regularly Scheduled Maintenance Item. Improvements in Drivability, Better Emissions and improved MPG (along with subsequent Fuel Cost savings) seem to justify this...

Happy Motoring!... Jim'99