When the air/fuel ratio is more than 14.7:1, lambda is greater than one and an engine has a “lean” fuel mixture since the relative proportion of fuel to air is lower than the stoichiometric ratio.  A “lean” fuel mixture improves fuel economy and lowers emission levels but reduces power.[ii]  If the air/fuel mixture is too lean (beyond 18:1), it may not ignite (“lean misfire”) which can result in a rough idle, loss of power, and increased hydrocarbon (HC) emissions because the fuel does not ignite and just blows right through the engine.  Extremely lean fuel mixtures can burn spark plugs and pistons and contribute to engine-damaging conditions such as pre-ignition.

When the proportions of air and gasoline are "just right," it burns efficiently and produces the fewest emissions.  However, an engine’s fuel requirements change with speed and load.  To go faster or when under a load, an engine needs more air and fuel to develop more power.  In that event a richer air/fuel mixture ratio is required.  When an engine is loafing along or coasting, the air/fuel mixture can be leaned to reduce fuel consumption and improve fuel economy.  The trick is to balance the air/fuel mixture ratio as driving conditions, loads, and temperatures constantly change.  That's where O2 sensors come in.  By monitoring the level of unburned oxygen in the exhaust, an O2 sensor quickly informs the engine’s computer when the fuel mixture is lean (too much oxygen) or rich (too much fuel).  The computer then adjusts the fuel mixture by quickly increasing the fuel when the mixture is lean (even by a little bit), or by quickly decreasing the fuel when it is rich (even by a little bit).  On average, this rapid flip-flopping back and forth allows the feedback fuel control system to maintain a more-or-less balanced mixture.

Why Normal Wear-and-Tear of the O2 Sensor Causes Less MPG

The speed at which an O2 sensor reacts to oxygen changes in the exhaust is critical for accurate fuel control, peak fuel economy, and low emissions.  The air-fuel mixture in an older carbureted engine doesn't change as quickly as that in a throttle body fuel-injected application, so response time is not as critical.  However, in engines with multipoint fuel injection, the air-fuel mixture can change extremely fast, requiring a very quick response from the O2 sensor.

A newer or good O2 sensor quickly responds to changes in the air/fuel ratio.  However, nothing lasts forever and O2 sensors are no exception.  As an O2 sensor ages, it doesn’t react as quickly as it once did.  The increased lag time makes the sensor sluggish and prevents the engine from keeping the air/fuel mixture in close balance.  Also, when a vehicle’s engine is operating, oil ash and contaminants from normal combustion accumulate on the sensing element, reducing the O2 sensor's ability to quickly respond to changes in the air-fuel mixture.  Thus, the O2 sensor will always slow down and become "sluggish" with normal engine operation over time.  Also, the O2 sensor's output voltage may not remain as high as it once was, giving the erroneous impression that the air/fuel mixture is leaner than it actually is.  Under various operating conditions this may result in a richer-than-normal air-fuel mixture that will cause an increase in fuel consumption and emissions.  Since this change occurs gradually, the problem may not be noticeable right away.  Over time, the situation will just keep worsening.  Therefore, to restore engine performance the O2 sensor must be replaced.  A new O2 sensor can improve fuel economy as much as 10-15% and will minimize exhaust emissions, reduce the risk of costly damage to the catalytic converter, and ensure peak engine performance (no surging or hesitating).

Why Sensor Failure Causes Less MPG

Aging and normal use of a vehicle will cause an O2 sensor to fail.  However, an O2 sensor can also prematurely fail if it becomes contaminated with lead from leaded gasoline,[iii] phosphorous from excessive oil consumption, silicone from internal coolant leaks, using silicone sprays, or using silicone gasket sealers on the engine.  Environmental factors such as road splash, salt, oil, and dirt can also cause an O2 sensor to fail, as can mechanical stress or mishandling.

A dead O2 sensor will prevent the vehicle’s onboard computer from making the necessary air-fuel corrections, causing the air-fuel mixture to run richer in the default "open loop" mode of operation (fixed air/fuel ratio setting).  This will result in much higher fuel consumption and emission levels.[iv]  Also, an O2 sensor failure may cause damage to the catalytic converter.  A rich operating condition causes the converter to run hotter than normal.  If the catalytic converter gets hot enough, the catalyst substrate that is inside may actually melt - forming a partial or complete blockage – causing the buildup of backpressure in the exhaust system.  That can cause stalling or a drastic drop in highway performance or stalling.  Therefore, to restore engine performance the O2 sensor must be replaced.  A new O2 sensor can improve fuel economy as much as 10-15% and will minimize exhaust emissions, reduce the risk of costly damage to the catalytic converter, and ensure peak engine performance (no surging or hesitating).

When O2 Sensors Must Be Replaced

To minimize encountering/suffering lower mpg and other dire consequences that can result from O2 sensor failure or aging/use, the O2 sensor must be replaced at the following intervals:

Unheated Thimble-type Zirconia O2 Sensor (LS):                             every 30,000-50,000 miles

1976 to early-1990’s (pre-OBD) vehicles

(1 or 2 wire connection)

Heated Thimble-type Zirconia O2 Sensor (LSH):                          every 60,000 miles

1st generation mid-1980 to mid-1990 (pre-OBD II) vehicles

(3 or 4 wire connection)

Heated Thimble-type Titania O2 Sensor:                                     every 60,000 miles

1986 to 1994 (pre-OBD II) vehicles

(3 or 4 wire connection)

Heated Thimble-type Zirconia O2 Sensors (LSH):                         every 100,000 miles

2nd generation mid-1990 and up (OBD II) vehicles

(4 or more wire connection)

Heated Planar-type O2 Sensors (LSF):                                     every 100,000 miles

1997 to present (OBD II) vehicles)

(4 or more wire connection)

Heated Wide-Band O2 Sensors (LSU):                                     every 100,000 miles

2004 to present (OBD II) vehicles)

(4 or more wire connection)

Some External Factors That May Trick an O2 Sensor

Since an O2 sensor reacts to oxygen in the exhaust and not fuel, any engine problem that allows unburned air to pass through the cylinders will also trick an O2 sensor into reading lean.  For example, a leaky exhaust valve - even a leak in the exhaust manifold gasket - may allow enough air into the exhaust to screw up the sensor readings.  It won’t damage the O2 sensor, but it will create a rich running condition that hurts emissions and fuel economy.

Something else you need to know about O2 sensors is that they have to be hot (617° to 662° F) to produce a voltage signal.  It may take a few minutes for the exhaust to heat up the sensor, so beginning in the mid-1980’s, most vehicles have been equipped with a built-in electrical heater circuit to get the sensor up to temperature as quickly as possible.  Usually, these are three-wire and four-wire O2 sensors.  Single and two-wire O2 sensors are unheated.  If the heater circuit fails, it won’t affect the operation of the O2 sensor once the exhaust gets hot but it will delay the computer from going into closed loop, which may cause a vehicle to fail an emissions test.

The Different Types of  O2 Sensors

Unheated Thimble-type Zirconia O2 Sensors (LS):  Introduced in 1976 for feedback fuel control on automotive engines, it consists of a zirconia ceramic "thimble" is encased in a protective tube which extends into the exhaust manifold.  Slots in the protective tube allow hot exhaust gases to reach the thimble.  It may take several minutes to generate a signal after a cold start because it relies solely on the heat from the exhaust to reach normal operating temperature.  Consequently, an unheated sensor may cool off at idle and stop producing a signal causing the engine control system to revert back to "open loop" operation (i.e. fixed air/fuel ratio setting).  This type of O2 sensor has a single wire connector, though some have two.

Heated Thimble-type Zirconia O2 Sensors (LSH):  Introduced in 1982, this sensor adds a heater element connected to a separate electric circuit so the O2 sensor achieves operating temperature between 30-60 seconds.  The heating element reduces cold start emissions and prevents the O2 sensor from cooling off at idle.  This type of O2 sensor has 3 or 4 wire connectors.

Heated Thimble-type Titania O2 Sensors:  Introduced in 1986, Titania O2 sensors use a different type of ceramic and instead of generating a voltage signal that changes with the air/fuel ratio, the sensor's electrical resistance changes.  It was only used on a few vehicles, less than 1% of all O2 sensor-equipped vehicles from 1986 to 1994.  No OBD II equipped vehicles (1995-96 to present) use it.

Heated Planar-type O2 Sensors (LSF):  Introduced in 1997, this O2 sensor has a flat, ceramic zirconia element rather than a thimble.  The electrodes, conductive layer of ceramic, insulation and heater are all laminated together on a single strip.  The new design works the same as the thimble-type zirconia sensors, but the "thick-film" construction makes it smaller, lighter and more resistant to contamination.  A new heater element warms it to operating temperature in only 10 seconds while using less electrical power than older type heating elements.  In 2004, these O2 sensors accounted for 30% of all O2 sensor applications.  By 2008, it is expected to account for up to 75% of all O2 sensor applications.

Heated Wide-Band O2 Sensors (LSU):  The newest O2 sensor technology builds upon the planar design and adds the ability to directly measure the air/fuel ratio via producing a signal directly proportional to the air/fuel ratio so the air/fuel ratios that can precisely measured from very rich (10:1) to extremely lean (straight air).  Instead of flip-flopping back and forth like previous O2 sensor designs, the engine’s computer adds or subtracts fuel as needed to maintain a steady 14.7 to 1 “stoichiometric” air/fuel ratio.  Another difference is a heater circuit that keeps a consistent operating temperature so the sensor will reach operating temperature in 20 seconds.  This sensor allows precision fine-tuning of the air/fuel mixture ratio so beside the Halo Spark Plug, it offers the greatest promise for increasing mpg.