Oxygen sensors explained

An oxygen (O2) sensor, also called a lambda sensor, measures how much oxygen is left in the exhaust. The engine computer uses the sensor before the catalytic converter to keep the air-fuel mixture at the chemically correct ratio, called lambda 1. For gasoline that is about 14.7 kg of air per kg of fuel (Bosch gives 14.66). A second sensor after the converter checks how well the converter is working.

Upstream and downstream sensors

Modern exhaust systems have at least two oxygen sensors:

• Upstream sensor (control sensor): before the catalytic converter. The computer reads it to adjust fuel delivery. If the mixture is rich, it injects less fuel; if lean, more.
• Downstream sensor (diagnostic sensor): after the catalytic converter. It checks that the upstream control is working, and the computer also uses it as a monitoring device. A converter whose efficiency drops below the threshold sets P0420 (bank 1) or P0430 (bank 2).

Bank 1, bank 2, sensor 1, sensor 2

Codes name sensors by bank and position:

• Bank 1 is the side of the engine that contains cylinder 1. Bank 2 is the other side, on V and flat engines. An engine with a single row of cylinders has only bank 1.
• Sensor 1 is the sensor closest to the engine on that bank (upstream). Sensor 2 is next along the exhaust, usually after the converter, and so on.

So "bank 1 sensor 1" is the upstream sensor on the cylinder 1 side. Which side is bank 1 depends on where the maker put cylinder 1; the service information shows it for your engine.

Types of oxygen sensor

Sensors are heated to reach working temperature; Bosch's older finger-type sensors are heated to about 350 °C. Newer planar sensors have the heater built into the sensing element, so they are ready sooner after a cold start.

What OBD-II checks

The OBD-II rules require the computer to monitor:

• The output voltage and response rate of the upstream sensors, including slow response in one direction only (rich-to-lean or lean-to-rich).
• The output, activity and response of the downstream sensors.
• The heater circuit of every heated sensor.

The computer must flag a sensor before its deterioration would make emissions exceed 1.5 times the applicable standard.

Codes an oxygen sensor can set

The same faults on bank 1 sensor 2 are P0136 to P0141. Bank 2 sensor 1 starts at P0150.

Codes the sensor reports but does not cause

The sensor is often the messenger. A working sensor that sees a real lean or rich mixture reports it, and the computer sets a fuel system code:

• P0171 and P0174: system too lean, bank 1 and bank 2.
• P0172 and P0175: system too rich, bank 1 and bank 2.

Air leaking into the intake or exhaust between the engine and the sensor makes the signal wrong. Bosch's test routine includes a leak test of the intake and exhaust for this reason. Check for leaks before replacing a sensor for a lean code.

Why oxygen sensors fail

Bosch groups failures into electrical faults, mechanical damage, poisoning or clogging, and overheating. The deposit on a removed sensor often points to the cause:

Fixing the cause matters. A new sensor fitted to an engine that burns oil or coolant will be contaminated the same way.

How to check an oxygen sensor

Bosch's workshop routine, in order:

1. With the engine warm, read the fault memory and look at the sensor's live values with a diagnostic tool.
2. Check the signal pattern against the specification for the vehicle.
3. Inspect the wiring and connectors.
4. Leak-test the intake and exhaust, especially between the engine and the sensor.

For a heater code, check the heater's power supply and whether the computer is switching the heater on.

Normal values and signal patterns depend on the vehicle, so compare what you see with the service information.

When to replace an oxygen sensor

Bosch treats lambda sensors as wearing parts and recommends checking them every 30,000 km (about 18,600 miles). Replace a sensor when the tests above show it is faulty, or at the interval in your vehicle maker's maintenance schedule if it lists one.

• Fit the same type of sensor as the original. Bosch notes that finger-type sensors must be replaced with the same type.
• New Bosch sensors come with their threads already greased.
• After replacing a sensor, clear the code and drive. The oxygen sensor monitor needs to run again before it shows complete; see drive cycles and readiness monitors.

A short history

Bosch put the first production lambda sensor on the road in 1976, on Volvo 240/260 cars for the US market. It made the three-way catalytic converter practical, because the converter only cleans all three main pollutants well when the mixture is held close to lambda 1. Oxygen sensor monitoring was one of the two minimum requirements of the 1990 US Clean Air Act Amendments for OBD, with the catalyst. See the history of OBD-II.

For other terms, see the OBD-II glossary.

OBD DashX reads and clears OBD-II trouble codes on Android with a Bluetooth ELM327 adapter.