Oxygen sensors tell the computer how rich or lean the exhaust is. When they age, fuel economy drops and emissions rise, and a smog check can fail. Testing the sensor and its wiring tells you whether replacement is needed.
Key takeaways
- Sensors wear with age.
- Wiring faults mimic sensor failure.
- Replace upstream and downstream sensors as needed.
What the sensors do
They measure oxygen in the exhaust so the computer can adjust fuel. Upstream sensors control fuel mix, and downstream sensors monitor the converter.
Signs of a bad sensor
A lit light, worse fuel economy, rough idle and a failed smog check are common.
Test before you replace
A scan tool shows the sensor's response. A slow or flat reading suggests an aging sensor. A wiring or heater circuit fault looks similar, so we test the circuit.
Replacement
Sensors thread into the exhaust and can be stuck. We use the right tools and the specified part.
After replacement
The computer relearns, and monitors take time. Plan a retest accordingly.
Upstream and downstream sensors
Most modern engines have an oxygen sensor before the catalytic converter, called upstream, and one after it, called downstream. The upstream sensor helps the computer adjust fuel mixture. The downstream sensor checks whether the converter is working. A code names which sensor and which bank of the engine is involved. Replacing the right one saves money, and replacing the wrong one leaves the light on.
Signs a sensor is aging
Sensors do not fail suddenly. They get slow. You may notice a slow drop in fuel economy, a faint rough idle or an occasional light that comes and goes. Because the effect is gradual, many drivers do not notice until a smog check. If your sensors are older than about 100,000 miles, testing them during routine service is cheaper than a failed test.
Need help in south Bakersfield?
Questions about smog repair? Read our Smog Repair page, call (661) 599-9098 or request a time. Peña's Auto Electric, Smog & Repair, 303 Taft Hwy, Suite B, Bakersfield, CA 93307. Mon to Fri 9:30 AM to 5:30 PM, Sat 8 AM to 1 PM.
Frequently asked questions
How long do oxygen sensors last?
Often 60,000 to 100,000 miles depending on the type.
Do I replace all of them?
Only those that fail testing.
Can wiring cause a sensor code?
Yes.
Will a new sensor fix my mileage?
If the old one was failing, often yes.