How to Test an Oxygen Sensor (P0420)

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(This testing applies to 4-wire Heated Oxygen Sensors only, not wide-band or A/F ratio sensors.)

What are the symptoms of a bad oxygen sensor?

• Check engine light

• Excessive exhaust smell

• Poor fuel economy

• Catalytic converter failure

Recommended Tools

Locate the correct oxygen sensor for testing

• Whether diagnosing a trouble code or testing function, first locate the correct/suspect sensor.

• Determine if it’s Bank 1 (cylinder #1 side) or Bank 2, and whether it’s upstream (before catalytic converter) or downstream (after converter).

• Use the to confirm the cylinder layout and bank orientation.

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Visual inspection

Difficulty: 1/5
Time: ~10 minutes
Tools required: Flashlight, inspection mirror

• Check for damaged wiring, loose connectors, or contact with hot exhaust components.

• Confirm the sensor is securely installed in the exhaust stream.

Test the oxygen sensor heater

Difficulty: 2/5
Time: 30–60 minutes
Tools required: Flashlight, multimeter

• Identify the two heater wires—on most 4-wire sensors, these are same-colored.

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• Using a multimeter set to Ohms, measure resistance between the heater pins.

  • Typical range: 4–25 Ω (consult service info).

• If resistance is out of range, replace the O2 sensor.

Suggested Tools:

Check the oxygen sensor signal

Difficulty: 3/5
Time: 30–60 minutes
Tools required: Multimeter or scan tool

• Warm the engine to operating temperature.

• Monitor O₂ voltage range (0–1.0 V typical).

  • Upstream (S1) should switch rapidly above and below 450 mV.

  • Downstream (S2) should fluctuate slowly and average between 500–700 mV.

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Test the oxygen sensor signal range

Difficulty: 3/5
Time: 30–60 minutes
Tools required: Multimeter or scan tool

• Verify the O₂ sensor can read lean (< 450 mV) and rich (> 450 mV).

• Create a small vacuum leak to induce a lean signal — voltage should drop quickly.

• Perform a snap-throttle to create a rich condition — voltage should spike high.

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Helpful Tool:

Replace an oxygen sensor

Difficulty: 2/5
Time: 30–60 minutes
Tools required: O₂ socket or 7/8" wrench

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• Disconnect the electrical connector and remove the sensor with an O₂ socket.

• Apply anti-seize if recommended and torque to spec.

• Reconnect harness and clear any stored codes.

Suggested Replacement Sensors:

Oxygen Sensor FAQ

How should an upstream (S1) sensor behave at idle?
It should switch rapidly above and below ~450 mV when the engine is at operating temp.

What’s normal heater resistance?
Typically 4–25 Ω (always verify the exact spec for the vehicle).

Can a bad catalytic converter mimic a bad O₂ sensor?
Yes. A failing cat often makes the downstream sensor switch actively like the upstream sensor.

Upstream vs downstream—what’s the difference?
Upstream (before the cat) reacts quickly to mixture changes; downstream (after the cat) is slower and averages ~500–700 mV during steady cruise.

Is it safe to induce a vacuum leak for testing?
A small, brief leak for testing is fine—watch for a quick drop below 450 mV—then restore the hose immediately.