How to Test an Oxygen Sensor (P0420)

Heated oxygen sensor (4-wire)

(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?

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 Firing Order Database to confirm the cylinder layout and bank orientation.
Tip: Exhaust routing can place sensors close together—verify the correct one before testing.
Upstream vs downstream O2 sensor locations

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.
Tip: Leaking fluids or heat exposure can corrode sensor connectors and cause signal issues.

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.
Identifying O2 sensor heater wires
  • 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.
Upstream O2 cross-count example
Tip: A failing catalytic converter can cause the downstream O₂ signal to mirror the upstream sensor pattern.

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.
Rich spike after snap throttle

Helpful Tool:
Live Data Scan Tool

Replace an oxygen sensor

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

Removing an O2 sensor with a socket
  • 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.