Oxygen Sensors



Wideband O2 Sensor

  • The A/F Sensor voltage signal is relatively proportional to the exhaust oxygen content. In other words, it is not a comparative measurement between the basic deltas of amount of oxygen in the exhaust vs. amount of oxygen in the atmosphere as in the Narrow Band O2 Sensor. Instead it is a signal that shows the proportions of oxygen in the exhaust. The A/F Sensor changes its current in relation to the amount of oxygen in the exhaust. A circuit then completes the translation, detects the direction and strength of the current flow, and puts out a voltage signal relatively proportional to the exhaust oxygen content.
  • It is important to really realize that with the A/F Sensor the voltage signal IS proportional to the change in the air/fuel mixture. Think of the A/F Sensor as a generator capable of changing polarity.
  • The A/F Sensor is designed so that at the stoichiometric set point of 14.7:1 there is absolutely no current flow and the resulting voltage signal put out by the circuit is called reference voltage.
    • The reference voltage varies from car to car, but is often 3.3v or 2.6v.
  • Rich condition  ➱ low oxygen in exhaust  ➱ A/F Sensor generates current in the negative direction (below ref voltage).
  • Lean condition  ➱ high oxygen in exhaust  ➱ A/F Sensor generates current in the positive direction (above ref voltage).
  • Notes
    • Diagnostic system runs functional test on a/f ratio  (O2) sensor at least once per each driving cycle.
    • Generic OBD II
      • Generic OBDII scan tools display about 1/5 of real A/F voltage.
      • Examples 3.8V=760mv, 3.3V=660mv, 2.8V=560mv.


Narrowband O2 Sensor

  • Old style oxygen sensors (implied “Narrow Band”) are made of zirconia, with platinum electrodes and a heater element. The oxygen sensor generates a voltage signal based on the amount of oxygen in the exhaust compared to the amount of oxygen in the atmosphere.
  • The zirconia element has one side exposed to the exhaust stream and the other side is exposed to the atmosphere. Each side of the zirconia element also has a platinum electrode attached. The platinum electrodes conduct the voltage generated. The way this works is when there is less oxygen in the exhaust, there is a large difference in oxygen content when compared to the amount of oxygen in the atmosphere. This in turn produces a higher voltage signal. On the other hand, when there is more oxygen in the exhaust, there is a small difference in oxygen content when compared to the amount of oxygen in the atmosphere.’
    • At the stoichiometric air/fuel ratio of 14.7:1, which is considered ideal for efficiency and emissions, oxygen sensor voltage output is approximately 0.45 volts which is right in the middle of lean or rich.
    • Rich condition  => low oxygen in exhaust  => 0.6V -0.9V.
    • Lean condition  => high oxygen in exhaust  =>  0.1V – 0.4 volts.
  • Limitations
    • Because of the obviously limited range of the old style oxygen sensor ( .1 volt up to .9 volts or so ) the old style oxygen sensor is indeed unable to detect exactly how “lean” or how “rich” the air/fuel mixture is. So essentially the old style oxygen sensor acts as a relatively rapid switch and simply switches from lean to stoichiometric to rich and back and forth and back and forth. This explains the oscillating effect of regular air/fuel gauges which are basically just showing the stoichiometric switch effect.
    • Also because of the obviously limited range of the old style oxygen sensor, small changes in the air/fuel ratio from the set stoichiometric air/fuel ratio of 14.7:1 will really radically change the voltage signal of the oxygen sensor. Again, the oxygen sensor cannot detect the small subtle changes in the exhaust stream oxygen content produced by changes in the air/fuel mixture. Therefore, in the presence of these really radical changes in voltage signals, the ECM will continuously add and subtract fuel producing a lean/rich cycle that oscillates back and forth over and over.


Troubleshooting O2 Sensors


đź“Ž Lean Fuel Trims, P0171, P1130 | Bad O2 | 2000 Lexus RX300

📎 Fuel Trims, P0170 | Bad O2 Sensor | Rover 45 

đź“Ž Surging at idle and poor running, O2 Sensors DTCs | Reverse O2 Installation | 1999 Ferrari 355

đź“Ž False Rich, False Lean | O2 Reverse Installation

đź“Ž O2 Sensor Heater B2S1, P0052 | Bad Reman PCMs | 2007 Jeep Wrangler, Dodge Durango
  • P0052 – Oxygen (A/F) Sensor Heater Control Circuit High (Bank 2 Sensor 1)
  • Daniel Lynn from IATN replaced PCM with 3 “new” computers from the dealer, one at a time, which didn’t work.  Instead of P0052, other oxygen sensor codes appeared. Problem was that the “new” PCM’s that he bought were remanufactured. Once he bought really new, not remanufactured unit, problem was fixed. Letter R on a part number means “remanufactured” Letter RL on a part number means “new”
đź“Ž O2 Sensor, P0161 | Non-OEM O2 Sensor | 2006 Durango 4.7L
  • P0161 Bank 2 Sensor 2 heater performance.
  • Two times sensor was replaced with a Bosch part, one time with NTK. Same code. Wiring checked out good. Heater circuit worked fine, except live data showed PWM was always near 100% for that O2S2B2 Heater, while the other side went down from 100% to about 80%. OEM sensor fixed the problem.
đź“Ž Wideband O2 Sensor Live Data | Toyota
  • Ref voltage is 3.3V
  • At idle or 2500 rpm readings should vary very slightly from 3.0V to 3.3V. Induce rich and lean condition and look for a quick response.
  • While driving at 25 MPH or more with above 1500 rpm, monitor sensor response. During acceleration, may read below 3.0V (fuel enrichment). During decel, may read above 3.3v (fuel cut), but readings should return to 3.3V during cruise throttle operation.
  • If sensor’s output remains at more than 3.8V, or less than 2.8V, or doesn’t change from 3.3V, ECM sets P1130.
  • If ECM sees constant 3.3V from a sensor, the circuit may be open.
  • If ECM sees more than 3.8V, or less than 2.8V, the circuit may be shorted.
  • Testing by propane. Put propane into the intake. Watch the fuel trim. If it was steady around +50%, it should sharply go down. If it doesn’t drop, then sensor is bad.
  • You cannot test a wideband sensor by connecting a scope. Neither amp nor voltage readings show any changes on the signal wires.
📎 No Start | Shorted O2 Sensor | 15′ Focus 2.0 GDI
  • Car failed to start (almost started), but felt like it lacked fuel. After the front O2 has been disconnected, started right up.



Testing O2 Sensors Tips

  • O2-Fuel Trim Comparison
    • Compare downstream and upstream O2 sensors with fuel trim readings.
    • If trims are high (indicating lean mixture) and rear O2 indicates very rich mixture, you can suspect front O2 sensor to be faulty.
  • Wide Open Throttle (WOT) Test
    • At WOT O2 should show rich condition. If it doesn’t, it might indicate bad O2 sensor, or dirty MAF…
  • Lean-Rich Response
    • While the engine is idling, temporarily disconnect a vacuum hose. You should see the STFT fuel trim readings jump immediately and go POSITIVE, and the LTFT should start to creep up in response to the artificial lean fuel mixture you have just created by disconnecting the vacuum hose.
    • Feed some propane vapor from a small propane tank into the throttle body or a vacuum hose connection on the intake manifold. This time, you should see a drop in fuel trim readings, with STFT going NEGATIVE, and LTFT creeping downward in response to the rich fuel mixture.
    • No change in fuel trim readings when you create an artificial lean or rich fuel mixture would tell you the engine computer is NOT operating in closes loop, or that the oxygen sensor(s) are not responding to changes in the fuel mixture.
  • Drive Test
    • Acceleration  = rich exhaust
    • Deceleration = lean exhaust
    • Engine runs leaner than 14.7:1 under cruise conditions.


False Rich
  • Spark plug wire that is too close to the O2 sensor might induce higher than normal voltage in signal wire
  • Loose O2 sensor ground
  • O2 Reverse Installation
  • O2 Contamination
  • Oil on a sensor can falsify O2 readings (2012-2014 Subaru)


O2 Contamination Causes
  • Due to additives in engine coolant. Has head gasket been recently replaced?
    • (O2 inspection, white sandy or gritty deposits, or green due to dye in coolant)
  • Due to silicon poisoning from bad fuel or wrong silicone sealant.
    • (O2 inspection, white chalky deposits)
  • Due to rich mixture.
    • (O2 inspection, black sooty deposits)
  • Due to excessive oil consumption from defective PCV or mechanical engine problem
    • (O2 inspection, dark brown deposits)
  • Contaminated from outside, blocking reference oxygen


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