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Tech: Time to Replace the O2 Sensor on your 3rd Gen Toyota? Short Cuts
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By: Dan Eddleman - May 2005

Check Engine Light

Is it time to replace the O2 Sensor on your 3rd generation Toyota with the 3.4L V-6 or 2.7L 4 cylinder engine? In this case the trusty "Check Engine" light may not be the best guide to tell you when this is necessary. I had noticed that my gas mileage had consistently dropped at least 10% or, about 2 mpg, without the usual explanation of winter oxygenated gasoline or any other symptoms. The Check Engine light had not come on, and I couldn't see any changes on the O2 Sensor voltage swing pattern using my OBDII scanner. So how does one know when it's time to replace this rather expensive sensor?

O2 Sensor Operation Overview
O2 Sensor Output, Engine Idling

If you are not familiar with the OBDII (On Board Diagnostic generation II) systems used on 3rd generation and later Toyotas, you might want to read this reference before proceeding further in this article. As described in the reference article, the oxygen sensor in the exhaust system is the key "feedback" sensor for correcting and maintaining the proper fuel mixture. It measures the post-combustion gases to determine the actual air/fuel mixture of the prior combustion cycle and then, the ECU adjusts the fuel trim percentage on the next combustion cycle, to keep the fuel mixture at the optimum value.

As the oxygen sensor ages, its responsiveness begins to slow down. Since I have a BR-3 OBD-II scanner that can read the voltages generated by the O2 sensor, I thought I would be able to see some indication in the voltage signal that my O2 sensor was not providing optimum performance. However when I examined the O2 Sensor Output with the engine idling, it appeared to be pretty much the same as I had seen when the vehicle was new. Also, the error codes applicable to the O2 Sensor had not shown up and turned on the Check Engine light.

However, if the Check Engine Light were to turn on with one of the following O2 Sensor codes stored, you should follow the diagnostic procedure and replace the O2 sensor if so indicated in the diagnostic procedure.

Oxygen Sensor Error Code Summary
Code Detected Condition Trouble Area
P0130 Oxygen Sensor Circuit Malfunction(Bank 1 Sensor 1)
Voltage output of heated oxygen sensor remains at 0.4V or more, or 0.55V or less, during idling after engine is warmed up (2 trip detection logic)
  • Open or short in heated oxygen sensor circuit
  • Heated oxygen sensor
  • Air induction system
  • Fuel pressure
  • Injector
  • Engine Control Unit
P0133 Oxygen Sensor Circuit Slow Response(Bank 1 Sensor 1)
Response time for heated oxygen sensor's voltage output to change from rich to lean, or from lean to rich, is 1 second or more during idling after engine is warmed up. (2 trip detection logic)
  • Open or short in heated oxygen sensor circuit
  • Heated oxygen sensor
  • Air induction system
  • Fuel pressure
  • Injector
  • Engine Control Unit
P0135 Oxygen Sensor Heater Circuit Malfunction(Bank 1 Sensor 1)
When oxygen sensor heater operates, heater current exceeds 2.35amps (2 trip detection logic)
  • Open or short in heated oxygen sensor circuit
  • Heated oxygen sensor
  • Engine Control Unit
P0141 Oxygen Sensor Heater Circuit Malfunction(Bank 1 Sensor 2)
When oxygen sensor heater operates, heater current exceeds 2.35amps (2 trip detection logic)
  • Open or short in heated oxygen sensor circuit
  • Heated oxygen sensor
  • Engine Control Unit

After doing some extensive reading, it began to come clear how the O2 Sensor performance could be off, yet not be able to see any symptoms (other than lower gas mileage) with the tools I was using. It turns out that the O2 swing cycle shown in the photographs is the swing time of the overall system and not just the O2 sensor itself. To really examine the performance the O2 sensor, for example to determine if it was developing a lean or rich offset, one would have to use a digital volt meter with an averaging feature connected directly to the sensor and look for the 450mV centerline signal in a known, properly performing system centered at the 50% duty cycle. To measure responsiveness, one would have to use a laboratory scope and setup specifically for this measurement as a properly performing O2 sensor has response times in the neighborhood of a few hundreds of a second, not the average 1.8 seconds seen as the overall system response time by an OBDII scanner.

So when do you replace the O2 sensor? The answer turns out to be simple. Per a trusted Toyota service manager's experience and, experiences posted in the Toyota 4x4wire Forum, the O2 sensor needs to be replaced on average every 75,000 miles to maintain optimum gas mileage performance.

O2 Sensor Replacement
The Replacement Oxygen Sensor

Replacing the O2 Sensor is a relatively simple process. Shown on the left is the new O2 Sensor that I purchased for $138.00 plus tax. The part number shown in the photo on the left is for a non California spec vehicle, which uses for Sensor 1, a narrow range oxygen sensor, simply called the oxygen sensor. The sensor 1 used on a California spec vehicle is a wide range oxygen sensor, called an air/fuel ratio (A/F) sensor. So when buying the replacement part, make sure you are specify your vehicle is a California spec or non California spec vehicle to get the correct part.

There are two sensors on your vehicle. Sensor 1 is located in front of the catalytic converter. Sensor 2 will be located behind the first and only catalytic converter on a non-California spec vehicle and behind the second catalytic converter on a California spec vehicle. Sensor 1 is the one that affects gas mileage as its condition degrades. Sensor 2 is there to monitor the condition of the catalytic converters and does not need to be replaced to correct the lower gas mileage problem.

The first photo below shows the location of Sensor 1, in front of the catalytic converter on the exhaust system, just to the right the transmission.

It turns out that disconnecting the Sensor 1 plug from it's mating connector is the difficult part of this job. This is a latched connector and if you simply try to pull the plugs apart, you either won't succeed, or will damage the wiring in the process. I first tried using one of the thin gauges from an old ignition point gauge set to push open the latch to disconnect the O2 plug. This was not successful primarily because I could not see the connector well enough as shown in the second photo below. It will be much easier if you first remove the bolt holding the connector bracket in place as shown in the third photo so you can swing the plug down for easier access. Then, you will be able to see much better the plug retainer latch. As soon as you have correctly moved the retainer latch away from the plug body, the plug should very easily pull out of the connector as shown in the last photo on the right.

Sensor 1 Location Attempting to Disconnect O2 Sensor Plug Removing O2 Sensor Connector Bracket Unlatching the O2 Sensor Plug from Connector

With the Sensor 1 plug disconnected, it is now a simple matter to use a 12mm box end wrench to remove the two bolts holding the sensor 1 and gasket on the exhaust system. Remove the two 12mm nuts as shown below and install the new Sensor 1 and gasket. The torque specs on the 12mm nuts is 14 foot-lbs, which is about the same torque used to install spark plugs.

Removing the two 12mm Nuts New Oxygen Sensor Installed Electrical Contact Cleaner

The installation is now complete. I did take one additional step to clean the Sensor 1 contacts using electrical contact cleaner before inserting the plug into the electrical connector. I am somewhat surprised that this critical connector is not an o-ring sealed connector similar to those used under hood for critical engine connections. If you like to off road in deep water or mud, this is something to think about. You should also consider the fact that the O2 sensor is actually two sensing elements, with one ventilated to the outside to sense ambient oxygen levels for use as a reference for the second internal element that senses the exhaust gas oxygen level.

At this point, you are ready to start the vehicle and check out the operation of the new sensor. I am going to pass along the following experience should you encounter the same situation. The FSM does not state anything about resetting the Main ECU computer, nor have I been able to find out any information indicating that new O2 sensors have a start up or break in period. But, the following experience makes me believe it might take the O2 sensor longer to fire up and operate the first time.

I started my vehicle with the BR-3 scanner attached and watched the new O2 sensor voltage output as the engine warmed up. It seemed that it was taking an abnormal amount of time for the system to go into closed loop and, for the characteristic sine wave swing of the O2 sensor to become evident. So I decided to take the vehicle on a short drive to really heat up the engine and sensor, and with the BR-3 scanner connected. The check engine light came on with a PO125 error indicating that there was insufficient coolant temperature for Closed Loop control. The details of the PO125 error is shown below.

PO125 Error Detail
Code Detected Condition Trouble Area
P0125 Insufficient Coolant Temperature for Closed Loop Operation

After the engine is warmed up, heated oxygen sensor (bank 1, sensor 1) output does not indicate RICH (greater than equal to 0.45V) even once when conditions (a), (b), (c) and (d) continue for at least 1.5 minutes:
  • (a) Engine speed: 1,500 rpm or more
  • (b) Vehicle speed: less than 62 MPH
  • (c) Throttle valve is not fully closed
  • (d) 140 seconds or more after starting engine
  • Open or short in heated oxygen sensors (band 1 sensor 1) circuit
  • Heated oxygen sensor (bank 1 sensor 1)
  • Air induction system
  • Fuel Pressure
  • Injector
  • Gas leakage on exhaust system
  • ECM

In essence, (and knowing that all else was in good condition), this error says that the Main ECU was not seeing the O2 sensor characteristic sine wave swing after waiting 140 seconds which, should have been enough time for the signal appear. I kept driving a little further and eventually begin to see the proper O2 voltage swing. I stopped and reset the PO125 error and have had no further problems. So this tells me there may be a longer first time heat up period for the O2 sensor to begin generating the expected signal. If anyone else has had any experience regarding this, I would appreciate you dropping me a line at the e-mail address at the top of the article.

Lastly, my gas mileage has increased back to what the vehicle was originally getting.


Based on other's experiences with declining gas mileage posted in the 4x4wire Toyota Forums, and a service manager recommendation, the O2 sensor does need to be replaced about every 75,000 miles to keep your Toyota running in optimum condition. It a fairly simple process to do and something anyone who likes to perform their own maintenance and service can do.

It never hurts to have the Toyota FSM for your vehicle which includes diagnostic procedures, possible causes and additional information on differences between California specification and non-California specification vehicles. Also included are precautions you should observe both for personal safety and to prevent other possible damage to the vehicle.

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