02 sensor

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toymr2
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02 sensor

Post by toymr2 »

Had the engine warning light come on- mk3 facelift borrowed a reader which says faulty bank one sensor two heater circuit, there are 3 02 sensors on the exhaust manifold, my question which one is faulty



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Re: 02 sensor

Post by jimi »

Any use to you Peter ? (courtesy of ROC)
Seems to be some confusion here as to which o2 sensor is which as far as I can tell from the technical information I have, there are three lambda (o2) sensors on the roadster (UK). The DTC codes and descriptions are as follows:

Bank 1 Sensor 1 is the top, pre-cat sensor on the right hand side of the manifold (when you are standing at the back of the car looking forward with the engine in front of you)
DTC Codes:
P0135 Oxygen Sensor Heater Circuit Malfunction(Bank 1 Sensor 1),
P0130 Oxygen Sensor Circuit Malfunction (Bank 1Sensor 1)
P0133 Oxygen Sensor Circuit Slow Response (Bank 1 Sensor 1)

Bank 2 Sensor 1 is the top, pre-cat sensor on the left hand side of the manifold (when you are standing at the back of the car looking forward with the engine in front of you)
DTC Codes:
P0155 Oxygen Sensor Heater Circuit Malfunction(Bank 2 Sensor 1)
P0150 Oxygen Sensor Circuit Malfunction (Bank 2 Sensor 1)
P0153 Oxygen Sensor Circuit Slow Response (Bank 2 Sensor 1)

Bank 1 Sensor 2 is the post cat sensor accessible from the bottom of the car just after the main cat. And by the way is a be-ach to get out.
DTC Codes:
P0141 Oxygen Sensor Heater Circuit Malfunction(Bank 1 Sensor 2)
P0136 Oxygen Sensor Circuit Malfunction (Bank 1 Sensor 2)
also
"The P0141 code is an advisory code, it has no effect on the actual running of the engine...

The code refers to what we call the 3rd O2 sensor, but is more accurately called bank 1, sensor 2. There are 2 other O2 sensors on the engine, known as Bank 1, Sensor 1, & Bank2, Sensor 1. These 2 are responsible for measuring the fuel air mix that has just been burnt and exhausted by the engine. There are 2 of them because Toyota's ECU actually runs this engine as 2 separate entities, i.e. it treats it as 2 two cylinder engines. Weird, but effective.

The 3rd O2 sensor is fitted downstream of all the cats, whereas the 2 metering O2 sensors are fitted upstream so that they can measure the raw, un-catalysed gas coming out of the engine. Its only job in life is to measure the O2 content of the exhaust gas after the cats. The ECU then compares this reading to the one it took from the Bank 1 sensor in the manifold. The logic it then applies to the 2 readings is astonishingly simple:

If the 2 readings are different, and more importantly the reading from the 3rd O2 sensor is more stable than that on the manifold O2 sensor, then all is well with the world. It means that the ECU knows your cats are working as they have changed the make up of the gas.

If the 2 readings are the same, then the ECU knows that the cats are doing nothing to the gas at all, and lights the CEL, which prompts you do go and have your wallet lightened by your local dealer...

Because there's no effect on the closed loop control of the Fuelling, the engine management makes no changes that would effect power, or economy.

You don't actually have a failed O2 sensor though, I'd bet good money that if I hooked up my OBDII reader to your car, I'd see sensible readings coming from the 3rd O2. what you actually have, is a failed heater element.

Each O2 sensor has a built in heater inside it. The heaters are used to get the Zirconia measuring sensors inside them up to operating temperature quickly so that they can get on with the job of measuring oxygen. This gains vital seconds in the process of gaining efficient emissions control on a cold start of the engine. Once the engine is hot, clearly anything in the exhaust flow isn't going to be in need of heaters! The ECU monitors these heaters by measuring the current going to them, to much or too little flags up the fault code you have at the moment. The sensor itself is still working, it just takes a while to warm up... Again, having a duff heater in an otherwise functional 3rd O2 sensor wont cause any problems with engine management.
So, what to do to fix it? Well, you can replace the unit, or you can ignore it... Either way your car will go just as fast as its always done, using exactly the same amount of fuel. The problem with just leaving it, is that with the engine light already lit, you wont see any new problems should they occur. Fixing the thing is expensive, especially considering all you are going to gain is clear dashboard.

I applied a bit of electrical tom-foolery to mine when it went, I simply connected a high power resistor to the heater circuit on the sensor, so that it drew the "correct" amount of current from the ECU, That way ,as far as the ECU was concerned, the heater was working, and after 2 full start -warm up - drive -stop- cool down cycles of the engine, the light goes out. Simple..."
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toymr2
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Re: 02 sensor

Post by toymr2 »

Cheers Jimi :th: :)


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Re: 02 sensor

Post by stephen.farr@btinternet.com »

Having been quoted £270 to change one by my main dealer on diagnosing why my engine light was on, I will share what I learnt to avoid paying it! Hopefully this puts more info in one place.
Whereas the garage said it was Bank 01 sensor heater failure, this didn't tell which one, there are two Bank 01 sensors! It was easy for me to find which sensor was at fault by simply measuring the heater circuit on the pre exhaust pair located on the exhaust manifold. (As above - Bank 01 is the RHS looking at the engine, Bank 02 is LHS; Bank 01 sensor 2 is post exhaust and located under the exhaust). I simply unpluged the pre sensors ( press the tab on the mounted socket and pull the plug out) to measure the resistance across the two black wires to the sensor. The RHS (Bank 01 sensor) was open circuit, the LHS was around 200ohm. So I didn't need to bother with measuring the 3rd. All actual sensors are identical.... but the cabling to pre and post is the differing factor and this determines all the different manufacturers codes. I went online and opted for Denso, and will use them as the example here: DOX0204 is the pre exhaust sensors with correct Toyota connector and 360mm lead,. DOX 0206 850mm lead for post. There is a 3rd version... DOX 0109 is "universal" i.e. no plugs! if you want to save a few quid and solder on your original plugs!
Finally proved very easy to take out the duff sensor, sprayed around the mount with Plus Gas at various temperature prior and came out easily. Denso come with pre greased threads and even had the torque setting. It and an O2 sensor tool ( a drive with a slot for the cable) cost £66! Oh... and finally had to disconnect the battery to reset the ECU to get the engine light to go off!

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