Failed Emissions On Mot

[androidGB]

Hi, I wonder if anyone can offer any advice on this one.

My 2000 Yaris (32000 miles) failed the emmisions test and the garage which carried out the MOT test(I'm in the UK) suggested I take it to a Toyota dealer.

No engine light had come on.

After "looking" at the car they suggested first trying to change the oxygen sensors (2) as that was the cheaper option and secondly if that didn't work changing the catalytic converter.

I couldn't believe this amateurish diagnostic approach and said surely there must be a way of testing the sensors.

Can anyone offer any advice

Many thanks

Andrew

I should have given the readings

Fast Idle

CO 1.5% Fail

HC 045 PPM Pass

Lambda 0.961 Fail

2nd Fast Idle

CO 1.0% Fail

HC 022PPM Pass

Lamda 0.971 Pass

Natural Idle Test

CO 4.85% Fail

[fast_silver]

Hi, I wonder if anyone can offer any advice on this one.

My 2000 Yaris (32000 miles) failed the emmisions test and the garage which carried out the MOT test(I'm in the UK) suggested I take it to a Toyota dealer.

No engine light had come on.

After "looking" at the car they suggested first trying to change the oxygen sensors (2) as that was the cheaper option and secondly if that didn't work changing the catalytic converter.

I couldn't believe this amateurish diagnostic approach and said surely there must be a way of testing the sensors.

Can anyone offer any advice

Many thanks

Andrew

I should have given the readings

Fast Idle

CO 1.5% Fail

HC 045 PPM Pass

Lambda 0.961 Fail

2nd Fast Idle

CO 1.0% Fail

HC 022PPM Pass

Lamda 0.971 Pass

Natural Idle Test

CO 4.85% Fail

←

possible catlic converter failure?

[GT4 BOOSTER]

Sensor Testing

The Lambda Sensor is at the heart of emission control continually monitoring the exhaust gas. Carbon Monoxide (CO) is produced by the engine. If the mixture supplied is too rich then CO will be high and visa versa. Having plenty of fuel (Rich) the engine will try to burn as much as possible, using up all available Oxygen, conversely if there is not sufficient fuel to maintain a correct burn then the excess Oxygen will pass into the exhaust system.

Rich mixture = High CO = Low Oxygen

Weak mixture = Low CO = High Oxygen

The Zirconia lambda sensor, by using precious metals, can determine the Oxygen difference between atmosphere and the exhaust gasses. The greater this difference, the higher the voltage it will produce, up to app 1volt. Lambda sensors will not operate until around 300 deg. They are heated by the exhaust and often have their own heater element. Single & twin wire lambda sensors have no heater element and are usually situated near or in the manifold, three & four wire sensors have an built in heater to aid rapid warmup and may be placed further downstream.

From the table it can be seen that the voltage produced changes significantly within the window lambda 0.97 - 1.03. The ECU monitors this voltage and can correct the mixture strength (by changing the injector open time) to try to maintain perfect combustion.

The sensor can be tested for output by connecting to it a digital voltmeter, set to a 2v scale. Connect the +ve test lead to the sensor signal wire [see wire colours] and the -ve test lead to earth/battery -ve. The voltage produced should swing from app 0.2v to 0.8v and back around once per second. If the voltage is stuck high then the engine is probably running rich for some other reason. If stuck low then usually either a faulty Lambda Sensor, air leak or an engine running very weak for some reason.

The newer type of lambda sensor (titania) work on a different principle, changing a supplied voltage as against creating one, but they still measures the Oxygen differences. These later types are generally more accurate and responsive.

not seen a yaris lambda as yet, but most toyota sensor wires are blue.