Emissions Sensors And Catalytic Converter

[toucan]

I have a perfectly good and ultra reliable 1995 Previa GS 2.4 auto but for the 2nd year running it has failed the MOT on emissions test.

Last year the MOT station recommended replacement patent cat' because they could not say whether it was the cat', oxygen sensors or both. The cat' was enough to get it through the MOT. I have only covered 2700 miles this year and it has failed again.

Once more the MOT station couldn't say which was at fault and inferred if the sensors were faulty last year they may have damaged the new cat' over the 2700 miles.

Went to Romford Toyota who persuaded me they had the diagnostic equipment that could identify the offending part and probably save me shelling out for parts I didn't need. On that basis I left it with them.

They were surprised to learn no warning light has ever shown up. They did a diagnostic test but couldn't get readings from the non manufacturer spec cat'. They replaced the oxygen sensors (there were 'stored fault codes' of some description) but the car still failed the re-test.

I had them take the sensors off. Now they want £1000 to replace the cat' with a Toyota genuine part without which they say they cannot accurately trace the fault/get a reading. Even then they cannot guarantee I won't still need their very expensive sensors refitted aswell.

The cost would then be heading towards £1600. I've told them to stop work while I get advice.

Help.

Isn't there anyway to determine which is the offending part?

Is there a patent part cat' they can get a reading from?

If we continue with this hit and miss method who knows where the bill will end up.

[propnut]

You haven't said what the emissions failed on, CO, HC or Lamda. This will be stated clearly on your MoT report and will provide the clue as to what is going wrong. If it is CO then chances are it is the Catalyst, If it is HC then it could be the sensors or something in the air/fuel ratio and again with the lambda test.

The role of your Lambda sensor is to detect the ratio of free oxygen in the exhaust gases, too much and the car is running too lean, too little and the car is running too rich. It sends this info to the ECU which then takes corrective action by increasing or decreasing the injector pulse duration for any given throttle load. However erroneous readings can be obtained due to other external conditions such as a blocked or missing air filter, low compression on a cylinder (which can be due to worn rings or a sticky/tight valve/incorrect clearances), incorrect fuel rail pressure.

The Catalytic converter uses the free oxygen in the exhaust gases to oxidise the noxious gases into something more benign e.g. CO becomes CO2, NO becomes NO2 etc. It is called a catalyst as the matrix (traditionally platinum) acts as the catalyst to invoke the reaction.

Your system may use a single or dual cat and 2 or more Lambda sensors. The first sensor closest to the exhaust manifold is the one that sends the corrective info to the ECU, the second one measures how well the first one is doing its job. It sounds as though you are not mechanically minded and as such I would suggest , as I have done elsewhere in this forum, that you go and find an old school mechanic who still has the ability (and experience) to diagnose problems without the use of an OBD scanner. In my opinion these scanners are the worse thing that could ever have happened to the consumer. They are meant as a trouble shooting aid for the mechanic but the problem is most modern mechanics have lost the traditional diagnostic skills and use the scanner as a verbatim gospel. If it throws up a code that it reads from the ECU then "that must be the problem". B*llS**t, in some cases the ECU gets it right but it is essentially a dumb computer with a system of variables programmed into it. If something fails that matches an exact code then great, but sometimes the ECU is not so sure so and hazards a guess as to what the problem might be and throws up the nearest code that it thinks fits the bill. In this instance it is up to the mechanic to diagnose based on proper trouble shooting techniques and MOST importantly experience what has gone wrong.

In my experience if he calls himself a mechanic then he is old school and knows what he is doing. If he calls himself a "technician" he is just a glorified code reader/part changer. The latter are the ones that are going to fix by process of elimination, changing parts AT YOUR EXPENSE, until the problem goes away.

[toucan]

You haven't said what the emissions failed on, CO, HC or Lamda. This will be stated clearly on your MoT report and will provide the clue as to what is going wrong. If it is CO then chances are it is the Catalyst, If it is HC then it could be the sensors or something in the air/fuel ratio and again with the lambda test.

The role of your Lambda sensor is to detect the ratio of free oxygen in the exhaust gases, too much and the car is running too lean, too little and the car is running too rich. It sends this info to the ECU which then takes corrective action by increasing or decreasing the injector pulse duration for any given throttle load. However erroneous readings can be obtained due to other external conditions such as a blocked or missing air filter, low compression on a cylinder (which can be due to worn rings or a sticky/tight valve/incorrect clearances), incorrect fuel rail pressure.

The Catalytic converter uses the free oxygen in the exhaust gases to oxidise the noxious gases into something more benign e.g. CO becomes CO2, NO becomes NO2 etc. It is called a catalyst as the matrix (traditionally platinum) acts as the catalyst to invoke the reaction.

Your system may use a single or dual cat and 2 or more Lambda sensors. The first sensor closest to the exhaust manifold is the one that sends the corrective info to the ECU, the second one measures how well the first one is doing its job. It sounds as though you are not mechanically minded and as such I would suggest , as I have done elsewhere in this forum, that you go and find an old school mechanic who still has the ability (and experience) to diagnose problems without the use of an OBD scanner. In my opinion these scanners are the worse thing that could ever have happened to the consumer. They are meant as a trouble shooting aid for the mechanic but the problem is most modern mechanics have lost the traditional diagnostic skills and use the scanner as a verbatim gospel. If it throws up a code that it reads from the ECU then "that must be the problem". B*llS**t, in some cases the ECU gets it right but it is essentially a dumb computer with a system of variables programmed into it. If something fails that matches an exact code then great, but sometimes the ECU is not so sure so and hazards a guess as to what the problem might be and throws up the nearest code that it thinks fits the bill. In this instance it is up to the mechanic to diagnose based on proper trouble shooting techniques and MOST importantly experience what has gone wrong.

In my experience if he calls himself a mechanic then he is old school and knows what he is doing. If he calls himself a "technician" he is just a glorified code reader/part changer. The latter are the ones that are going to fix by process of elimination, changing parts AT YOUR EXPENSE, until the problem goes away.

[toucan]

Thanks for your advice.

I hope you won't mind if I pick your brains again........

When I had the cat replaced last year I'm sure we had the one or two cat discussion and it transpired it had only one. Certainly only one was replaced.

The car failed the crypton gas analysis catalyst (closed loop) test on both first and second 'fast idle' tests.

In the first test it registered .50% CO, 10ppm HC and .99 Lambda.

In the second it registered .89% CO, 10ppm HC and .97 Lambda.

In the last 'natural idle test' it passed?, the CO level measuring .08%

I'm not a mechanic as you will have reasoned by now. I don't know anyone locally (Essex) who has the knowledge and capability to troubleshoot and as you quite rightly say, the majority of service centres are only interested in replacement and fixed price quotes.

I can order a cheaper cat and sensors on the net but am I right in thinking there would be no guarantee come next year that I will not be faced with the same emissions test failure and cat or sensors issue and what effect is that likely to have on the ECU/EMU, will it respond despite not working with genuine Toyota parts?

You should reasonably expect more than a year from any replacement parts, but it seems the cat I had fitted last year hasn't survived if your CO theory is correct.

Can a faulty sensor or sensors cause the cat to break down after only 2700 (mostly town driving) miles?

How much might a dirty air filter play any part? I don't know the condition of mine. Would it be useful to change this?

It looks like there are patent cats of different quality on the net, constructed with different materials.

Toyota's own must be gold plated for the price they want. Can you recommend the construction type I should go for?

Finally, if I can afford to replace the cat and both sensors more reasonably off the net do you suppose, given the figures above, I could reasonably expect the car to pass the emissions test or are there other parts/issues I have to consider?

I ask because Toyota told me my vehicle has 4 sensors whereas everyone has otherwise only referred to two.

[propnut]

Here's what you do

1. Lie down under the driver side of your car around the middle section with a torch and peer at the exhaust pipe. You should see in the mid section a fat piece which is the Cat. If you have only one there obviously will be only one.

2. Either side of this (assuming only one) you will see a device sticking vertically up. The front one should have 4 wires and the rear two wires. These are your O2 sensors.

Now a sensor can be checked both inside the car and externally for operation, my personal choice is externally. Whilst this is not hard to do it requires bench vice, a blow torch (simple butane one from B&Q will suffice), a test meter and unfortunately for you a medicore of experience. The experience is required for two elements.

1. To get the sensors off without buggering up the threads that hold them on-----depending on how rusted these are this can be one swine of a job.

2. The ability to use a multi test meter

Unfortunately I gather you may not be up to either and I mean no disrespect its just an observation which you have confirmed.

There are primarily two types of CAT, a cheap ceramic and a more expensive Platinum matrix type. The ceramic type also has precious metals but these are impregnated onto its sub matrix rather than comprising the main constituent. Both work equally well.

The main cause of failure is coating of the matrix with carbon or silicon. Now as far as a Previa goes by far the most likely cause is a head gasket that is on its way out and is slowly leaking coolant into the combustion chamber. Coolant contains silicon and as this passes over the Cat coats the matrix and stops it working. It can however also be contaminated by carbon due to either burning of oil or running consistently rich and a dirty oil filter will most definitely cause this, however your HC readings seem quite low and your Lamda seems to indicate that the sensors are working alright.

I would probably start by getting someone to check the head gasket in two ways

1. A water jacket combustion test. This tests for the presence of CO in the water system. If you have noticed the car using water and or the temp guage sitting higher than normal then I would suspect this.

2. The other is a compression or cylinder leak test, both will indicate issues especially if adjacent cylinders read low.

These cars MUST be service VERY regularly so I would give it a damn good service changing everything i.e. Plugs, Air Filter, Oil

AND check the PCV valve, this allows the crankcase to breath and needs to be working efficiently.

If you can find a family friend or someone you know who is a mechanic and is prepared to do this for you then I would take this route. That way you will know that money will not be his/her main motivation.