O2 sensor questions

PDelux

A couple of questions for the motor experts please ;
1. can a bad o2 sensor lead to damage of the catalytic converter(i.e. so damaged it needs to be replaced)?
2. is it possible the o2 sensor is damaged and no error code is given by the diagnostic machine hooked up by the garage?

paddy2008

yes a faulty 02 sensor can cause this fault and may not show up any DTCs on a scan tool but a simple emmissions test will prove this.

lifer_sean

PDelux wrote: »

A couple of questions for the motor experts please ;
1. can a bad o2 sensor lead to damage of the catalytic converter(i.e. so damaged it needs to be replaced)?
2. is it possible the o2 sensor is damaged and no error code is given by the diagnostic machine hooked up by the garage?

1 - yes it can. The o2 sensor is used to control the quantity of fuel into the engine. If the sensor incorrectly signalled that there was oxygen in the exhaust (mixture too lean) then the ECU will compensate by over-fuelling the engine. This would cause unburned petrol to pass into the exhaust, and can damage the cat if in sufficient volume.

2 - A correctly connected diagnostic tool should flag this. Is the diagnostic tool communicating with the car, ie does it "see" the ECU ? No communication is not the same as no fault code !

Darragh29

lifer_sean wrote: »

1 - yes it can. The o2 sensor is used to control the quantity of fuel into the engine. If the sensor incorrectly signalled that there was oxygen in the exhaust (mixture too lean) then the ECU will compensate by over-fuelling the engine. This would cause unburned petrol to pass into the exhaust, and can damage the cat if in sufficient volume.

2 - A correctly connected diagnostic tool should flag this. Is the diagnostic tool communicating with the car, ie does it "see" the ECU ? No communication is not the same as no fault code !

An example of this happening would be if you had a vacuum leak causing excess air to get into the inlet tract, say for axample a vacuum leak at the pipe supplying the brake servo assist assembly witha vacuum from the inet manifold.

Air has approximately 23% oxygen. If you have a vacuum leak, this causes excess air and on this basis, excess oxygen, to enter the combustion process.

Your lambda/O2 sensor will detect this excess oxygen and adjust the air fuel ratio to be rich (excess fuel), which will end up being burnt off in your cat.

Your cat is designed to do this, but at a specific rate. If it is constantly overloaded with unburnt hydrocarbons over and above what it designed to deal with, it will burn out.

As Sean has said above, a diagnostic system will flag this with an emissions test, (you need to do an oxygen test that is independent of the vehicle lambda/O2 sensor, and you'll also need to do a HC and CO test to see if the air-fuel ratio is running rich or lean or what is going on here), but you'll still have to hunt down the vacuum leak that is causing the problem, the computer or emissions tester won't find this for you...

PDelux

Thanks for the good replies.
Is there any other sensor involved in calculating the air/fuel mix ratio other than the o2/lambda sensor?

Darragh, as regards the vacuum leak if this was the case i guess the air/fuel ratio would be bad the whole time during the running of the car, right? So i would see excess fuel usage, maybe some stuttering of the engine in idle and maybe alot of smoke from the exhaust?

Darragh29

PDelux wrote: »

Thanks for the good replies.
Is there any other sensor involved in calculating the air/fuel mix ratio other than the o2/lambda sensor?

Darragh, as regards the vacuum leak if this was the case i guess the air/fuel ratio would be bad the whole time during the running of the car, right? So i would see excess fuel usage, maybe some stuttering of the engine in idle and maybe alot of smoke from the exhaust?

Yeah, definitely fuel consumption is a sign that this is happening. I've seen some cars though with major emissions problems and it would not be obvious from just listening to the engine that the emissions are all over the place.

Most cars have at least 2 lambda/O2 sensors, one upstream of the cat (between the engine at the cat inlet), and another one downstream of the cat (between the cat outlet and the middle exhaust box). Using two lambda sensors, one operating at each end of the cat, the operational efficiency of the cat itself can be computed and fed back to the ECU. Some cars only have one lambda sensor and some cars for example Maxima's have 4 of these sensors.

Another common issue that can fu*k up a cat in jig time is if you have a coil pack problem or an ignition coil problem. On some VW Passats, the way the coil is configured, one half of the coil pack supplies an ignition spark to cylinders (1) and (4) and the other half of the coil pack supplies a spark to cylinders (2) and (3). I've seen one or two cases of this coil pack acting up and the engine running on only 2 cylinders and what happens here is the air-fuel mixture that is passing through the 2 non-firing cylinders, goes straight into the cat as unburnt fuel and starts a little furnace of it's own in there. One car I saw this happening on nearly went up in flames because the customer kept driving the car and only stopped it when he felt heat coming around his feet at the pedals. When he turned up at my garage, the cat was white/yellow with heat and the engine bay was starting to catch fire so we had to use a carbon dioxide fire extinguisher to cool everything down and stop the car going up in flames. The cat in this case was scrap.