[Sankar]

The the post cat or downstream O2 sensor in a modern OBD2 vehicle is supposedly there to determine that the catalytic converter is working.

I have read that in some cars which use a wideband O2 sensor as the upstream sensor then the input of the downstream narrow band is also taken for fueling during certain situations.

I have also read that in the case of a vehicle with narrow band O2 sensors both upstream and downstream the downstream sensor values are not taken for fueling. If that is the reason then why do some people (on various forums on the internet) report lower FE when they have a P0420 error in their cars with with narrow band O2 sensors?

Any thoughts guys?

[SS-Traveller]

Originally Posted by Sankar

The the post cat or downstream O2 sensor in a modern OBD2 vehicle is supposedly there to determine that the catalytic converter is working.
...the input of the downstream narrow band is also taken for fueling during certain situations.
...report lower FE when they have a P0420 error in their cars with with narrow band O2 sensors?
Any thoughts guys?

Sankar, I'm no expert in this, but a couple of thoughts:
- Wouldn't lower FE (or any other change) depend on the ECU codes written for the specific car/engine where the error occurs?
- Would P0420 error indicate only a downstream O2 sensor issue? This site seems to suggest otherwise...

Causes
A code P0420 may mean that one or more of the following has happened:

• Leaded fuel was used where unleaded was called for
• A damaged or failed oxygen sensor (HO2S)
• Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
• The engine coolant temperature sensor is not working properly
• Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
• Retarded spark timing
• The oxygen sensors in front and behind the converter are reporting too similar of readings
• Leaking fuel injector or high fuel pressure
• Cylinder misfire
• Oil contamination

Possible Solutions
Some suggested steps for troubleshooting a P0420 error code include:

• Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
• Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
• Inspect the downstream heated oxygen sensor (HO2S), replace if necessary
• Replace the catalytic converter

Overall probably the biggest mistake vehicles owners make when they have a P0420 code is to simply replace an oxygen sensor (H02S). It is important to do proper diagnosis so you're not wasting money replacing parts unnecessarily.

[SPIKE ARRESTOR]

The wideband lambda sensor is used upstream and is one of the main inputs for fuel control. The downstream lambda sensor are primarily present for emissions and OBD. Additionally, they also fine tune the inputs for fuelling (signals given by upstream sensor) so that a stable and well controlled close loop is maintained.

Various combinations of 2 point lambda and wideband lambda sensor are used by different manufacturers, depending on cost and other relevant parameters.

Spike

[Mpower]

A failed downstream O2 sensor should not affect FE.

If it is used for setting fuel trim, then its probably under a transient condition.

Which Indian cars are using wideband sensors?

[Sankar]

Thought i'd post this here, found from some forum while googling. This is not applicable to our emission norms but still found it interesting.

Credit to the actual uploader in that forum.

Originally Posted by SS-Traveller

Sankar, I'm no expert in this, but a couple of thoughts:
- Wouldn't lower FE (or any other change) depend on the ECU codes written for the specific car/engine where the error occurs?
- Would P0420 error indicate only a downstream O2 sensor issue? This site seems to suggest otherwise...

Causes
A code P0420 may mean that one or more of the following has happened:

1. Leaded fuel was used where unleaded was called for
2. A damaged or failed oxygen sensor (HO2S)
3. Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
4. The engine coolant temperature sensor is not working properly
5. Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
6. Retarded spark timing
7. The oxygen sensors in front and behind the converter are reporting too similar of readings
8. Leaking fuel injector or high fuel pressure
9. Cylinder misfire
10. Oil contamination

Yes, the lower FE will depend on the specific cars and models, if the downstream sensor is being used for fueling calibration it will definitely affect FE. Some cars use the downstream sensor for fueling and some don't, but in the case of my K12M VVT Swift (2012) i'm not sure if it does or not. The vehicle supports P2270 and 2271 DTC which mean Stuck Rich or Stuck Lean, both are associated with the downstream sensor. But there is no failsafe operation mentioned for this code.

From what i understand, if the P0420 doesn't accompany any other error codes and is on its own its mostly due to a bad cat, leak or bad sensor

1) Leaded fuel damages the cat so it can be teamed with inefficient cat
2) Bad sensor - might throw other codes related to that sensor like voltage or resistance too high or low depending on the problem. Mostly only 420.
3) Will throw other codes related to that sensor like voltage or resistance too high or low depending on the problem.
4) Will throw an error code for coolant sensor
5) And will prolly richen the mixture too since sensor is reading more oxygen.
6) If the knock sensor is gone it should throw an error for that too.
7) Failed or deleted cat con only error 420 will be reported, like it was in my case when i went for headers.
8) Leaking fuel can be detected upon inspection. The too high fuel pressure should generate a code me thinks.
9) Error code will be given for cylinder misfire.
10) Physical check only i guess.

Originally Posted by SPIKE ARRESTOR

The wideband lambda sensor is used upstream and is one of the main inputs for fuel control. The downstream lambda sensor are primarily present for emissions and OBD. Additionally, they also fine tune the inputs for fuelling (signals given by upstream sensor) so that a stable and well controlled close loop is maintained.

Various combinations of 2 point lambda and wideband lambda sensor are used by different manufacturers, depending on cost and other relevant parameters.

Spike

Usually a narrow band is used as the upstream and downstream O2 sensor in most vehicles, but some cars do use a wideband upstream sensor like the post 2005 Impreza. If a wideband is used then the downstream narrow band sensor is also used for fueling trim correction, this is what i've understood so far. My car uses a 4 wired narrow band upstream and downstream sensor, 4 wired sensors are narrowband and wideband sensors are 5 wired.

I'm interested in understanding whether the car manufacturers in India (for eg: Maruti in their new BS4 engines) use both sensors for fueling or only the upstream one. Why would they limit the use of 2nd O2 sensor for emissions check? Unless constrained by bean counters i guess they will use it for fine tuning the fueling too.

Originally Posted by Mpower

A failed downstream O2 sensor should not affect FE.

If it is used for setting fuel trim, then its probably under a transient condition.

Which Indian cars are using wideband sensors?

I too would like to know which Indian cars come with a wideband upstream sensor. The mango cars i've come across all use narrow band upstream.

[Arch-Angel]

Hi Sankar, i just have a question regarding this, since the downstream o2 sensor is used to determine whether the catcon is working(i think its a wideband), wouldn't its removal cause the ECU to detect that the cat is gone? Another question would be if a narrow band sensor is used, wouldn't the pcm get a wrong input and start running rich AFR?

Alright, is this method of reasoning correct? If the downstream o2 sensor detects that there is a presence of COx, the pcm compensates by trying to run on a leaner mixture?

[Sankar]

Originally Posted by Arch-Angel

Hi Sankar, i just have a question regarding this, since the downstream o2 sensor is used to determine whether the catcon is working(i think its a wideband), wouldn't its removal cause the ECU to detect that the cat is gone? Another question would be if a narrow band sensor is used, wouldn't the pcm get a wrong input and start running rich AFR?

Alright, is this method of reasoning correct? If the downstream o2 sensor detects that there is a presence of COx, the pcm compensates by trying to run on a leaner mixture?

I'm no expert on this, but the following is what i've learnt so far on this subject.

The downstream sensor is narrow band, operating voltage between 0 to 1. The removal of the second O2 sensor would first give an error code indicating that the O2S2 heating circuit is open (i have this error now) and after some time it will give another error code (i had it once before when i disconnected the O2S2 before forgot the exact code now) which i will update when i get it now that i don't have the 2nd O2S.

Narrow band is good enough to get the fueling done by the ECU because it can tell the ECU how the engine is running. Wideband is used when the correct AFR needs to be known on a modified car or while getting a tune done. Wideband gives the actual AFR where as narrowband tells whether its lean, rich or stoichiometric.

What down stream sensor detects is O2, if the level of O2 read by the upstream and downstream sensor is the same then ECU interprets it as bad cat. The downstream sensor should read only lower amounts of O2 due to the oxidation taking place inside the cat.

The ECU cycles the fueling between slightly rich and slightly lean always because that is how the three way catalytic converters used in the cars now like it. So the voltage generated in the upstream O2 sensor will cycle between above .46 and below .46 volts, .46 volts is considered as stoichiometric. The downstream narrowband O2S shouldn't switch like the upstream sensor if the cat is efficient, if it switches like upstream one then ECU will give error code telling cat is not efficient.

Even with the removal of O2S2 the engine will not run rich, it still runs in closed loop mode using the upstream O2 sensor.

[SPIKE ARRESTOR]

I forgot to ask, are we discussing Gasoline or Diesel engines here. The lambda control is slightly different for both.

Which lambda sensor is used will depend primarily upon

a) the nature of aftertreatment device used,
b) the nature of control required around L < > 1.

Wideband lambda sensor downstream is a very rare configuration (I have only heard and not seen one yet). It is mostly used upstream when the lambda values fluctuate considerably from stoichiometric range, e.g. a gasoline engine running on homogeneous and stratified (lean burn) modes.

How is the role of a downstream lambda sensor different for diesel and gasoline engines wrt -
a) Catalyst heating and component protection
b) Regeneration
c) Exhaust gas temperature models
d) Dew point temperatures
e) OBD

There may lie the answer.

BTW, what catalysts are found in Indian cars - BS-IV (gasoline / diesel)?

Spike

[Sankar]

I'm looking for answers on the influence of downstream O2 sensor has in fueling in a Petrol engine.

My Swift Petrol which is a BS4 compliant vehicle has a 3 way cat.

[SPIKE ARRESTOR]

Ok, then you most probably will be having 2 point lambda sensors (upstream + downstream) as 3 way catalysts require precise lambda control.

As I said earlier, the downstream sensor is more to fine tune the exhaust gases downstream. It tells the controllers if the downstream exhaust gases are running lean/rich in addition to the inputs provided by the upstream sensor for different engine operating points.

Suppose the engine is running slightly rich (say L = 0.98 due to some reason, although it is supposed to be controlled around L ~ 1) and you do not have this sensor. You get a rich signal from the upstream sensor, but none from downstream (to fine tune). There may be a case that the amount of fuel leanness / richness detected downstream may not be required (also for plausibility reasons) . So that small correction factor is gone na? That small factor translates to a very small change in FE, as the engine is not supposed to run rich at that particular operating point. This is my understanding, hope I am not confusing.

Hope resident experts chip in with their insights, I am still learning vehicle calibration.

Spike

[Sankar]

Yes, 3 way catalysts work in a narrow stoichiometric band, and yes the FE difference might be due to that correction factor like you said. Maybe that is why the service manual gives no failsafe operation for issues related to O2S2. Makes sense now.

[Mpower]

Originally Posted by SPIKE ARRESTOR

I forgot to ask, are we discussing Gasoline or Diesel engines here. The lambda control is slightly different for both.

Correct me if I'm wrong, but AFAIK diesel engines don't have O2 sensors in the first place!

[Arch-Angel]

Originally Posted by Mpower

Correct me if I'm wrong, but AFAIK diesel engines don't have O2 sensors in the first place!

Hi Mpower, i think they do, they are present in order to detect NOx gas in the exhaust in order to manage the egr flow if i remember properly.

[SPIKE ARRESTOR]

Originally Posted by Mpower

Correct me if I'm wrong, but AFAIK diesel engines don't have O2 sensors in the first place!

They have, sometimes 2 depending on the exhaust system configuration. The second one downstream is mounted right next to the DPF and in front of the SCR.

Why do you think it is not required, any reasons?

Spike

[Mpower]

I'm not sure what do you mean by 'next to the DPF' and 'in front of SCR' ? Does it appear 'next to' in a picture?

Its either upstream or downstream of the exhaust !