# 2.5 Emissions frustrations



## gootz (Dec 29, 2015)

Hi guys, I am new to this forum but love my past 4 VW's - all mkiv's. 
This is my first mk5. I live in NY and sadly have to have emissions. I bought this mk5 Jetta value edition manual. Love the car but it had a check engine code which said p04020. The old owner showed me as i didn't take his word for it. Sure enough that was it and he said he would sell it after fixing the Catalytic converter. He got a new one on it and within the day, the light came back on. He won't help me with it as he said he replaced the cat (which I know he did as I saw the old and new). 

Anyways a few weeks have gone by and I have looked up some of this issues and I believe it can be the upper 02 sensor... I bought a bosch 16698 which is for the lower according to my engine code (BGQ due to being sold here in NY and NY emissions). So I crawl under my car to take a look at the lower 02 sensor and see 2 under there! I have one in the Cat and then one after it! Why does my car have 3 02 sensors? My question is...what 02 sensor should I replace and which Bosch/Denso/etc to use??

Thank you for helping!


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## Master_P (Feb 10, 2012)

BGQ and BGP engines are both 2.5 in-line 5 cylinder engines. The only difference (that I'm aware of) is that BGQ has three O2 sensors, whereas BGP has two. There is one sensor before the cat, and one after the cat. BGQ engines have an additional third sensor in the middle of the cat, while BGP does not. BGQ engines are sold in states with strict emissions requirements eg. NY and CA.

P0420 indicates that the catalytic converter efficiency is problematic. Either the converter is bad (most common), or there is faulty readings with the O2 sensors leading the ECM to think the converter is not working correctly.

If I had to make a wild guess without any OBD scanners/VCDS/Torque, I would probably start with the middle O2 sensor, then the rear sensor after the cat. It's probably not the front sensor because that sensors primary job is to measure fuel mixture from the engine. If that sensor was faulty, you would see codes indicating the fuel mixture is too rich or lean. You might also see hits on fuel economy as well. I have used Torque Pro to watch this sensors behavior on SWMBO's MKV Jetta. It will cycle between rich and lean conditions after the engine warms up and enters closed loop.

The middle sensor measures converter efficiency. From my readings with Torque, this sensor should give a constant output voltage around 0.5v. If it's reading too high or low, there might be a converter problem, but since you've swapped the converter, that leaves the sensor as suspect. The rear sensor varies a little bit too, but not as much as the front sensor. The ECM uses the voltage differences from the three sensors to measure converter efficiency.

If you want to know which sensor to chase, I suggest buying an ELM OBD2 Bluetooth adapter. They only cost $15-20 online. Use the free Torque app with your smartphone and watch the second and third O2 sensor behavior when the car is warmed up. You can also check/clear CEL lights as well.


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## gootz (Dec 29, 2015)

Thank you Master P. I have been researching for some time the ELM OBD2 Bluetooth adapters. I have a PC and an iPhone...no android. Will my PC suffice? 
Anyways, I do plan on buying one that will work with a PC...it seems you can't use an iPhone with these obd2 adapters. 

p04020 bank 1 doesn't specify which 02 sensor it is?? I have read that the upper one is a culprit many times. Like I said, I have the BOSCH 16698 02 sensor and have read its after the catalytic converter. I see the Bosch 16002 should work too?? I don't get how multiple models should work??

Lastly, lets say I get my ELM adapter and get voltages for the 3 02 sensors which i have (GOD I HATE NY), how will it be apparent that they are defective...??
Thanks for your time.

David


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## Master_P (Feb 10, 2012)

gootz said:


> Thank you Master P. I have been researching for some time the ELM OBD2 Bluetooth adapters. I have a PC and an iPhone...no android. Will my PC suffice?
> Anyways, I do plan on buying one that will work with a PC...it seems you can't use an iPhone with these obd2 adapters.


A laptop might work. I own a Droid phone and use Torque with the ELM adapter, and cannot offer personal experience with laptops. Having said that, a quick google search indicates that iPhone/iPad solutions are marketed. You will need to research this further.



> p04020 bank 1 doesn't specify which 02 sensor it is?? I have read that the upper one is a culprit many times. Like I said, I have the BOSCH 16698 02 sensor and have read its after the catalytic converter. I see the Bosch 16002 should work too?? I don't get how multiple models should work??


In-line cylinders with one exhaust manifold utilize a single bank. For a V-6 engine, there will be two cylinder heads with three exhaust ports per head. Each head has a dedicated three-port exhaust manifold, and comprises one bank. In this case, the bank assignment tells you which converter and/or O2 sensors are suspect. This isn't applicable with our cars, so the bank assignment is not relevant.

Your DTC code indicates the ECM is not satisfied with the catalytic converter efficiency. That is different from general O2 sensor functionality. Either the sensors are working normally and the catalytic converter is not, or the converter is working properly but one or more O2 sensors is telling the ECM that it's not working.

If a specific sensor does not output voltage within a defined specification (i.e. all 0.0 volts or 1.0 volts in the case of narrow-band AFR), the ECM will throw a code dedicated to the parameter for that specific sensor, and it will tell you which sensor is the problem. If the heater voltage is too high, low, or the wrong resistance, the ECM would throw different codes. In these cases, you can definitively identify the problematic sensor because the ECM continuously monitors those parameters on each sensor, and there are dedicated DTC codes for each value on each sensor. This isn't your case.

Finally, I think both of the Bosch sensors you listed are used in BGQ engines:
http://www.boschautoparts.com/auto

From what I gather, 16698 is used in BGQ engines after the primary converter. 16002 is used in both BGQ and BGP engines after the secondary converter. Bosch offers two other O2 sensors for our cars. One is a wideband primary sensor (before the cat), and a universal sensor which can work after either downstream positions. This would suggest that 16698 - used only on BGQ engines - goes in the middle of the converter given that only BGQ engines support a sensor in this location. 16002 would go at the converter exit. The differences between the PN's could be the length of the wire loom, connector style, or grommets and fittings in the wire loom. The sensors are likely functionally equivalent.

On my old Acura, the front and rear O2 sensors were the exact same functioning narrow band sensor with the same connector, but the rear sensor required longer wire length to reach the wiring harness - and the harness was tucked under the floorboard. The sensor had a built-in grommet to keep dirt out of the cabin. Since the form was different, the PN was different. I think something similar is going on here.



> Lastly, lets say I get my ELM adapter and get voltages for the 3 02 sensors which i have (GOD I HATE NY), how will it be apparent that they are defective...??


My first post should have answered this. When the engine is warmed up and enters closed loop operation, the primary sensor should fluctuate between rich and lean conditions. This sensor voltage will vary between 0.1-0.2 volts and 0.8-0.9 volts (for a narrow band). The middle sensor should hold steady around 0.5 volts. The rear sensor should probably follow the middle sensor, but might vary a little bit.

In open loop (cool engine), the engine will run rich. The primary O2 sensor voltage will reflect a rich condition. Once the car warms up and you depress the gas pedal, you should see the O2 sensor quickly react to the added fuel.

If the downstream sensors mirror the behavior of the primary sensor, you have a bad converter.

If one of the downstream sensors constantly reads too high (>0.8 volts) or too low (<0.3 volts), it may be defective.

If you push the gas to create a rich condition and one or both downstream sensors react too slowly, that sensor may be defective.

Read this for additional insight:
http://www.autoserviceprofessional....y-air-fuel-and-oxygen-sensor-diagnosis?Page=1


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## gootz (Dec 29, 2015)

*Many Thanks*

Master P - I really really appreciate your time here. I ordered the mini elm327 which is iPhone compatible tonight and researched some apps which people recommend. You rock man. that was super informative. I understand 02 sensors now...thanks for breaking it down. Enjoy your night and thanks again


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