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Hello,

This is my first post on this forum, although, I have been reading up on it for months now as a reference (Thanks!). I have been working on older diesel cars for the majority of my life which is why switching over to the 3.8L gasoline jeep has brought me into some unfamiliar territory when emissions codes are being pushed.

Recently my jeep has been kicking a P0420 code "Catalyst System Efficency Below Threshold (Bank 1)".

Doing my due diligence, I started troubleshooting the error code with the simplest of equipment to replace; the O2 sensors. I was able to monitor both the Upstream and Downstream O2 Sensors via the OBDII port for both of the banks to find the Bank 1 Sensor 1 had a semi-random behavior. Bank 2 Sensor 1 (working side) had a more cyclic behavior ranging from ~.2V to .8V. Viewing both of the Sensor 2's outputs it appeared the Catalytic converters are working by the removal of O2 in the exhaust by the downstream (Sensor 2) on both banks staying high ~.8V. Below is a screenshot of the raw sensor data.


Thinking I narrowed it to a faulty O2 Sensor in the Bank 1 Sensor 1 area, I replaced it (upstream sensor on passenger side of car). Upon running the car and resetting the error code, I hooked up my OBDII reader and noticed the output of bank 1 sensor 1 was still outputting seemingly random data compared to an expected sinusoidal curve. Attached is a screenshot of this as well.


All of this data was taken following warming the car ~45 mile highway drive back from work. All spark plugs and distribution for spark plugs was replaced within the past 5k miles. I have been using fuel injector cleaner but it has not proven to help.
Would it be possible the new O2 sensor is faulty or would this be more upstream such as an issue with the fuel injectors?
Of course this starts kicking emission errors as inspections come due ~Murphy law

Any advice into the next step of troubleshooting would be GREATLY appreciated!!

Thanks for the help and the previous help I've obtained from you guys through the forum.

Matt
 

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Here is some info for your code, from the JK service manual:

P0420-CATALYST EFFICIENCY (BANK 1)



Theory of Operation


The State of Change (SOC) catalyst monitor uses the signals from both the upstream and downstream O2 sensors to detect aging of the catalyst. Based on the fact that when a catalyst ages, it loses some of its Oxygen Storage Capacity (OSC). As a result, part of the untreated exhaust gases can breakthrough the catalyst and causes the downstream O2 sensor to deviate from its neutral (Stoichiometric) position. By observing the activities in the downstream O2 signal, the degradation level of catalyst can be detected. In general, the higher the downstream O2 sensor SOC value, the more exhaust gas breakthrough and the lower the OSC of the catalytic converter.

When Monitored:
The monitor will run at between 1400 and 2300 RPM. It also runs between 40 and 70 kPa.

Set Condition:
If the final State of Change index is within the calibrated fail threshold. Two trip fault. Three good trips to turn off the MIL.

Possible Causes
O2 RETURN UPSTREAM CIRCUIT SHORTED TO GROUND
O2 RETURN DOWNSTREAM CIRCUIT SHORTED TO GROUND
O2 RETURN UPSTREAM CIRCUIT OPEN
O2 RETURN DOWNSTREAM CIRCUIT OPEN
EXHAUST LEAK
ENGINE MECHANICAL CONDITION
AGING O2 SENSOR
CATALYTIC CONVERTER

NOTE: A new rear O2 Sensor along with an aging front O2 Sensor may cause the DTC to set. Review the repair history of the vehicle before continuing.

NOTE: If an O2 Sensor DTC set along with the Catalytic Converter Efficiency DTC diagnose the O2 Sensor DTC(s) before continuing.

NOTE: Check for contaminants that may have damaged the O2 Sensor and Catalytic Converter: contaminated fuel, unapproved silicone, oil and coolant, repair necessary.
 

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I had a long series of issues with O2 sensor readings. I swapped sensors multiple times, tore the engine down looking for vacuum leaks, replaced the plugs and numerous other things. I recently discovered the issue to be the connection of the engine harness at the ECU. Wiggling the main harness would cause my O2 sensor readings to fluctuate.

A disconnect and reconnect of the levered connector made the issue go away for several months, but it recently returned. Another unplug, and re-plug and it's fixed again. I'm thinking of using some zip ties to help keep the strain off the connector. Beyond that I'm not sure what to do for a long-term fix. Replacing the main harness and/or the ECU seems like overkill, but ultimately might be my only choice.

Wiggling the harness at the ecu with the engine off, and monitoring the O2 readings would let you know if that's part of your issue.

Good luck.
 

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Discussion Starter #4
Thank you so much for your responses.

It was a cold spell up here in the New England area for the past couple months but finally had a break a weekend or two ago that I was able to play around with the jeep a bit. Turns out from the warmer weather (~55F) the MIL turned off after driving for about an hour. The MIL light came back on the next week when it dropped to 20 outside.

Looking at the waveform while the light was off - it still appears as the previous images I uploaded with sensor 1, bank 1 outputting seemingly arbitrary values. My thoughts now would be that the problem is more upstream than the upstream O2 sensor.

Could it be that a cylinder is fouling except not tripping any error codes? I replaced all of the spark plugs less than 10k miles ago and the distribution block. None of the spark plugs showed signs of fouling when removed. I guess my understanding on the upstream O2 sensor would be it measures how clean of combustion is taking place - could this be an issue with the fuel injectors then? I'm out of ideas on how to diagnose the problem aha any points would be greatly appreciated!

I also traced all of the O2 sensor cables out and reseated them; unfortunately, this did not seem to change the sensor data.

Thank you again for your input!
 

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Hello,

This is my first post on this forum, although, I have been reading up on it for months now as a reference (Thanks!). I have been working on older diesel cars for the majority of my life which is why switching over to the 3.8L gasoline jeep has brought me into some unfamiliar territory when emissions codes are being pushed.

Recently my jeep has been kicking a P0420 code "Catalyst System Efficency Below Threshold (Bank 1)".

Doing my due diligence, I started troubleshooting the error code with the simplest of equipment to replace; the O2 sensors. I was able to monitor both the Upstream and Downstream O2 Sensors via the OBDII port for both of the banks to find the Bank 1 Sensor 1 had a semi-random behavior. Bank 2 Sensor 1 (working side) had a more cyclic behavior ranging from ~.2V to .8V. Viewing both of the Sensor 2's outputs it appeared the Catalytic converters are working by the removal of O2 in the exhaust by the downstream (Sensor 2) on both banks staying high ~.8V. Below is a screenshot of the raw sensor data.


Thinking I narrowed it to a faulty O2 Sensor in the Bank 1 Sensor 1 area, I replaced it (upstream sensor on passenger side of car). Upon running the car and resetting the error code, I hooked up my OBDII reader and noticed the output of bank 1 sensor 1 was still outputting seemingly random data compared to an expected sinusoidal curve. Attached is a screenshot of this as well.


What app are you running ?

All of this data was taken following warming the car ~45 mile highway drive back from work. All spark plugs and distribution for spark plugs was replaced within the past 5k miles. I have been using fuel injector cleaner but it has not proven to help.
Would it be possible the new O2 sensor is faulty or would this be more upstream such as an issue with the fuel injectors?
Of course this starts kicking emission errors as inspections come due ~Murphy law

Any advice into the next step of troubleshooting would be GREATLY appreciated!!

Thanks for the help and the previous help I've obtained from you guys through the forum.

Matt
Here is some info for your code, from the JK service manual:

P0420-CATALYST EFFICIENCY (BANK 1)



Theory of Operation


The State of Change (SOC) catalyst monitor uses the signals from both the upstream and downstream O2 sensors to detect aging of the catalyst. Based on the fact that when a catalyst ages, it loses some of its Oxygen Storage Capacity (OSC). As a result, part of the untreated exhaust gases can breakthrough the catalyst and causes the downstream O2 sensor to deviate from its neutral (Stoichiometric) position. By observing the activities in the downstream O2 signal, the degradation level of catalyst can be detected. In general, the higher the downstream O2 sensor SOC value, the more exhaust gas breakthrough and the lower the OSC of the catalytic converter.

When Monitored:
The monitor will run at between 1400 and 2300 RPM. It also runs between 40 and 70 kPa.

Set Condition:
If the final State of Change index is within the calibrated fail threshold. Two trip fault. Three good trips to turn off the MIL.

Possible Causes
O2 RETURN UPSTREAM CIRCUIT SHORTED TO GROUND
O2 RETURN DOWNSTREAM CIRCUIT SHORTED TO GROUND
O2 RETURN UPSTREAM CIRCUIT OPEN
O2 RETURN DOWNSTREAM CIRCUIT OPEN
EXHAUST LEAK
ENGINE MECHANICAL CONDITION
AGING O2 SENSOR
CATALYTIC CONVERTER

NOTE: A new rear O2 Sensor along with an aging front O2 Sensor may cause the DTC to set. Review the repair history of the vehicle before continuing.

NOTE: If an O2 Sensor DTC set along with the Catalytic Converter Efficiency DTC diagnose the O2 Sensor DTC(s) before continuing.

NOTE: Check for contaminants that may have damaged the O2 Sensor and Catalytic Converter: contaminated fuel, unapproved silicone, oil and coolant, repair necessary.
I had a long series of issues with O2 sensor readings. I swapped sensors multiple times, tore the engine down looking for vacuum leaks, replaced the plugs and numerous other things. I recently discovered the issue to be the connection of the engine harness at the ECU. Wiggling the main harness would cause my O2 sensor readings to fluctuate.

A disconnect and reconnect of the levered connector made the issue go away for several months, but it recently returned. Another unplug, and re-plug and it's fixed again. I'm thinking of using some zip ties to help keep the strain off the connector. Beyond that I'm not sure what to do for a long-term fix. Replacing the main harness and/or the ECU seems like overkill, but ultimately might be my only choice.

Wiggling the harness at the ecu with the engine off, and monitoring the O2 readings would let you know if that's part of your issue.

Good luck.
I'm having similar issues chasing down bank one efficiency issues. I just changed both upstream sensors yesterday. I had changed the downstream sensors a while ago. I will try checking the ECU connectors. before I changed the upstream sensors I disconnected the battery and shorted the leads together, i then pulled the sensors and from wut i could see the cats looked ok (they were all there and didn't look clogged). I reinstalled the sensors, reconnected the battery and reinstalled an 87 tune from my Superchips tuner. I started up the jeep and checked for leaks and went for a test drive, no codes and drove fine, the wife and I drove around all day with no issues. on my way to work this morning, i got a CEL about 15 miles into my 20-mile commute. so the bank 2 code is back. i have a dash command app to monitor some obdII sensors. ( i updated my flashpaq and lost data logging:mad:) the dash command app cant monitor all 4 O2 sensors. do you guys know of a IOS app that will have more obdII data logging? anyways this is frustrating as hell, I hate the damn CEL being on .... a new set of cats from O'Reilly's is $800 but rockauto has replacements for around $400. or a set of pacesetter long tubes and off road y pipe is about the same $400, but the headers will require a tune to delete the cel for no cats.
 

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The Torque app can monitor all 4 O2 sensors.
 

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can you post a screenshot? the Torque app on ios has terrible reviews

I can't, the Jeep in in the shop having a new Metalcloak 5.5 lift installed.

As I understand the ios app requires a wifi OBDII adapter, because Apple locked down the bluetooth features. I bought a cheap Android tablet to keep in the Jeep. It works pretty good with the direct bluetooth connection.
 

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However, there's this:

 

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ok cool. I downloaded another app and was able to log some data during lunch today. and all 4 sensors were all over the place (like the ones on the left in your video). I'm not sure if it's good or bad though. looking at the live data is great but I'm not sure what I'm looking at. ill record data on my commute home today just to see if I can download it and ill try to post it.
 

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ok cool. I downloaded another app and was able to log some data during lunch today. and all 4 sensors were all over the place (like the ones on the left in your video). I'm not sure if it's good or bad though. looking at the live data is great but I'm not sure what I'm looking at. ill record data on my commute home today just to see if I can download it and ill try to post it.
You want to see the voltage changing between 3 and 5 volts on each sensor, that shows that the sensor is working in the right parameters.

Sent from my SM-N910V using Tapatalk
 

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O2 sensors measure the oxygen content in the exhaust. With regards to narrow band O2's they are essentially only accurate at stoich. O2 in the exhaust at stoich is 1.5% which translates to 450mv or .450 volts.

Upstream O2's are used for primary fuel control. In closed loop the ECU commands a rich command when the O2 level is lean <.450 volts; and a lean command when O2 level is rich >.450 volts. This results in the typical O2 switching which results in a waveform. The amplitude and frequency is important for proper cat operation.

The rear O2's basically monitor cat performance. There is about 21% O2 in the atmosphere and about 1.5% in the exhaust ahead of the converter. The converter uses this oxygen to combine with bad gases like CO and HC's to create harmless elements like H20.

Oxygen is stored in the pores of the cat and the frequency of switching affects the efficiency of the converter. Since the rear O2 sees less free O2 the amplitude is lower than the front O2. If the amplitude and frequency is the same as the front obviously the cat is not working.

So what you want to see is both front O2's switching rapidly, if not you may have a bad O2 or mechanical problem. With no converters the O2's will continue to switch. Exhaust leak, weak compression, bad injector, intake leak, stuck egr valve, pcv malfunction...... can all cause codes.

I'd say most of the time P0420 and 30 is a bad cat; however, cats are usually the symptom of a cause. Burning oil can whipe out a cat by coating the cat. Running too lean or rich can melt the cat metals sealing the pores. Running superchargers or headers can make things worse.

If your front O2 readings are erratic address that first.
 

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data logs

I was able to pull some screenshots on my commute today. I got the CEL P0430 at about 15 minutes into the commute again.
 

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I have been fighting this myself. Started with the P0420 code last year.. Right before my emissions check of course. I figured my cats were bad or broken but the Jeep was running fine.
I eventually cut out my stock cats (don't do this) after buying an ebay set and new O2 sensors all around. I still need to fine tune some exhaust adjustments but this took care of my P0420 code for a year.
Fast forward to August 2018 and driving around town and bing, P0430 code. This is now bank 2.

I don't drive the Jeep daily, so I started poking around and will have to try some connections.

2016 I installed new exhaust manifolds, so it shouldn't be those...
 

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I was getting p0420 last year in my '08. Swapped the sensors around & it remained the same, so it was a bad cat.

After checking the price for the Y pipe here, (~$2500AU!*), I figured I could get an exhaust shop to replace the twin cats on each side, but I haven't had good experiences with any of them that are still around. So I went whole hog & bought a set of the RIPP Long Tube Headers and their catback exhaust. A little more than an OEM Y pipe after shipping & GST, but not by much.

Had a p0430 for a while after replacing them, but I put that down to contamination that has since been burnt out from either some silicone or having the headers ceramic coated before I installed them. Either way, the code would set if I drove along the motorway for any distance after I installed them, but it hasn't come back for 6-9 months, so I'd say they are all good now.

*It's what we in Oz refer to as the Australia Tax. It's the 50-100% markup companies put on any product they sell in Oz. (This includes software downloaded over the internet, by the way! They seem to figure they got away with upping the price due to 'shipping costs' for so long, now they do it for everything.) In the case of FCA, typical markup compared to US pricing is ~500%.
 
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