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2009 STS 4.6
2009 STS 4.6
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22 Posts
Discussion Starter #1
I have a 2009 sts northstar. I got the code po430 catalyst system efficiency below threshold bank 2. I’m assuming it’s an 02 sensor. I had the one below the passenger cat replaced a few months ago. It wasn’t throwing a code but the car was misfiring and they ended up replacing it and a spark plug and coil. I’m just trying to figure out what’s going on. I love this car but it’s been non stop small things since I bought it this September. Any help is appreciated.
 

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2006 STS4 4.6L
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537 Posts
Can anyone say if the extended coverage applies to the 2009?
There was a sort of extended warranty on the cats for the STS, although I can't remember what years it applied to. Your cat may be done for.
 

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'93 SedanDeville 60 Special
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230 Posts
You did not mention how many miles engine and smog parts have as answer would change

Always best to change both such as the front ones as a set rather then one working faster then the other switching slower but I'd look for a exhaust leak first.

If possible swap the 2 fronts and see if the DTC is same one or now for the other side.
Make sure the dealer work was proper as seating wiring connectors, plug wires, etc
  • The catalytic converter is no longer functioning properly
  • An oxygen sensor is not reading (functioning) properly
  • There is an exhaust leak
  • Check wiring for O2 is OK, no cuts or crimps and if wanting to ring out the wires as if there is 4 wires
1. Goes to ground
2. gets 12 volts from a fuse for O2 heater
3. PCM sends the O2 0.450 mVolts
4. would be output of the O2 back to the PCM


Using a thermal temp tool, after engine is fully heat up
measure the temps right before the CAT and right after it

Do the same for the left side and see if the temps are about the same across both CATs.
 

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'93 SedanDeville 60 Special
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230 Posts
I believe for best function performance of O2 sensors have a maximum lifespan of about 30,000 miles due to functioning in about 600-800 degrees and get lazy after that so I would have replaced both fronts so there is no real imbalance of reporting the AFR of each side.
With 90,000 plus miles could be possible the CAT is caking up but tests above would help decide if CAT is at issue or not.
 

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2006 STS4 4.6L
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537 Posts
I highly doubt your upstream o2 sensors have little to any affect on the code you're getting; provided there is perhaps a small margin which it could be affecting it, I highly doubt it would be significant or noteworthy.
"A maximum lifespan of 30,000 miles" LoL, no. I've heard 50,000 and 100,000, but 30k is just ridiculous. The o2 sensors are built to withstand extremely high temperatures, they are heated for crying out loud.
There is a reason GM had the extended coverage on the cats. I'm not saying with 100% certainty that it is a bad cat, but I am saying it with a greater certainty than the idea of just a bad o2 sensor.
 

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'93 SedanDeville 60 Special
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I highly doubt your upstream o2 sensors have little to any affect on the code you're getting; provided there is perhaps a small margin which it could be affecting it, I highly doubt it would be significant or noteworthy.
"A maximum lifespan of 30,000 miles" LoL, no. I've heard 50,000 and 100,000, but 30k is just ridiculous. The o2 sensors are built to withstand extremely high temperatures, they are heated for crying out loud.
There is a reason GM had the extended coverage on the cats. I'm not saying with 100% certainty that it is a bad cat, but I am saying it with a greater certainty than the idea of just a bad o2 sensor.
I suggest you re-read what I posted "best performance"
OP's car is a 2009, O2 sensor designs were not best to handle what Ethanol does in degrading O2s.

Common for vendors to state 30-50K and O2 my function but are degrading and the fact they only function up to only 1 volt, it is easy when degrading for the reported AFR to be incorrect
The fact the 02 sensors are used for other functions of what the PCM does, including determining the state of the CATs to deciding on startup what the average injector ON time will be per drive cycle.

Try doing a internet search but I know what Bosch told me when I worked at GM about 02s designed in early 2000 and effects of the hotter exhaust temps due to Ethanol and PCM calibration for smog purposes

Collective exhaust temp with a V8 and due to leaner AFR or Ethanol easy be 300 deg per cylinder thus 1,200 plus degrees input to a CAT but can be even higher so PCM tries to protect the CAT by using the 02s to monitor temps and if need be command more fuel via COT mode to cool the CATs down

The fact is the PCM is deciding on the P0430 DTC by using the up/down stream O2s and if they are degraded can also then incorrectly state a CAT issue when it is the o2s reporting

As I posted, there are others ways to check what the true cause of the DTC is but in many cases it is not the CAT failed but an exhaust leak or degraded O2s ( up or down stream)

575184


575183
 

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2006 STS4 4.6L
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Right, I was referring to "best performance". The man is welcome to change all his o2 sensors, but he shouldn't be surprised if he still has the aforementioned code set afterwards. I certainly believe that as an o2 sensor is subjected to wear (whether by regular use or contaminants), and that a new o2 sensor may report more accurately than an older one, there is a reason GM extended the coverage for the cats. I have heard plenty more times of bad cats vs bad o2 sensors on our vehicles, although it isn't absolute. Changing the o2 sensors could probably only hurt OP's wallet (at worst), but in my opinion he would likely just be throwing parts at it.
 

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'93 SedanDeville 60 Special
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230 Posts
Changing a degraded O2 as they are cheap saves in fuel use as the PCM is dictating what the injector pulse with ON time is on what a O2 reports.
P0430 is dictated on what the O2s output is and from that the PCM is doing routines to decide what the internal temps of a CAT and deciding to trigger COT to dump excess fuel to cool the CAT down so degraded 02s are more apt to cause the P0430 then the CAT itself
Again only testing for exhaust leaks, bad AFR, O2 sensors and then the CATs themselves but replacing at least the front O2s can only better performance and less fuel use.
If in fact the rear O2s are not tripping DTCs would sway that the CAT output is within exhaust makeup range and more point to front O2s, AFR or exhaust leaks.,

Vehicle owners for some reason think if the nameplate says sparkplugs are good for 100,000 then they do not change them, yet when nameplate says the engine oil of today can work for 7-15K miles the owners are still changing the oil very 3,000 miles.
Same thing, O2s not really designed for Ethanol and it's effects on O2s and CATs are being ignored as a maintenance replacement
 

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2006 STS4 4.6L
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I feel like the subject is focusing more on o2 sensors now than whatever the actual problem is that OP is experiencing. I'll wait for him to actually do some homework and respond to us before I continue the conversation. It's kind of pointless to discuss it until we actually know what the real issue is, honestly.
 

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2009 STS 4.6
2009 STS 4.6
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22 Posts
Discussion Starter #11
The downstream passenger side sensor was replaced. I’m hoping that it’s just the driver side sensor acting funny since it’s still original. I’m going to buy a new one today and give it a test to see if that’s it. I’ll post what happens. If that’s not it I’ll take it to a mechanic. But for $50 I think it’s a good place to start.
 

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'93 SedanDeville 60 Special
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The downstream passenger side sensor was replaced. I’m hoping that it’s just the driver side sensor acting funny since it’s still original. I’m going to buy a new one today and give it a test to see if that’s it. I’ll post what happens. If that’s not it I’ll take it to a mechanic. But for $50 I think it’s a good place to start.
Check the wiring of the O2 sensors while your at it.
Make sure none are cut, melted, bound up.
Also check for any exhaust leaks or signs of carbon leaks.

When taking a O2 sensor on or off make sure the wiring harness connector is unplugged so that the wires do not get rapped up and cut off the airpath in wires

Best case using a OBD-II scanner is monitor all the O2 sensors and see if the values left/right are about equal and how those values compare to the rear O2 sensors.

With engine warmed up the readings at idle should be kinda around 0.500 mVolts

All O2 sensors are not equal as to the brand,
I'd suggest OEM ones over Generic types may report incorrectly
 

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2006 STS4 4.6L
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The downstream passenger side sensor was replaced. I’m hoping that it’s just the driver side sensor acting funny since it’s still original. I’m going to buy a new one today and give it a test to see if that’s it. I’ll post what happens. If that’s not it I’ll take it to a mechanic. But for $50 I think it’s a good place to start.
Were you getting a code before bank 1 was replaced?
 

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2009 STS 4.6
2009 STS 4.6
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22 Posts
Discussion Starter #14
I wasn’t getting any lights when the guy replaced the first one. The car was just running really rough. It’s idling rough now and feels choppy when accelerating. I’m hoping that maybe since he replaced one it’s making the old one look bad in the computers eyes
 

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2006 STS4 4.6L
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The downstream sensors will have little to no affect on the idle. Provided, the downstream sensors will work more or less in conjunction with the upstream sensors to monitor system efficiency, but the downstream sensors would be like the last place I would look as an answer to a rough idle. Rough idle/poor acceleration would most likely be fuel or air; MAF, air filter, vacuum, spark, coils, injectors, fuel pump/filter, throttle body, etc. After all those I would start looking at the upstream sensors. Certain conditions and symptoms will help point in one direction or another.
 

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'93 SedanDeville 60 Special
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The long and short term fuel cells along with the O2 sensors need to be monitored by a OBD-II scanner and see what the values are
If using a good scanner should be able to see if the PCM is going into closed loop once engine reached normal coolant temps which means PCM is not seeing some type of sensor issue.
Also possible the shop used a generic O2 and most do not work well with the PCM's hardware quad driver for O2s.
 

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2006 STS4 4.6L
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ACDelco is always my number one choice when it comes to parts, some people like Bosch o2 sensors but I've never used them. I've seen new parts bad out of the box, since then I always try to stick with OEM and/or reputable companies.
 

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2008 STS Northstar RWD, 2000 Trans Am WS6 6 spd, 1968 LeMans convertible 6.6
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141 Posts
A lot of mis-information in this thread.

OP, your misfiring was the coil/plug. IDK why the upstream O2 was replaced, maybe it had a heater circuit code. If it was misfiring long enough, that may have melted the converter substrate enough to degrade it. Also, I have never seen a degraded O2 sensor throw a false P0420/430. I have replaced plenty of O2 sensors in my career. Sometimes the customer will come back a few weeks later with the Check Engine light on and the only code will be a P0430/420, and I have to explain why that code is now there when it was not before the O2 replacement.

A catalyst efficiency code is just that, an efficiency issue. Since 1996 or so, when OBD2 came out, the primary purpose of the O2 sensor downstream of the catalytic converter is to check the function of the converter (there are some exceptions, cough cough Chrysler minivans) and the upstream O2 is for fuel trim. If the downstream O2 sensor is pretty much mirroring what the upstream O2 sensor is doing, that is a signal to the PCM that the converter is not doing its job. Before the PCM runs the P0420/430 monitor, it will first check the function of the O2 sensors. So, if the PCM thinks there is an O2 sensor issue, it will not run the converter efficiency monitor.

Remember that less efficient does not equal clogged or plugged. Quite a few cars come in my service repair bay reporting low power, and a clogged/melted down converter is the problem. No exhaust flow out, and no power, sometimes they won't even start. These cars almost never have a Check Engine light on. I've seen several V6 Ford Escapes with plugged converters. Sometimes the EGR valve on these have a blowout in the casting because the exhaust backpressure is so high it blasts a hole in it, from the plugged converters.

As long as there are no exhaust leaks, a less efficient converter has always been the cause of the P0420/430 codes. I am a Master ASE, L1 certified auto tech and have been fixing these issues over 30 years.
 

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2009 STS 4.6
2009 STS 4.6
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22 Posts
Discussion Starter #19
Alright. So I put the new oxygen sensor on this morning. The light has went off on the dash and the car is running much better. It accelerates and idle good again. The old sensor wire was twisted up. I didn’t remove the sleeve over the wires but I figure there’s an issue in it somewhere. The only issue I’m having is although there is no dash light when I hook up the scanner it still shows the p0430. If I erase it it just pops right back up. But the car is running good now. Before it had choppy acceleration and it would idle like it was misfiring.
 

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2006 STS4 4.6L
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It's interesting that your car is running better from a downstream o2 sensor, I'm going to wait and see how this turns out with that pending code.
Good luck.
 
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