P0420 code has me stumped

N8Dogg98 · Apr 25, 2012

I keep getting the P0420 code and each time I reset the ECU it comes back after 500 miles of driving. I realize the code is telling me the bank 1 catalyst is below efficiency, but I don't have cats and during my dyno tune my tuner shut off the rear O2 sensors.

What else could be causing this to happen? I recently replaced the IAC motor, I cleaned the MAF, I've checked for vacuum leaks and haven't found anything, there are no exhaust leaks, and the PCV valve was replaced last week (though not sure if it would have anything to do with this).

Now there is another issue here. My car won't idle well once its warmed up. I thought it was an IAC problem, but even after replacing the IAC it still doesn't idle well. If I come to a stop after the car has warmed up the RPM's bounce between 500 and 1300 and sometimes the car dies. If it doesn't die, the RPMs settle around 900. WTF?

AZmustang98 · Apr 25, 2012

How is your EGR system?

N8Dogg98 · Apr 25, 2012

No clue... she's 14 years old and has 140K miles. Could replacing the EGR valve fix it?

sneaky98gt · Apr 25, 2012

I have also had this pop up on me a couple of times, even though the rear O2s are turned off in the tune. It almost always happens to me after I've been running the car hard, like at the track.

FEARNO1 · Apr 25, 2012

An exhaust leak was the cause of mine throwing this code.

trinity_gt · Apr 25, 2012

You say " but I don't have cats": The absence of properly functional cats is the cause of the code. I would offer that the rear O2s are not actually "turned off", at least not in a way that prevents the PCM from monitoring them and/or setting the code. Don't throw any more random parts at it. The PCM is telling you what the problem is (i.e. cats or the absence thereof.) Find out why the tune doesn't appear to have properly "shut them off" or get some cats or MILs.

As for the idle, try this: With the engine idling, unplug the IAC valve. What happens to the idle? If the engine almost stalls, you may want to increase the base idle air by adjusting the throttle stop screw to achieve an idle of, say, 500rpm. When you plug the IAC in again it should rise the normal level. An unstable idle may suggest that the IAC system is trying to operate in closed loop outside its control region because, for instance, there's too much (e.g. leaking throttle plate in the closed position) or too little air (e.g. throttle plate closed too far at idle).

nine2oneGT · Apr 25, 2012

trinity_gt said:
You say " but I don't have cats": The absence of properly functional cats is the cause of the code. I would offer that the rear O2s are not actually "turned off", at least not in a way that prevents the PCM from monitoring them and/or setting the code. Don't throw any more random parts at it. The PCM is telling you what the problem is (i.e. cats or the absence thereof.) Find out why the tune doesn't appear to have properly "shut them off" or get some cats or MILs.

As for the idle, try this: With the engine idling, unplug the IAC valve. What happens to the idle? If the engine almost stalls, you may want to increase the base idle air by adjusting the throttle stop screw to achieve an idle of, say, 500rpm. When you plug the IAC in again it should rise the normal level. An unstable idle may suggest that the IAC system is trying to operate in closed loop outside its control region because, for instance, there's too much (e.g. leaking throttle plate in the closed position) or too little air (e.g. throttle plate closed too far at idle).

^This^

I had this problem on my stang a couple years ago. The idle would bounce and even die sometimes. I ended up having to replace mine but it wasn't too expensive.

I guess you replaced it already therefore, not the prob.

N8Dogg98 · Apr 25, 2012

Thanks Trinity. What's most unusual is that the car ran fine with no codes for over a year with this tune. I removed the EGR today and cleaned it with brake clean and it seemed to help, but now its doing the same thing again. Us it even possible for anything other than the rear o2s that would cause this code?

Sent from my LG-P999 using Tapatalk 2

trinity_gt · Apr 25, 2012

Not that I know of. The PCM looks at the signal from the rear O2s. If it is varying in a manner similar to the fronts then the indication is that the exhaust stream after the cats still has varying levels of oxygen consistent with catalysts that are malfunctioning or absent. That's the bottom line with these codes.

The fact that your car has no cats means that the sensors are going to see this. Unless you put cats back in or filter the signal output with a MIL the PCM is going to think the cats are bad and set this code. Revisit the tuner to verify that the rears and/or the code-setting logic are disabled.

N8Dogg98 · Apr 26, 2012

trinity_gt Thanks again. It is possible my tuner didn't shut off the rear O2's, as I had a magnaflow catted x-pipe the day of the tune and later switched back to my o/r h-pipe. I thought he said he shut off the rear O2's but who knows. I'll go back and check to verify if he did in fact shut them off.

Now on to the other issue. I unplugged the IAC and the car dies right away. Now keep in mind I do have stage 2 N/A cams so it already has a somewhat lumpy idle anyways. I'll try messing with the throttle stop screw, but do you know if it should turn it to allow more air, less air, or just try and see what works?

N8Dogg98 · Apr 26, 2012

I increased the throttle opening by moving the throttle stop screw back about 3/16". The idle doesn't bounce around like it used to, but I am still having a slight bounce in the idle. Its maybe a 100 rpm bounce until the car finds its groove and idles nicely. My tuner set the idle speed very high (IIRC 900 RPMs) to keep it from dying. I'll have to go back to my tuner and have him turn it back to a more reasonable 600-700 RPM.

FEARNO1 · Apr 26, 2012

So you cant get this code if the rear o2's are shut off? I ask this because I dont have cats either. The tuner said the o2's were shut off. I got this code. After I fixed my exhaust leak cleared the code it has never returned and that was about 2yrs ago.

95Vert383AOD · Apr 26, 2012

Wouldn't Bank 1 Be your Passenger Upstream O2 sensor? Theres 4 on the 4.6 right? Disabling the 2 rear has nothing to do with the O2 (upstream) right at the header.

So its either an exhaust leak like said above on the passenger side or the upstream O2 sensor on the same side.

trinity_gt · Apr 26, 2012

95Vert383AOD said:
Wouldn't Bank 1 Be your Passenger Upstream O2 sensor? Theres 4 on the 4.6 right? Disabling the 2 rear has nothing to do with the O2 (upstream) right at the header.

B1 (bank 1) is indeed the passenger side but the catalyst efficiency codes deal with the rear (S2) sensors, not the fronts (S1.)

B1S1 -- passenger side front (A/F feedback)
B1S2 -- passenger side rear (cat monitor)
B2S1 -- drivers side front (A/F feedback)
B2S2 -- drivers side rear (cat monitor)

The PCM isn't seeing anything wrong with the sensors themselves but rather with the operation of the catalysts residing between S1 and S2.

95Vert383AOD · Apr 26, 2012

Sensors can slowly lose their effectiveness over time. They don't always just have a hard fail. I didn't see if the OP stated that it was s1 or s2. Seeing that S2 has been disabled it should naturally either be a leak or a worn O2 sensor no longer reading correctly. The ECU has no idea of the physical condition of a cat or whats going on in the exhaust other than the O2 sensor.

I vote that B1S1 is out of range due to age or theres a leak throwing off the values. If you have a scan tool that gives you live data you can compare the LIVE values of B1 to B2. B1 will probably be fairly steady while B2's values will jump all over the place. (Not a totally dead O2 but an incorrectly reporting one)

trinity_gt · Apr 26, 2012

The code is P0420. Its a cat efficiency code being seen because (a) there are no cats so the rear O2 sensor signals look pretty much like the fronts and (b) the rear O2s are not shut off in the tune.

There are S1 sensor specific codes and he's not reporting any of these. He's not getting adaptive trim limit codes, "indicates rich" or "indicates lean", "lack of upstream switch", "slow response" etc etc. If there were a problem with one or both S1 there would be code(s) for it.

95Vert383AOD · Apr 26, 2012

Well at 14 years and 140k .... I personally would change the O2 sensors anyways for good measure. Especially if i was at the point where i was getting tunes. But thats just me. Mustang O2 sensors are alot cheaper than a tune and for me it would be cheap insurance.

N8Dogg98 · Apr 27, 2012

95Vert383AOD said:
Well at 14 years and 140k .... I personally would change the O2 sensors anyways for good measure. Especially if i was at the point where i was getting tunes. But thats just me. Mustang O2 sensors are alot cheaper than a tune and for me it would be cheap insurance.

You're missing the point. Trinity is probably right that the rear O2's are not shut off in the tune. When I got a tune, I had a catted x-pipe and I thought my tuner said he shut off the rear O2's but I don't remember for sure. Why would I waste money on replacing rear O2 sensors that have nothing to do with the code I'm getting?

trinity_gt So it does idle better now Trinity, but still bounces up and down. I did replace the EGR valve and while it seems to run better at part throttle, the idle is still somewhat erratic (didn't really expect the EGR valve to make a diff). Should I continue to back out the throttle stop screw? I don't have very far left to go with it.

trinity_gt · Apr 27, 2012

To be fair, O2 sensors should generally be considered wear items and replaced at reasonably regular intervals as part of regular maintenance. I would certainly replace any O2 sensor that had 140K miles on regardless of whether there was a code or not. But I strongly doubt that the sensors on the car are that old and I also, as stated, don't see a link between P0420 and front O2 sensors.

Regarding the idle, I would disconnect the IAC and attempt to set the idle to ~500RPM. This may be difficult to get with aggressive cams (mine gets pretty unhappy even with just "stage 1" Comp 262AHs) but you should at least ensure the car doesn't just die outright when the IAC is unplugged. With this setting, you should allow the IAC to operate in closed loop without pushing any control limits.

If the idle is still wonky you may wafting an unlit propane torch (i.e. letting the gas out of the thing with no flame) in the area of the ducting between the MAF . Look for changes in engine note/speed indicating it's pulling in and combusting the flammable gas. The other thing you may check is the PCV valve. It is supposed to be closed and sealed at idle. You can disconnect the hose at the throttle body/plenum and plug the vacuum port there and see if the idle improves.

N8Dogg98 · Apr 27, 2012

trinity_gt said:
To be fair, O2 sensors should generally be considered wear items and replaced at reasonably regular intervals as part of regular maintenance. I would certainly replace any O2 sensor that had 140K miles on regardless of whether there was a code or not. But I strongly doubt that the sensors on the car are that old and I also, as stated, don't see a link between P0420 and front O2 sensors.

Agreed, and FWIW the fronts were replaced about 2 years ago.

trinity_gt said:
Regarding the idle, I would disconnect the IAC and attempt to set the idle to ~500RPM. This may be difficult to get with aggressive cams (mine gets pretty unhappy even with just "stage 1" Comp 262AHs) but you should at least ensure the car doesn't just die outright when the IAC is unplugged. With this setting, you should allow the IAC to operate in closed loop without pushing any control limits.

I'll keep trying to back out the set screw, but I don't know that I have much further to go. Right now if I unplug the IAC it will idle for about 5 seconds before dying, which is better than before when it would die right away.

trinity_gt said:
If the idle is still wonky you may wafting an unlit propane torch (i.e. letting the gas out of the thing with no flame) in the area of the ducting between the MAF . Look for changes in engine note/speed indicating it's pulling in and combusting the flammable gas. The other thing you may check is the PCV valve. It is supposed to be closed and sealed at idle. You can disconnect the hose at the throttle body/plenum and plug the vacuum port there and see if the idle improves.

Thanks again Trinity. I'll give it a shot. The PCV valve is brand spankin new (bought a Motorcraft one) but I'll try your suggestion.

P0420 code has me stumped

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