Please confirm my diagnosis of one bad O2 sensor

[Timbert]

99 stock at 122k with CEL and five different O2 sensor codes. Using older Durametric 5 software and Gen 1 cable. I've read as many posts here on the problem as I could and watched the helpful fuel trim videos, but here's my readings after reconnecting the battery and starting up and getting warm for at least ten minutes when the cooling fans came on:

At idle FRA and TRA are flat at .98, .98, 0.0 and 0.0
Bank 1 O2 sensor ahead of cat voltage is flat at .43
Bank 2 O2 sensor ahead of cat voltage swings from .17 to .89 every four or five seconds.

Inducing lean by removing oil filler cap increases the revs but the O2 sensors follow the same pattern above, with bank 1 not responding at all and just holding at .43 or .45 volts

I happen to have three MAF sensors I have been swapping in and out and while at least one of them is bad, causing different fuel trim values, there is a common denominator so I want to isolate the O2 sensor problem. I did a smoke test for vacuum leaks with a DIY paint can which was a lot of fun but did not reveal any leaks that I could find on my first try.

[Jamesp]

Upstream sensors bounce up and down with exhaust pulses. If it's not doing that, the sensor is toast.

[Timbert]

Thanks, I confirmed the bad O2 sensor in the following way but need to move onto my next problem.

Pulled the two bank 1 O2 sensors and the pre-cat had an open circuit on the heater, toast indeed. I did the propane torch test and the voltage would move up, but when it went down it moved rapidly to .3 volts then slowly the rest of the way, unlike the properly working one in this test: https://www.youtube.com/watch?v=YL6xkOZxHZ0

Installed a new $75 Bosch sensor with the factory harness and it had a heater resistance of 2.5 ohms out of the box. My rear sensor heater resistance was 12.5 so I just left it. (I also happened to order a generic sensor as well for $15 and the heater resistance on it was 5.5).

I still get error codes for the heater on both sides (P1115 and P1119), it says "resistance limit" although the Durametric is reporting 5.5 ohm under the actual value. I turned it off and disconnected the sensors on the opposite bank and both sensors measured about 15 ohms on the heater circuits. Higher than the new one, but I'm not sure what the normal range is.

Anyway, I now get good voltage swings on both sides but after a few minutes the TRA levels creep up to -0.38. I have three MAF sensors to play with. Static voltage on each, with the ignition-on values are .82v (generic), .98v (Bosch ending in 123), and 1.15v (eBay $19 unit). The one that puts out .82 stalls as soon as I start the car. I've swapped each of them in after a battery disconnects and started a few times to warm everything up each time. Voltage while idling them is 1.29 to 1.51 which seems right. But why do the TRA levels keep coming back to -0.38? And the idle is crummy, about every five seconds it hesitates and almost stalls. The .spec air mass value is 11.75 to 13 so that seems in range as well.

Also, with one of the MAF sensors (the Bosch) the O2 voltage does not swing, it just stays low on both sides. Resetting and installing the other MAF puts the voltage swings on the O2 sensors back to normal, but still crummy idle.

The car hesitates badly around 4k when on a short test drive, and of course with the MAF unplugged it idles fine.

With the MAF unplugged and the battery just reconnected, the Durametric shows normal O2 sensor voltage swings and the idle is very smooth. The fuel trims are at .98, .98, .0 and .0 even after fifteen minutes of idling so I assume the DME doesn't adjust fuel trim without input from the sensors. The CEL will certainly come on after a few cycles and I need to get my smog test done.

What are my next steps? Thanks in advance.

[Timbert]

So I have now replaced both front O2 sensors with Bosch and eliminated the "resistance limit" warning so my O2 heaters are working. According to the "DME setpoints" pages in the factory manual several values are still out of spec:

Mass Air Flow is between 17.5 and 18.5 when it's supposed to be 15 +/- 1.25.
Ignition Timing is 3.8 to 9, supposed to be 5.3 +/- 0.5
Spec. air mass is 11 to 12, supposed to be 15.5 +/- 0.25
Spec. air adaptation is 0, supposed to be 1.5 +/- 0.05
Injection time is 2.6 to 3.7, supposed to be 2.2 +/- 0.1

But now the 02 sensor voltages no longer swing up and down, and the fuel trims are at the baseline. The idle is smooth, but shouldn't I be seeing O2 sensor voltage swings? I have two different MAF sensors, either of which MIGHT be good or bad.

When I remove the oil cap the fuel trim levels do not change, as the example in the manual indicates they should. I am stumped, can anyone help?

[jcslocum]

After a reset I think you need to drive the car a bit to allow the ECU to relearn and get settled.

[Timbert]

Replaced the MAF with a new genuine Bosch one from Rockauto, disconnected battery for ten minutes, and after a drive cycle the CEL came on and still reports 4 bad O2 sensors. No voltage swings from O2 sensors, Durametric shows -0.41 volts steady. The O2 heater resistance reports 4.1 ohms when the spec calls for 3.1 +/- 0.3 so I'm off a bit there but can't believe that's the problem. The front O2 sensors are brand new and idle is smooth.

I pulled the oil fill cap and still the fuel trims did not change nor did the O2 sensors vary their voltage.

What am I doing wrong? Would a bad DME do this? If I had a vacuum leak or fuel delivery problems I should see the fuel trim responding, right?

[Mig]

My understanding is the (narrow-band) O2 sensors are out of the picture until the system is fully warmed up and running closed loop.

If the engine is 'cold' the system is open loop; the injection is not being modified by inputs from the O2 sensors.

Are you having idle/drivability issues when the engine is cold and running open loop?

[Timbert]

Mig, thanks and my situation occurs when fully warmed up: no voltage swings from O2 sensors. The idle is smooth and I've just driven it pretty gently over the last 300 miles or so and it seems fine. I just fixed a vacuum leak at the J-tube by sealing it with new o-rings from dealership and some small amount of blue RTV. Also turns out there's an O2 sensor relay to check as well as voltage to the sensors. Tomorrow I'll get the multimeter back out and check the outputs at the DME per the factory manual, as well as the wiring and connectors on the O2 circuit.

Again, when I put in two new front O2 sensors a month ago I got good voltage swings and now nothing...

[Mig]

No voltage swings could indicate open-loop operation even though the engine is up to operating temperature.

Confirm this by using a basic OBD scan tool while the engine is running at operating temperature.

I would also use the scan tool to reset the codes, then go for a drive, and download the codes and conditions which triggered the CEL

[Timbert]

Progress: I swapped back in the O2 sensor that had the 12 ohm resistance, rather than the 3.1 which is spec. The voltage swings on both front sensors came back before the engine was even hot, fuel trims kicked in, and the loop is working. CEL came on after a few drive cycles, and lo and behold we got a resistance limit code for the front sensors (the heaters are on the same circuit). So, either the sensor plug or the sensor wiring or the (new) sensor itself that I removed is bad (though the heater resistance is close enough at 3.5 ohms).

[Mig]

What's the actual OBD code?

Quite the head scratcher.

[Timbert]

I think I've solved most of this. Durametric shows codes now are P1117, or Porsche 14 on the Durametric 5.2.2, which calls it "tank empty" but based on the what I've seen elsewhere the description is not accurate and it's the rear sensor heater. That makes sense because the resistance on those I checked directly and it's quite high. The other codes are P1115 and P1119, or Porsche's 13 and 5, which are "Resistance limit O2S ahead of cat exceeds limit value" and that reflects what I measured directly at the sensors.

I conclude that one of the front sensors is good on all fronts, and that the two other sensors I have been putting in and out of the front of bank 2 -- it appears one has a good heater but bad O2 sensing and the other bad heater and good sensing.

When I get the new one from Rockauto and confirm everything's solid I'm going to test the "new" Bosch one I bought on Ebay one more time and then return it to the seller. Might also heat it up with propane and see what the output is, rather than putting a known-bad part back in the car.

[Mig]

It's a mystery to me why OBD codes are not standard across different manufacturers. Wayne over at Pelican Parts has posted the Porsche codes for anyone interested.

P1115 Oxygen Sensor Heating 1 After Catalytic Converter - Above Upper Limit
P1117 Oxygen Sensor Heating 1 After Catalytic Converter - Below Lower Limit
P1119 Oxygen Sensor Heating 2 Ahead of Catalytic Converter - Below Lower Limit

So, one O2 heater upstream of the cat, and one O2 sensor downstream of the cat is might be faulty.

Seems odd that sensors would fail at the same time.

[Timbert]

Solved. Thanks to everyone for the help.

Finally got a second set of legitimate Bosch 13806 sensors from Rockauto and installed them after the cats. All four that I replaced (no-name sensors) were either shorted on the heater circuit or at more than 12 ohms (the Bosch ones were 2.5 to 2.8 out of the box and my Durametric report on the resistance with the four Bosch ones installed is right at spec). Plus, at least one had a shorted signal lead that threw lots of errors and Durametric showed a negative voltage rather than the proper swings of positive current. All four of the bad ones would generate voltage with the propane torch, but that's not good enough to keep the CEL off.

I now have proper voltage swings, and the idle fuel trims are .08 and .12 or so, so I may have a very small vacuum leak but not enough to cause a CEL.

So case closed, I hope, on the major issue of rough idle and bad sensors causing the CEL. Note this was a car I purchased recently as a known basket case, so normal troubleshooting procedures had to be modified. I didn't know what had been done to the car, but I learned right away the sensors and the MAF were suspect, and all non-Bosch, but I also had a major vacuum leak at the j-pipe that didn't show up with the smoke test. That's because I used the j-pipe connection to attach my ghetto smoke machine. Duh.

I also learned an easy-to-overlook fact that the heaters are on the same circuit. So an open circuit on one sensor, either front or rear, will also throw the code for its mate on the other side of the car. Like Christmas tree lights wired in series: when one bulb goes out they all turn off.

[flouese]

TIMBERT,

Care to share how you constructed your "ghetto" smoke machine.

[Timbert]

I Googled "smoke machine" or "vacuum leak" and did the one on Youtube using a one gallon paint can. Two 3/8 holes drilled in the lid, attached a couple air hose fittings that came with my compressor tools. One connects to the compressor, the other to a washing machine fresh water hose (smaller diameter than a garden hose). Jam the other end somewhere in the intake. Not the J pipe because that's a likely source of a vacuum leak. Do this all with the engine off, and cold, as warm engine parts can expand enough to close off the leak you're trying to find.

If I had to do it again, I would remove the MAF which you have to do any way to keep it from getting contaminated, and shove some rags in there to keep the pressure from coming back out the air filter. But I'd attach the smoke end of the hose to the manifold. My best idea is to use the rear-facing port on the top of the manifold behind the passenger (US) seat. It's the part that's used on the opposite manifold to connect to the brake booster.

Then put some mineral oil on some rags and light them, wait a few seconds and stomp out the flame. Drop the smoking rags in the can and hammer the lid closed. Tough to close completely if there's a lot of old paint on the lid but do your best. Have someone turn your compressor on for a few seconds at a time, or rig up an in-line valve, or use an electric tire inflator to generate some low but steady pressure, I think a couple PSI is enough. Fresh air being pumped in will keep the smoky rags going, but mine went out a few times. Then look for smoke coming out of the motor, and have a flashlight handy.

Apparently you can use the same thing to locate exhaust leaks.

You're looking for air that cheats its way in after the MAF, so maybe prop the throttle body open to make sure the smoke can get to the resonator and rest of the intake before the TB, maybe there's a leak there.

But don't take my word for it, I did all this and did not find the leak but may get back in there depending on how my fuel trims look later on.

[JFP in PA]

Originally Posted by Timbert

I Googled "smoke machine" or "vacuum leak" and did the one on Youtube using a one gallon paint can. Two 3/8 holes drilled in the lid, attached a couple air hose fittings that came with my compressor tools. One connects to the compressor, the other to a washing machine fresh water hose (smaller diameter than a garden hose). Jam the other end somewhere in the intake. Not the J pipe because that's a likely source of a vacuum leak. Do this all with the engine off, and cold, as warm engine parts can expand enough to close off the leak you're trying to find.

If I had to do it again, I would remove the MAF which you have to do any way to keep it from getting contaminated, and shove some rags in there to keep the pressure from coming back out the air filter. But I'd attach the smoke end of the hose to the manifold. My best idea is to use the rear-facing port on the top of the manifold behind the passenger (US) seat. It's the part that's used on the opposite manifold to connect to the brake booster.

Then put some mineral oil on some rags and light them, wait a few seconds and stomp out the flame. Drop the smoking rags in the can and hammer the lid closed. Tough to close completely if there's a lot of old paint on the lid but do your best. Have someone turn your compressor on for a few seconds at a time, or rig up an in-line valve, or use an electric tire inflator to generate some low but steady pressure, I think a couple PSI is enough. Fresh air being pumped in will keep the smoky rags going, but mine went out a few times. Then look for smoke coming out of the motor, and have a flashlight handy.

Apparently you can use the same thing to locate exhaust leaks.

You're looking for air that cheats its way in after the MAF, so maybe prop the throttle body open to make sure the smoke can get to the resonator and rest of the intake before the TB, maybe there's a leak there.

But don't take my word for it, I did all this and did not find the leak but may get back in there depending on how my fuel trims look later on.

I'll give you points for creativity on making your own smoke generator, but connecting it to your intake and expecting to see where it leaks out is not how these things work.

The generator produces smoke, which is emitted through a small wand that is run along vacuum connections while the engine idles. What you are looking for is for the smoke to suddenly disappear as the wand tip approaches the leak sources. I seriously doubt you can find a vacuum leak by flooding the intake system with smoke, expecting to leak out, unless the leak source is absolutely massive.

[Timbert]

JFP I get what you're saying, and that makes perfect sense, but I looked at about a dozen sources and they all pumped the smoke into the intake. The several machine kits that I saw come with plugs and rubber funnels for the intake to accept the smoke hose. Here's an article from Popular Mechanics supporting the effort (not the DIY, but how to use it once I made it) that I pursued:

How to Find a Vacuum Leak - Smoking Out Vacuum Leaks

And here's a pro setup from Redline that goes for $1500 and the video showing how they connect it to the intake to fill the engine with smoke to find the leak.

[JFP in PA]

I'm sorry, but that is complete nonsense. Vacuum leaks occur due to pressure differential, there is lower pressure inside the system (engine vacuum) than outside (atmospheric pressure). So unless you use a machine to pressurize the engine above atmospheric pressure with smoke, there will be no leak. And if you did internally pressurize the engine with smoke, you would have smoke pouring out the air intake and exhaust, and probably would start blowing off every vacuum connection on the engine.

We smoke test cars for vacuum leaks just about every day, and is not done by blowing smoke into the engine, period.

[Timbert]

I certainly accept there's more than one way to do it. But I went with what I researched. Engine off, one or two psi, plug the intake. Look at the video I linked to above, done by the company that sells the machine they sell at AutoZone. All I was trying to do is replicate that. It's not complete nonsense, is it really?

I don't have a dog in the fight, just trying to offer my two cents on my one-off experience with this thing because a member on this thread asked me to.