Hennil

Hi
I´m quit new to the forum and have some trouble understanding faulty codes and O2S readings. Today I found three faulty codes (CEL not on): P0161, P1117 and P1118. I decided to take some live data for all four sensors. The data with warm engine gets me puzzled though. It shows as follows:

O2S bank 1 S2 max 0.815 and min 0.010
O2S bank 2 S2 max 0.495 and min 0.440
O2S bank 1 S1 max 2.56 and min -0.51
OS2 bank 2 S1 max 125.99 and min -128.00

It’s the readings for sensor 1 on both banks that makes me worried/doubtful. I´m not especially familiar with O2 sensor readings and would greatly appreciate any input on this. Perhaps it´s just me being overly worried for nothing, but I´m starting to think that the engine don´t run as good as last season. My car is a 996 tt My 01.
Looking forward for your insights on this and apologize for my English (living in Sweden).

Regards, Henrik

dprantl

It's either the wiring to the O2 sensors after the cat, or the sensors need to be replaced. I would be replacing all four if I got those codes; O2 sensors are wear items that have a finite lifespan.

Dan

Macster

P0161: Oxygen Sensor Heating After TWC, Bank 2 – Open Circuit.

There are some tests. Too involved to type in. See attached PDF.

P1117: Oxygen Sensor Heating After TWC, Bank 1 – Signal Implausible.
P1118: Oxygen Sensor Heating After TWC, Bank 2 – Signal Implausible

As with P0161 there are tests to be done. See 2nd attached PDF.

I'm a bit "troubled" by the errors. An O2 sensor heater can fail. Had one fail on my 2003 Turbo at around 132K miles. Had all 4 sensors replaced. (Engine ran better afterwards.)

But to have 3 sensors with pretty much the same problem... Given this I'm reluctant to just recommend you replace all 4 sensors. There could be another problem.

r6vr6

Henrik,

Does you scan tool distinguish between active and history codes. If they are history codes I just would delete them and drive it around und and see if it comes back. If it does then it is a good idea to inspect the wiring as mentioned above. The 02 sensor code you had was for B2S2. So that is the rear passenger side sensor. The sensor pigtail goes from the cat to the engine bay by the motor mount. Also disconnect the connection and look for any corrosion on the terminals or any backed out terminals. While in there you could measure the heater circuit on the suspect sensor and compare it to the other side for reference. If there is a distinct difference in the resistance then perhaps the sensor has failed. The service manual says the bank 2 plug is the more rectangular one of the 2. If the sensor checks out then your in for some fun chasing wires between the o2 junction area and the engine controller and looking for shorts or opens.

The second sensor in a bank is just for checking the operation of the cats. It will not affect how the engine runs. For me as long as it wasn't causing a MIL in the dash I would not care about a s2 code too much after checking some if the stuff above. S1 code would cause me to loose some sleep until I figured it out.

Macster

The DME will based on the #2 sensor readings adjust fueling to try to get the expected O2 sensor readings. Very likely it will lean out the mixture to feed more oxygen to the cylinders with the "hope" the converter retains this oxygen.

Thus the engine is not being fueled optimally. The engine may not show any real signs of any issues but the loss of power/torque is real.

'course, under hard (enough) acceleration the DME can ignore the #2 senosr readings and fuel the engine to maximize torque output so under hard acceleration the engine is unaffected by the #2 sensor readings.

powdrhound

Is this your opinion and conjecture or are you basing this on factually supported data?? If so please post the documentation published by Porsche supporting your claim that the rear sensors have an effect on fueling. I may have missed it but nowhere have I personally seen Porsche ever claim this.

In the official Porsche documentation regarding the engine on the 996TT/GT2 they go into 6+ pages of detailed workings of the primary O2 sensors and their effect on fueling control but only provide a 1 line description of the post cat sensors with zero mention of fuel control. Everything I have read simply implies that the rear sensors are for catalyst efficiency only because using them for anything else would directly depend on the quality of the catalysts and their ability to scrub which continually degrades as they age. As a result the signal the secondaries "see" would not be reliable and change as the cats on the cars age. In other engines like the 986 for example, Porsche deleted the secondary O2 info altogether in their ROW tunes but has them on their US cars which would further indicate that the secondaries do not have fueling control. So if you have information to the contrary, please show it.

Thank you..

Hennil

So if I´m getting this rights the faulty codes indicates problems with the sensors after the catalytic converters. It seems rather strange that both sides gone bad at the same time? The included PDF checklist will be a good help for me. Thank you for your answer and the included checklists /Henrik

r6vr6

if both heater circuits have issues it is worth checking the fuse. Fuse holder "c" fuse c2 should be 30 amp per the wiring diagram I found.

Hennil

This reading was active codes. When I first had the CEL come on a few weeks ago the message was for P0420 which could indicate that the cat on bank 1 could be shoot. I think that the car has been running for about 3 hours since then. So a bad sensor after the cat do not cause any harm for the engine if I keep driving the car a bit more before repair?

Thanks for your answer! / Henrik

Hennil

Hi

So even though the car did not have any codes for primary O2 sensors I do think that the readings seems odd. It is a big difference between bank 1 and 2. Do you think that this may indicate other problems in the fuel/air mixture and if so, what could cause this problem? I would not like to drive around with that kind of problem going on. Perhaps would the CEL come on if a have a major and potentially harmful problem?

Thank You. /Henrik

powdrhound

If it was me, I would simply change out all 4 sensors to start. They're cheap and it can't hurt especially if your current ones are original. It will also answer a lot of questions by looking at the values with new sensors.

You could also swap the existing sensors left to right to see if the odd readings follow. If so then the sensors are the problem. If not, you need to look elsewhere. Good luck.

Carlo_Carrera

+1

Hennil

Ok. This is probably what I will do. On a 17 year old car it seems like the smartest thing to do. If one sensor goes I probably end up with changing one every year. In Sweden we just have about 6 months season so ending up with the car sitting in the garage every season is not an option. This was my situation last year when the transmission (first and second gear) gave up and I was waiting for a input shaft for over 4 months. This part was backordered in Europe. That was no fun at all
Thank You! /Henrik

Macster

This is covered in the Bosch handbook.

From the Bosch Automotive Handbook, 5th edition, page 522 under the section that starts with: “Lambda closed-loop control”. Located at the bottom of the page the 2nd paragraph: “Closed-loop control for 𝛌 = 1”, has this to say:

“This concept is the most effective for minimizing pollutants. The engine must be operated within a very narrow range in which “lambda” [I’ll use the word so I don’t have to cut/paste that lambda symbol everywhere], in which “lambda” equals 1 +/- 0.005 (catalytic converter window). Such precision can only be achieved with precise closed-loop control of the A/F mixture with the Lambda oxygen sensor installed upstream of the converter.”

The last sentence in the paragraph reads: “The second Lambda sensor downstream of the catalytic converter increases precision even further.”

To add to the above, this is my understanding.

The emission system is based upon the requirement to feed exhaust gas of the proper composition to the converters. They are very efficient at processing exhaust gases when the exhaust gases are of the proper composition.

It has been determined that a healthy engine running a 14.7:1 air fuel mixture generates the proper composition of exhaust gases. (The 1st paragraph of the section I named above covers this.) The emissions system is therefore designed to fuel the engine at 14.7:1 except in special cases, like hard acceleration during which time the DME is alllowed to ignore the #2 sensor readings and fuel the engine with a richer mixture which helps the engine develop more power and thus meet the torque demanded by the driver.

Normally the #1 sensor controls the fueling. The DME varies the fueling to just slightly (very slightly) rich to just slightly (very slightly) lean about every second at ldie. (This is with engines with narrow band #1 sensors. With engines -- like the Turbo -- that use wide band the sensor reading is able to indicate how too rich or lean from ideal the mixture is and the DME then adjusts the fuel pulse width to bring the reading within spec. The result is the Turbo engine is even more precisely fueled since with the wide band sensor the DME knows by how much to add or subtract fuel.")

From the #1 sensor readings, showing an excess of oxygen in the exhaust gas from the combustion chamber or a lack of oxygen in the exhaust gas the DME knows it has control of the fueling and based on the injector pulse widths necessary to obtain these readings knows the engine is operating properly.

The #2 sensors are monitored to confirm that even when excess oxygen is in the exhaust gas coming from the combustion chambers there is no oxygen in the exhaust gas from the converter. The assumption is the oxygen was used by the converter to process the exhaust gas.

However, when the #2 sensor readings indicate the converters are not operating within the acceptable envelop the fueling is adjusted. If the #2 sensor readings then fall into the acceptable range the adjustment stops. The engine then may be running with a fuel air mixture leaner than or richer than 14.7:1 but as long as the converters are happy...well, the converters and the engine doesn't misfire...

If the #2 sensor reading are still not acceptable the adjustment continues until the readings are acceptable or until the fueling adjustment goes too far and the adjustment limit is reached in which case the DME sets an error code and turns on the CEL. In some cases the engine may misfire before the adjustment reaches its maximum limit.

(Actually had a chance to view this once: When my 2002 Boxster sufffered a bad VarioCam solenoid/actuator one of the things I noticed while monitoring engine telemetry was the short term fuel trims on the affected bank were swinging very wildly. I noted this behavior. Later at the dealer talking things over with the tech I mentioned this and he told me this was normal behavior under these circumstances as the DME was trying to find a fueling setting that would have the #2 sensor reading acceptable for that bank. But because the problem was mechanical no fuel adjustment setting would help and eventually the engine would go into misfires. This never happened as I shut off the engine then upon a restart the solenoid/actuator were ok and remained ok the 30 mile drive home.)

Might add that I never noticed any degradation in the ability of the original converters on my Boxster even after hundreds of thousands of miles. Granted while once in a while a P0430 error code would appear this was due to a loose brick that would shift out of position and I guess affect gas flow through the brick and this would effect the efficiency of the converter.

Upon engine shut down and cool down the brick would become loose -- I could hear it knocking about on cold starts -- and most of the time then upon restart assume an optimum position. The engine passed a number of smog tests all but the last one used the sniffer.

However, one of the times with the sniffer the engine did fail one smog test with I think high HC. (Smog test records are at home.) After I left the smog station I dashed over to the dealer where I spoke with both senior techs and both -- unbeknownst to the other -- recommended the same "solution". Give the car an "Italian" tune up.

So I did. Well, I didn't get all that crazy with the car -- for about 20 miles I was behind a CHP cruiser -- but I did run the engine harder than normal, and I might add I drive my cars fairly spiritedly normally.

But the advice was spot on. Just a couple of hours later back at the same smog station the engine passed with flying colors.

Even with all the miles the numbers were still quite good well within the limits.

Some converters do degrade I'm sure. Tracking subjects the converters to very high levels of heat which can cause the converter to physically break down. The use of oils with large amounts of ZDDP can degrade the converters. (The ZDDP forms a glass like covering on the hot working surfaces of the converter.)

In my Boxster's case the converter brick became loose because the converter suffered from a road debris impact some years prior.

The left side converter was just fine but to rid myself of the irritating noise: knocking when cold, buzzing with hot; I replaced both original converters with used ones which proved to be just fine. No more knocking. No more buzzing. And no more P0430 (or any other error codes).

The converters on the Turbo are up to 156,130+ miles and still work just fine. The Turbo has been through the smog test with the exhaust gas sniffer thingie more than once and the numbers are superb.

T10Chris

Is there a fuel trim PID based on the voltage output from the rear O2 sensors in our cars? If not, they don't do anything to alter fueling. I do not recall seeing fuel trim variation output/voltages available based on the rear O2. I could be wrong since I didn't go very in depth with it.

John, I imagine Kevin could look into this and tell you straight away since he is well versed in the stock computer and its code. If there is no programming in the ECU to alter any fuel trims based on the output voltage of that sensor, it has no effect in our application.


Of the very very few manufacturers that were using Fore/Aft Oxygen Sensor Control in the time the 996TT was developed (I can only think of 2-3 automakers worldwide), none of them were German automakers and the most range of variance that could be applied to the fuel trim was <2%. If they play any role it is miniscule. 2% range variation at stochiometric burn would be a window of 14.55~14.85. That is nothing.

Adjusting fuel trims and emissions requirements is big money and heavily guarded secret in the industry. This is way more valuable than horsepower secrets or racing tricks to OEMs. Every maker does it differently and doesn't want to share how they do it with anyone. All an O2 sensor does is send a signal... What the ECU does with those signals is dependent on how the OEM sets it up.

So with that being said, where does it state that those factors you list apply specifically to a Porsche application? Because everything below this point in your post is just opinion and hearsay based on random experiences.