O2 Sensor challenge

I replaced the O2 sensors in my 1999 Boxster and after a couple hundred miles, my check engine light comes on and indicates O2 sensors , both banks. I used good Bosch sensors (put not Porsche OEM) for replacement. The engine runs perfectly when codes reset but, invariably, the light pops on again in about the same number of miles. Before I try a new set of sensors, has anyone run into other issues associated with the O2 sensor code that I may be overlooking? Thanks, Dave

Hi Dave, and Welcome to Planet-9, the Friendly Porsche Forum We need some info: Your exact model (S or base)? What sensors did you use? Do you have a Porsche capable code reader? If you intend to DIY on your car, having a code reader that can read the Porsche specific codes, and monitor things like the O2 sensor outputs, fuel-trim, etc is a must-have. They range from $100-$3,000. Otherwise, you're stuck replacing parts (as you've already done) until the issue is resolved, or taking it to a mechanic who has a diagnostics tool, or selling the car. Doing repairs by parts replacement (based on best, well-intentioned GUESSES by people on the web) can become quite expensive since it's always the last part you buy that fixes it. The path to that last part can be long and expensive. A general bit of advice when posing questions like this to the forum.. include all the info you can, even things that might not seem significant (like when did you last replace your fuel cap?) Be tter questions get better answers (me)

Thanks for the reply, Don. It is a 2.5 litre base model. The code reader I have is the Durametric Diagnostic Tool running on a laptop. The exact model of the Bosch sensors I cannot recall, only that they were guaranteed-fit-appropriate for my model year and engine type. The codes were O2 sensor , both banks. I was just not sure what else it could be with those fault codes if the sensor replacement did not take care of it. Appreciate ideas from you and others that have done the repair and maybe had similar delayed re-triggering (150 to 200 miles after code re-set)of the same fault codes. I have never replaced the gas- cap but it appears in perfect condition pretty much like the rest of the car. It is original, however! Dave

Also, price paid for sensors was about $110 each.

A friend had the same issue with his 2000 986 2.5L. The marvelous shop I used in Bordentown NJ fixed it for him, and it involved one of the valves in the emissions system. Cost of the repair was around $700. I'll ask him if I can see his receipt for the work and that might reveal which valve was replaced, if I had to guess - probably the one for the evaporative charcoal canister. But, that's just my guess, don't take it as real diagnostics. I just talked to my friend, his car is covered, the receipt is in the car, he'll dig it out when he next removes the cover. Probably not today since it's ungodly humid and hot in NJ, so he doesn't want to drive his lovely sports vehicle.

My friend must have felt guilty about not responding. He went and found the invoice for his repair. His receipt said: "Charcoal canister shutoff valve replaced".. That's where I'd head next . Please take pics as you're doing the job, we can make a DIY out of it. From a bit of reading I did - the valve is located behind the right headlight, and accessible from the wheel well on that side once the wheel and fender liners are removed. For a search - you might try Google. It can come up with answers, some right, some wrong. The trick is figuring out which is which.

Thanks for chasing this data down, Don. Looking at the components of this system, there is the charcoal canister, some tubes connecting to another module that looks like it has 2 sensors in it. I will investigate this avenue which I think is the likely culprit. The parts are fairly pricy so I will try to find the sticky item. f not, the whole assembly may need fixing. At least there is a viable path now. I will document for the forum. Dave

Ordered shut off valve for canister and will document with photos and steps and testing. I am thinking that this will get it done.

I finished the write up and photos on the Evap canister shut-off valve repair. I tried to attach it but it said it was not an allowable file extension. It is a Word document (.docx)

"Print" it as a PDF, and then attach that instead. Windows 11 comes with a Microsoft PDF printer, but if you have another OS there are free alternatives out there... I used to use CutePDF in the past.

Update on repair. Finally threw in the towel. When after the last repair, replacement of the EVAP Canister shut-off valve, the same code came back again. I simply could not figure out what was going on with the Check Engine Light. A local European indie shop found that my upstream O2 sensors that I had replaced were malfunctioning. So, my very first shot at this was replacing the O2 sensors ! Here's the thing, I think. Even though many of the sources for O2 sensors indicate that there is no difference between the upsteam and downstream sensors (all four that I put in were the same Bosch model), apparently there definitely is a difference. Fortunately, they only had to replace 2 of them insrtead of four. Also, I was up to the point of needing my spark plugs done so I had them do that, too, and the brake fluid service. A couple thousand bucks later, all seems well and perky. I am pretty sure the plugs were original from 1999 since the car is still under 30,000 miles. Amazing that the car ran so well after 27 years on those original units. Here's the question, will the CEL blink on after 100 miles like it did every time I tried to sort it out? I am hoping not. Here is a question for the group: Can a Durametric scanning application tell if any particular O2 sensor is not functioning correctly? The shop clearly had a device that pointed to the operation of these (said that one was stuck).

Yes - the Durametric and any Porsche specific tool should be able to ID which sensor is bad. It wouldn't surprise me if a cheapo OBD -II tool also would ID it.