Aging O2 sensor

[oldgold]

A while back I had a check engine light come on. The OBD showed an aging o2 sensor. I reset the computer to suppress the check engine light for the o2 reading. Question - does the ECU adjust the fuel mix / etc.. based on the aging o2 sensor reading? The car has seemed a bit sluggish lately.

[RFM]

Yes, O2 sensors are parameters for the DME, to get the programmed fuel mixture.

[Baconaire]

Depending on the age of your car (or the sensors) its worth replacing them as they do impact air/fuel mix and thus performacnce and mpg. You can get Boxster specific Bosch sensors for about $130 anywhere ( I got mine at Autozone, took one day for them to order). Avoid the universal sensors. If your current ones are high mileage just replace both pre-cat sensors. I've read that post cat are not as important. Installation is easy DIY with a 22mm or 7/8" wrench. I have a specific socket but an open end wrench is easier. Be gentle with the wire harness as I've had wires pull out of brand new sensors and throw an error immediately. Use anti seize but very sparingly.

If performance is sluggish a spring tuneup might be in order. Now this is my cheapo procedure than can be accomplished for the price of a bottle of throttle body cleaner provided there are no significant problems. Be prepared to get dirty.

-Change or clean air filter

-clean throttle body and butterfly nice with proper cleaner

-If really ambitious remove throttle body

-Clean idle valve

-Clean air plenum (reach in with a rag wet with throttle body cleaner, DO NOT DROP ANTHING IN!)

-Check plugs(but I don't do this often unless there is a real problem) but if you do this go ahead and replace them

-Clean MAF if old with proper cleaner, just be careful with it.

-If I'm doing all of this I usually wipe out the whole intake system.

-vacum engine bay

-vacum drain points

-vacum radiators(grills in front bumper)

-And replace O2 Sensors if needed

-Of course observe proper oil/filter change intervals, trans oil as well

-Maybe snake oil but I run a bottle of STP injector cleaner on a fresh tank of gas

Clean the car nice inside and out using proper protectants and cleaners.

Take a shower. You should be filthy if you have not done this in a while. Don't be afraid to see lots of carbon and oily residue.

Go for a spirited ride (this really does help clean carbon out of the engine)

Just be careful with all the sensitive sensors and electronics. MAF,02 and plugs should not need replacing often.

[oldgold]

That's a very good list. I'll skip ahead to the "take a shower" part.

[Glyn]

I have just had the same fault code for the post Cat sensor.

The durametric software detailed the fault, i e-mailed Durametric for their evaluation of the fault, this is their reply:-

Here is a recap for the fault P0139 on Motronic 7.2 is a fault that indicates the measurement of the 02 sensors voltage may no longer be accurate. This fault is triggered when certain conditions, give a reading from the 02 sensor that is out of range.

Function

The diagnosis function is responsible for checking the voltage position of the sensor after the catalytic converter.

This happens in 2 areas:

1. After 5 seconds of the deceleration phase, the sensor voltage must be less than 0.1 V.

2. If the sensor voltage remains above or below 0.6 V for longer than 80 seconds, the mixture is made lean or

enriched → the voltage must change.

Diagnosis conditions

– Air flow rate from 25 to 120 kg/h

– Oxygen sensing after TWC is active

– Basic adaptation has reached steady condition

– No secondary air diagnosis

– No fuel tank ventilation diagnosis

– EVAP canister not highly loaded

– No fault in EVAP canister purge valve, fuel tank

ventilation system, secondary air injection, oxygen

sensor heating after TWC, rpm/crankshaft position

sensor and fuel supply system

Trobleshooing steps show to clear the fault code then road test the vehicle, if the fault code returns then replace the sensor.