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Old 10-25-2012, 11:53 AM   #1
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Join Date: Sep 2012
Location: Ipswich UK
Posts: 16
Boxster Misfire Cured. Coils and O2 Sensors

I posted a problem about a misfire a couple of weeks ago and am pleased that I have now cured it.

I hope the following is of assistance to somebody having similar symptoms...

It started with a misfire and a fault code indicating misfire on 123. This coincided with the weather getting a bit colder. I cleared the codes and then it recorded a misfire on 456.

Live OBD data then started to show problems. First of all the left side started showing Fuel System OL and 0% on STFT and LTFT. The right side was showing Fuel System CL and various readings on STFT and LTFT.

Then both sides started showing Fuel System OL and 0% on STFT and LTFT on both sides.

My OBD reader came with no advice regarding how to interpret the data and I received advice from several members of this forum.

At this stage the car would start and idle OK but driving it with the engine cold was almost impossible and when warm it would barely rev above 2500 and any load would lead to loss of power.

I changed the coils thinking that this was the most likely cause but this had no effect on the running of the car.

As I understand it the OBD readings relating to STFT and LTFT relate to the amount of fuel that is being injected into the cylinders; it is a +% when it is trying to enrichen the mixture and -% when it is trying to weaken it. It does this in response to information the ECU receives from the O2 Lambda sensors which measure the amount to unburned oxygen in the exhaust gases.

I was fairly certain the problem was O2 sensors and I took the car to a nearby official Porsche dealer who confirmed this was the problem. They wanted almost 700 to replace two sensors!!

So, if the system is showing OL all the time and 0% STFT and LTFT it is an indication that the O2 sensor has failed.

I bought a couple of sensors from a reputable Porsche specialist for 55 each +VAT and this completely cured the problem and the car runs perfectly again. It took about 15minutes per side, fortunately the old sensors came out easily. Quite how Porsche justify almost 700 for this job is beyond me.

So, the solution was new coils (155) and new O2 sensors. I am not sure why this started off with a misfire and then went to almost simultaneous failure of both O2 sensors. I suspect that the persistent and bad original misfire was caused by failing coils (they were 14 years old) and may have caused the O2 sensors to get contaminated and fail. If I had changed the coils at the outset of the problem I might still be running on the same O2 sensors.....
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