986 Forum - The Community for Porsche Boxster & Cayman Owners - View Single Post - Please confirm my diagnosis of one bad O2 sensor

Timbert · 10-10-2015, 05:16 PM

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.

10-10-2015, 05:16 PM	#3
Timbert Registered User Join Date: Sep 2015 Location: Phoenix Posts: 15	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.