oxygen sensor codes

[silverfarmer]

Hi everyone.  I have a 1998 stock manual boxster.  My car was impacted by a mini-van in the driver's rear quarter.  I pick up my repaired car at the collision center and 30 miles (of 120 mile) trip home I get a solid CEL.  Car was not giving any indication of a problem, runs fine rest of way home.  Hook up durametric and get P0140 O2 sensor behind cat conv cyl (4-6).  No other codes.  I erase code.  Next day I take car out for a drive, 8 miles out another solid CEL.  Again, drive home, hook up durametric and get codes P0140, P0160 and P1121.  I erase codes.  I swap the post cat sensors from side to side and go for a drive.  38 miles no problem, get out of car to open garage door at home, get back in and solid CEL is illuminated.  Pull in garage, leave car running and hook up durametric.  One code:  P0140. So code didn't follow sensor.   I then actual value graphed the pre and post cat voltages.  I have attached the screen shot.  I really could use some help interpreting it and/or suggestions on what path to take from here.  

 

Could the cat have suffered internal damage from the accident and need replaced?  The post cat sensors seem fine.  It is quite variable as to when the CEL appears, I don't begrudge the shop as their test drive after repairs was short according to the mileage out.  Nothing was replaced w.r.t. the exhaust system.  Repairs were mainly suspension components and cosmetics.

 

I did try posting on another forum, but my post never showed up!?

Thanks!

[mikefocke]

The P0140 and P0160 are "interruption of signal". 

 

 

Brett posted on PPBB:

The O2 sensors do just that, sense the amount of O2 in the exhaust gas relative to the amount of O2 in ambient air. Perfect combustion of a perfect mixture of air and fuel (around 14.7/1 air/fuel ratio) leaves behind only CO2 and water as products of combustion. All the oxygen gets consumed in the combustion and combines with all the carbons and hydrogens. If there is not enough fuel (lean mixture), then all the fuel gets burned leaving some oxygen left over. Conversely, if there is too much fuel (rich mixture), then all the oxygen gets burned leaving behind extra hydrocarbons (fuel). Now an oxygen sensor outputs a voltage between 0 and about 1 V depending on the difference between the amount of oxygen in the exhaust and the amount of oxygen in normal air. If there is a lot of oxygen in the exhaust (lean mixture condition), the sensor outputs close to 0 volts. Conversely, if there is no oxygen in the mixture (rich condition), then the output is close to 1 V. These O2 sensor voltages are read by the computer. This is the feedback loop that tells the computer how the engine is performing with regard to air/fuel mixture. It's impossible for the computer to hold the exact perfect air/fuel mixture constantly, so the way mixture control is designed is for the computer to continually adjust the mixture from very slightly rich to very slightly lean and back again using feedback from the pre-cat O2 sensors. This means that the pre-cat O2 sensor signal will oscillate back and forth from high to low to high to low voltage as the computer adjusts the mixture. In a normal running engine at idle the signal goes from low to high voltage and vice versa about every 1 second, with a transit time from low to high (or vice versa) being about 200-300 milliseconds. This transit time is important because as an O2 sensor ages, the transit time gets longer, and eventually it can get too long such that the computer will call it a malfunction and signal a check engine light and fault code for a slow responding O2 sensor. O2 sensors need to respond to mixture changes quickly so that the computer can keep up with the proper mixture adjustments.

So the bottom line is that the pre-cat O2 sensors should oscillate between about 0.2 to 0.8 volts regularly (about every 1 second at idle) in a healthy engine.

The post-cat O2 sensors are identical to the pre-cat O2 sensors (same voltage outputs). They are there only to monitor the performance of the catalytic converters. So, as discussed, the pre-cat sensor signals are oscillating between 0.2-0.8 volts. Once the exhaust gasses pass through the catalytic converter, most (all, in theory) excess fuel (hydrocarbons) will be combusted thus reducing hydrocarbon emissions. The cat uses oxygen in the exhaust to combust the fuel. So what you end up with in the exhaust after passing through the cat is a gas mixture that is reduced in hydrocarbons and reduced in oxygen relative to the mixture entering the cat. The post-cat exhaust gas mixture should be CONSTANTLY low in oxygen if the cat is doing its job of burning excess fuel. Therefore, the post-cat O2 sensor signal should be a constant lower voltage reading (not oscillating). So, if the post-cat O2 sensor is seen to oscillate just like the pre-cat O2 sensor, that means that the post-cat sensor is seeing the same gas mixture as the pre-cat sensor meaning that the catalytic converter isn't doing its job of burning excess fuel. The computer monitors the post-cat sensor and compares it to the pre-cat sensor. If the signals are similar, it assumes the cat is bad.

What I recall...it has been years .....

 

The connection to the actual sensor is crimped, not supposed to be soldered. The Porsche ones come with the pigtail. The pigtails are of different lengths for the pre and post cat sensors. Bosch makes em for Porsche. Buy the version with pigtail crimped at the factory.

 

Sometimes people buy a bare sensor and use the old pigtail. Not good.

 

 

[silverfarmer]

Thanks for the reply Mike,

 

So am I correct looking for an electrical problem/disruption somewhere from connection point of bank 2 post cat sensor to ECU?  

 

 

[silverfarmer]

 I found this from JFP in PA (02/2019):   This is a great example of why you shouldn't replace anything before you know what the codes are telling you. P0134 is the code for the interruption of the signal from the O2 sensor ahead of the three way cat on bank #1. Usually, this is caused by a problem with either the signal or ground wire connected to the sensor, and is rarely caused by the sensor itself. The diagnostic tree in the factory OBD II manual for the car involves checking the wiring by using a multimeter to test the ohms between pins #1 and 2 (should read 1.8-2.5 ohms at 20 degrees C), and test the ground continuity between each of these pins and the sensor housing itself (each should read infinite ohms). The harness connector pins 3 and 4 should read ~450 mV, and pin 3 to ground should read ~750 mV. Harness connector pins 1 and 2 should show battery voltage. If any of these tests are out of bounds, you have a wiring fault, not a sensor fault.

 

My questions regard the harness testing.  A.)  Should the car be running during the test of the harness connector?  B.)  Am I reading testing PIN 3 TO PIN 4 should read ~450mV? 

3.)  PIN 3 to ground (any body bolt?) should read ~750mV? 4.)  PIN 1 TO PIN 2 should read battery voltage?  

 

I did the sensor tests:  PIN 1 to PIN 2 read 1.7 ohm (at 12 degree C) and grounding PIN 1 and then PIN 2 read OL on each reading.  So I think my sensor and the accompanying wiring is OK.

 

Thanks for the help, lest I fry some electronics!  

[JFP in PA]

OK, here goes:

 

It is easiest if the car is running.  Yes on the pin to pin tests.

 

P0140: Interruption of signal on O2 sensor after the main cat on bank #1.  Diagnostics are to check the continuity of the signal wires back to the DME terminal #4 at the sensor to terminal 77 at the DME; terminal 3 at the sensor to terminal to terminal 46 at the DME.  Also check both wires for potential short to ground.  P0160 is the same tests on the other bank sensor.

 

P1121 is for low heating circuit current on the sensor ahead of the cat on bank #2, again, a possible interruption or short to ground of the circuit.

 

It is also useful to check ALL of the O2 sensors for normal voltage curve function; the sensor behind the main cat should be drawing a fairly straight voltage curve over time, while the one ahead of the cat on the same bank should be fluctuating voltage in a sine wave fashion:

 

 

I my experience, it is very odd to have multiple O2 sensors fail all at the same time; usually this is a harness problem.

[silverfarmer]

Thank you for the quick reply.  I really appreciate your expertise and willingness to help.  I am headed to the shop right now!  Merry Christmas!

[silverfarmer]

So my results:

 

Engine running:

PIN 3 TO PIN 4 ~453mV

PIN 3 to GND ~720mV

PIN 1 TO PIN 2    never settled, jumped all over on multiple attempts !

 

So am I looking for a bad ground from the harness to the DME?

 

Engine NOT running:

PIN 1 to GND ~6.75kohm

PIN 2 to GND  ~6.75kohm

PIN 3 to GND ~1.13kohm

PIN 4 to GND  ~52.7kohm

 

Your suggestions appreciated!

 

 

[silverfarmer]

Sorry for lag time, getting over some flu bug finally.  I went over all the voltages and resistances and connections again.  I visually checked the harness connections back to where they were last accesible.  Both sides.  Nothing out of the ordinary.  Applied a bit of dielectric grease to each pin and reconnected sensors.  I erased the P0140 code and went for almost 70 mile drive:  back road, highway, stop and go in town traffic.  No CEL, left car running and got out to open garage, returned to CEL?  Pulled in hooked up Durametric without turning off car.  P0140 again.  Compared voltages as you suggested pre and post cat each side.  While my plots are not as smooth or consistent (with regard to amplitude), but I think they look correct.  Pre on both oscillate between 0.05 and 0.75 and post 0.98 and 1.02.  

Would you suggest investigation further up the harness from where it disappears in the trunk to the direction of the DME?  Is there a diagram of the path anywhere?

 

On an aside:  The Durametric I have readout for P0140 is cyl 4-6?  Is that considered bank 1 or 2?  I'm checking both sides, but trying to see if there is a bug\error on my readout.

 

Thanks!

[JFP in PA]

If the car was in my shop, I would be testing the harness between the sensor and the DME for continuity, resistance, etc.

 

The codes are specific to banks: P0140 is bank# 1, cylinders 1-3; P1121 is for bank # 2, cylinders 4-6.  As both are signal interruption related, I would be checking the harness, both electrically, and visually for any damage.

 

These kinds of faults can be a pain to locate the problem, as it can be anywhere along the harness (rodent damage, wire insulations becoming brittle and splitting, etc).

[silverfarmer]

Thank you.  I'll give it a go!  Again, I appreciate your help.

[silverfarmer]

Just a follow up.  I went through all the harness I could get to up to the DME and I found nothing problematic.  Checked wiring, checked relay connections, essentially anything I could find/trace in the trunk after removing the covers and wood floor.  I re-swapped the 2 post cat sensors back to their original positions.  I cleaned and dielectric greased the harness connections and sensor plugs of both.  I removed the driver seat and cleaned and dielectric greased the ecu harness connection, nothing appeared faulty with the wiring.  

I had some nice weather so I went for about a 100 mile drive today (highway, back roads, stop and go, stopped for gas).  No CEL, no problems with anything!?  Ran like a top.  

I don't have a lot of hope yet for this "fix", but mileage will tell.  It would be nice if it is this simple.

Thank you.