Oxygen Sensors & Engine Management

Replacing an oxygen sensor–  Before 1996 the 993 has one oxygen sensor, from 1996 on the 993 has four oxygen sensors. The sensors measure oxygen content before and after, in the case of 1996 on, for the two catalytic converters and are integral to the emissions and mixture adjustment functions of the engine. They only work properly when heated up. Over time they accumulate deposits that slow the time it takes to warm into their operating range. 

After X number of start-ups where the time to heat was in excesses of Y seconds the OBDC system will light the check engine light and present an oxygen sensor slow to respond code. Not very surprisingly, this seems to happen to most owners in the fall when the weather is a little cooler and the sensor warm-up period is extended by a few seconds. The fix involves simply clearing the code and seeing if it comes back any time soon. Think of the money you will save by not unnecessarily replacing all four of your sensors one or more times over several years.  

My 96’ had three of its original four sensors up until 100K miles on the odometer. At some point you may receive an OBD Code indicating the sensor or its internal heater is dead or in combination with other codes, for example, ones related to the secondary air injector system, suggesting one or more of the sensors are too far out of specification to do its job. At this point, they need to be replaced. The OBD Codes encountered with a slow to warm sensor are: 

Diagnostic Trouble Codes (DTC) 15,16,17,21,22 and 23 or 

"P" codes P0133, P0139, P0153 and P0159 

A description of the codes can be found here 

depending on which oxygen sensor is running a little slow to warm up.

Keep in mind your car uses data from the oxygen sensors to evaluate the function of other components in your car’s emission system. Some of the OBDC codes such as those related to your catalytic converter’s function or the air injector system can be erroneous if the sensors are a bit out of specification, even if the system doesn’t display a sensor-specific code. For this reason and if you have not replaced your originally installed oxygen sensors before always replace any old sensors on the exhaust bank exhibiting a code and clear all the codes waiting for them to reappear before engaging in any diagnosis of invasive/expensive actions to resolve issues related to the codes.

The easiest way to replace an old oxygen sensor is to:

1. Remove the under-engine tray if still present;   
2. Put the rear of the car on jack stands;  Here is a page on how to lift your car safely   
3. Spray a  little acidified penetrating oil such as PB Blaster on the sensor's bung (threaded mount), and let it sit overnight;   
4. Cut the sensor's wire just behind the sensor; 
5. Remove the rubber grommet that passes through the above engine tin;  
6. Slip a closed-end boxed end wrench over the sensor and tap it loose with a mallet;  
7. Screw in the new sensor, using care not to twist their wire pigtail. If anti-seize is present on the new sensor use care to not contaminate the sensor head with it.
8. The before and after CAT sensors have different part numbers, use care to not install them in the wrong positions; 
9. Tape the new pigtail wire to the old one and use the old wire to pull the new wire up to the electrical plugs in the engine compartment above. 
10. Match the pigtail plug color black or grey to the same color socket on each side of the engine bay. 
11. Reinstall the engine tin rubber grommets. They are easy to put back in position;  
12. Replace the engine under-tray or just store it somewhere for a future owner of your car. 
13. Clear any OBD Codes and see if they return. 

#OBD #OBDC #OBDC-II #On board diagnostic