# Knock Sensor AND Rear o2 Sensor



## Handyman2005 (May 19, 2006)

My 1995 Altima GXE just sailed thru NYS Inspection with flying colors yesterday. This afternoon, t he check engine light came on. I ran the diagnostics and they are showing a bad Knock Sensor circuit AND an open Rear O2 Sensor open circuit. The car is running FINE!!, I cannot clear the codes. Any suggestions???


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## smj999smj (Jan 1, 2006)

The rear O2 sensor is a monitoring sensor for catalytic efficiency and does not affect drivability. The go bad now and then. You probably need a new rear O2 sensor, but the correct way to determine that would be to check the circuit back to the ECM and do a componant check of the sensor.

Same really applies to the knock sensor, as well. Probably a bad knock sensor, but a componant test and circuit tests would needed to be performed to confirm.


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## x60079 (Aug 20, 2007)

Chances are that you have had a bad knock sensor for a while because it will not turn on the malfunction indicator light. Its sole purpose in life is to retard the timing up to 10 degrees if it senses engine knock. So....as long as you run the recommended octane gas there should be no problem (assuming the engine runs well). The component check is simply measuring the resistance 500 - 620 K ohms. I have noticed people selling nissan factory knock sensors on ebay for about $60. (but you may have to weld together your own tool to get it out).


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## MickeyKnox (Jun 17, 2006)

interesting. a lot of people had rear O2 sensor and knock sensor lights on. 

I had the same thing. I changed both O2 sensors, air filter. and the rest of tune up. and check engine lights went away. O2 sensors go bad. I think you need to replace rear O2 sensor every 100K. I change my front O2 sensor every 20k because it monitors fuel/air mixture. 

knock sensor failure will result in hard knocks at all rpm. especially when you use low octane fuel. use 89 octane when you do emissions test.


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## spat (Oct 15, 2005)

MickeyKnox said:


> interesting. a lot of people had rear O2 sensor and knock sensor lights on.
> 
> I had the same thing. I changed both O2 sensors, air filter. and the rest of tune up. and check engine lights went away. O2 sensors go bad. I think you need to replace rear O2 sensor every 100K. I change my front O2 sensor every 20k because it monitors fuel/air mixture.
> 
> knock sensor failure will result in hard knocks at all rpm. especially when you use low octane fuel. use 89 octane when you do emissions test.


Fault codes are just that! It means a fault has been detected and helps the technician know what system to trouble shoot. *Fault codes do not tell you that a component is bad* only that it is sending data that is out of spec! I have run a few O2 sensors over 200K miles with no problems.


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