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Unread 06-24-2013, 12:44 AM   #1
Angelkath
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2001 XJ Cherokee 
 
Join Date: Jun 2013
Location: LITH, IL
Posts: 2
o2 spike

I have a 2001 Jeep Cherokee. I have no mods to date. I am expercing a o2 sensor spike. I just replaced the o2 sensor so i know that is not the current issue. I would like to know what are the volts the are supposed to run through the o2 sensor and the best way to look for a spike. Any info would be helpful including anyone who has had this issue and has fixed it. I know i will need a volt meter to test this. Any ideas? Also have a misfire in cylinder 3 and wondering if the o2 spike could be a cause for this. Have replaced the spark plugs but not the coil pack. Please help.

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Unread 06-24-2013, 06:37 PM   #2
CJ7-Tim
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2000 XJ Cherokee 
 
Join Date: Apr 2002
Location: out in the garage - Minneapolis, Minnesota
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What is an O2 sensor spike, and how did you discover/measure this ?



For the HO years, 1991-2001, the O2 sensor has 4 wires. One wire is the 12-14 volt power that comes from the O2 sensor heater relay on the passenger side firewall (unless they moved them in later years to the relay & fuse boxes?), and that powers the internal heater in the sensor so that the sensor can work at idle, and almost immediately after start up. Loss of that power will hurt gas mileage even with a good O2 sensor.

Two black wires are grounds for the heater power and O2 signal to the ECU. A poor ground will limit current to the O2 sensors heater or cause an error in the O2 sensor output voltage read by the PCM (ECU) causing poor mileage even with a good O2 sensor. Test the 2 ground wires with power off. Test between the wire end at the sensor and the battery negative ground post. It should read less than 1 ohm.

The last wire, #4, is a signal feed wire, 0-1 volts, from the O2 sensor to the PCM (ECU) sensor. The O2 sensor is an O2 concentration sensitive variable voltage generator. At optimal O2 concentration the O2 sensor puts out 0.45 volts.

At idle that voltage should read 0.1-0.9 volts oscillating quickly back and forth roughly once every second. At 2000 rpm it should run between 0.4 and 0.5 volts max (in park). A digital meter can NOT be used for reading the O2 sensor voltage, but it can be used to test the grounds and the 12-14 volts to the heater. You must use an old style analog meter with the needle gauge on the display to see the voltage swing back and forth.

If the O2 sensor readings are not right, say they read .1 volts or .8 volts steady, you have a problem. BUT before you blame the O2 sensor make sure it has good wiring, and make sure the proper voltage is feeding it, by turning power on, engine off to read the engine off voltage feeds (12-14 on one, and ensure the ground wires (power off) reads less than 1 ohm to the battery negative post.

If any one of the grounds or the O2 sensor to ECU/PCM wires reads a high voltage chances are the wiring harness is damaged and the 12-14 volts for the O2 sensor heater is getting through a bad spot in the wires insulation and contacting a ground wire or the O2 sensor wire feed to the ECU/PCM. This is not the O2 sensors fault, but a wiring problem.


A leaky exhaust system or leaky fuel injector(s), or bad compression or leaky valves, bad plugs, wires, cap, rotor, HV coil, and so on, or combination of these, can also cause a lean or rich condition that gives you high or low O2 sensor readings that are not the O2 sensors fault, so try and verify those other items also before buying parts like an O2 sensor to fix your problem.
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Unread 06-24-2013, 08:57 PM   #3
Angelkath
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2001 XJ Cherokee 
 
Join Date: Jun 2013
Location: LITH, IL
Posts: 2
My jeep is throwing the code after replacing the o2 sensor
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Unread 06-25-2013, 12:12 PM   #4
CJ7-Tim
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2000 XJ Cherokee 
 
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Location: out in the garage - Minneapolis, Minnesota
Posts: 22,080
Specifically, by the number, what code or codes did you have before, and have now ?
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Between 2009 when Obama took office, and 2013, median annual household incomes have fallen by more than $2,100.

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