Hey guys. My 99 Tahoe came up with a o2 sensor code and catalyst below threshold. If my cat was plugged up wouldn't my rig have no power? Mine still has a ton of power and seems to be running fine. I'm thinking that my rig may just need a tune up. Any thoughts?
99 cat or tune up?
- Thread starter MJH0815
- Start date
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MO Viet Vet
Full Access Member
I am thinking you are getting a warning that the cat is not efficient and the system is telling you before it clogs and you lose power. If oxygen sensors are all the same age you can cross fingers and just replace them. I think what will happen is that the cat is going bad and the new sensors will again likely throw a code about cat efficiency. Sometimes you just have to bite the bullet. Here is what I found and copy/pasted here:
Testing Sensors:
First, a note on multi-meters: Testing should be done with a good quality digital multi-meter (DMM). Analog meters impose too much impedance on the circuits and lead to false readings.
In order for the sensor to function, it must be above 600 degrees. For this reason, there is a heater built into most O2 sensors to speed up the heating process. That is why the sensor has four wires. Two are for the ground (gray) and signal (black) and the other two are for the heater (white).
On Vehicle Test
Plug the DMM into the sensor’s wiring. This is probably easiest to do at the sensor connector, but may be possible at the computer. Testing at the computer includes all the wiring too. Set the DMM to read DC voltages below 1 volt. Start the engine and wait for it to warm up. You may have to rev the engine a bit to speed this up. The O2 sensor will indicate no voltage when it is cold. It warms up in 20 seconds and will start to indicate a voltage between 0 and 1 volt. 0v is a lean condition and 1 is rich. A value of .45 is optimum. After the engine has reached normal operating temperature, the voltage value should fluctuate around the .45V value. It should respond instantly to changes in the mixture. This can be tested by first removing the vacuum line from the fuel pressure regulator and watching for the voltage to approach 1V. With the vacuum hose reattached, disconnect the EGR valve from its hose. This should make the sensor read lean and well below .45V. If the sensor passes these checks, it is probably good. If this does not produce the required results, check the wiring for continuity. If that checks out OK, remove the sensor from the vehicle and run a bench test on it.
O2 Sensor Bench Test
Remove the sensor from the vehicle and attach a DMM to the Grey and Black leads. Heat the sensor with a propane torch. Within 20 seconds the sensor should produce a voltage when in the flame. Now pull it from the flame. The voltage should drop off to near zero within a second. Now put it back into the flame and check that the voltage climbs rapidly. If it passes so far, let it set in the flame for several minutes. If its reading stays consistent, its good. If it fails these bench tests, its time for a new one.
O2 sensor replacement
Before you replace this sensor, try to determine the cause of failure. In most cases contamination kills the thing. Non-sensor safe RTV, coolant and excessive carbon quickly kill these things. For best results use the exact replacement part. These are more expensive than the generic Bosch or Borg/Warner, but they are more accurate.
Testing Sensors:
First, a note on multi-meters: Testing should be done with a good quality digital multi-meter (DMM). Analog meters impose too much impedance on the circuits and lead to false readings.
In order for the sensor to function, it must be above 600 degrees. For this reason, there is a heater built into most O2 sensors to speed up the heating process. That is why the sensor has four wires. Two are for the ground (gray) and signal (black) and the other two are for the heater (white).
On Vehicle Test
Plug the DMM into the sensor’s wiring. This is probably easiest to do at the sensor connector, but may be possible at the computer. Testing at the computer includes all the wiring too. Set the DMM to read DC voltages below 1 volt. Start the engine and wait for it to warm up. You may have to rev the engine a bit to speed this up. The O2 sensor will indicate no voltage when it is cold. It warms up in 20 seconds and will start to indicate a voltage between 0 and 1 volt. 0v is a lean condition and 1 is rich. A value of .45 is optimum. After the engine has reached normal operating temperature, the voltage value should fluctuate around the .45V value. It should respond instantly to changes in the mixture. This can be tested by first removing the vacuum line from the fuel pressure regulator and watching for the voltage to approach 1V. With the vacuum hose reattached, disconnect the EGR valve from its hose. This should make the sensor read lean and well below .45V. If the sensor passes these checks, it is probably good. If this does not produce the required results, check the wiring for continuity. If that checks out OK, remove the sensor from the vehicle and run a bench test on it.
O2 Sensor Bench Test
Remove the sensor from the vehicle and attach a DMM to the Grey and Black leads. Heat the sensor with a propane torch. Within 20 seconds the sensor should produce a voltage when in the flame. Now pull it from the flame. The voltage should drop off to near zero within a second. Now put it back into the flame and check that the voltage climbs rapidly. If it passes so far, let it set in the flame for several minutes. If its reading stays consistent, its good. If it fails these bench tests, its time for a new one.
O2 sensor replacement
Before you replace this sensor, try to determine the cause of failure. In most cases contamination kills the thing. Non-sensor safe RTV, coolant and excessive carbon quickly kill these things. For best results use the exact replacement part. These are more expensive than the generic Bosch or Borg/Warner, but they are more accurate.
Thanks man. That was nice information to have. I replaced the o2 sensor that was bad and had autozone clear the codes. It went away for about a week but my service engine light came back yesterday while I did some Highway driving. I'm gonna go to autozone tomorrow and check the codes again and if it says catalyst below threshold I'm gonna assume my cat is getting bad and gonna bite the bullet. I was just looking for other opinions before I spent the money.
MO Viet Vet
Full Access Member
Which sensor did you replace? The front one or the rear one? You said "a" sensor and there is more than one. On an OBDII system there will be a sensor in front and a sensor in back of each cat.
