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The CEL has been on for quite a while in my 99 RX300. I've cleaned the MAF, replaced the air filter, checked for vacuum leaks, etc. I'm wanting to look at the O2 and A/F sensors now, but would like to know how to test them before I go blowing money replacing what may not be defective. Any advice would be appreciated.
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The easiest way to test the sensors is to read the code that triggered the check engine light with an OBDII (On-Board Diagnostics II) reader that plugs into the recepticle on the bottom left of the dashboard. These can be purchased, or borrowed from AutoZone stores and others.
The other way is to measure the resistance values on the leads of the sensors. I've heard that it is usually the heaters in the sensors that burn out. I think that the sensors have four contacts. I have the service manual and could look up the resistance values at room temperature and normal operating temperature, but as I recall the resistance between the non-heater contacts is in the range around roughly 10 ohms.
I pulled the codes and want t make that one last check with the ohmmeter. 10ohms for the non-heater contacts? If you could grab the other values, that would be great. Where can I get one of those service manuals?
You can use an analog voltmeter (the one with the needle). Tap into the signal wire of the o2 sensor. Run the engine at 3000rpm for 30secs to a minute. The needle should start moving and eventually go back and forth. A bad o2 sensor will just stay there, dead, or at a certain figure, without bouncing back and forth.
If it is a heated o2 sensor (4 wire), you don't need to race the engine as much as with a non heated (1 wire) sensor.
By the way, it's those lovely p0171 and p0174 codes.
I doubt seriously that it is your A/F ratio sensors. More likely maf bad or vacuum leak somewhere. I know you said you cleaned the maf. Did you take it off to clean it? Is the little wire still intact? is the tube on the back of the air box plugged on? Are the clamps on the intake tube from the air box to the throttle body tight and the tubing in good condition? Go over the vacuum tubing with a fine tooth comb. Could be wrong but I don't think this has anything to do with the A/F sensors. Bad maf or vacuum leak somewhere.