# Easiest method to test O2 sensor? **URL:** \ **Category:** Maintenance/Repairs\ **Created:** [October 22, 2013, 7:21am UTC](https://community.cartalk.com/t/easiest-method-to-test-o2-sensor/81889 "2013-10-22T07:21:45Z")\ **Posts on this page:** 1\ **Showing post:** 14
**Author:** ![keith](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/keith/32/5518_2.png) [@keith](https://community.cartalk.com/u/keith)\ **Post date:** [October 23, 2013, 4:21am UTC](https://community.cartalk.com/t/easiest-method-to-test-o2-sensor/81889/14 "2013-10-23T04:21:33Z")
About once a second for those old sensors. Tester, the impedance of a meter is placed in parallel with the load and therefore affects the total impedance of the load. The lower the impedance, the greater the effect. The source and the load form a Kirchoff’s loop with the source impedance in series with the load impedance. This forms a voltage divider. If the load impedance goes down, then the source impedance drops more of the supply voltage, so the meter will read a lower voltage than the true voltage. It “swamps” the supply voltage. DVMs are more accurate because they have an almost infinite impedance, therefore they have little to no effect on the load impedance. A good VOM will usually have an impedance of 20kohms/volt DC and 5kohms/volt AC. Some lower cost meters will be 5k and 1k respectively and some super cheap small meters will be 1k and 200 ohms/volt. You see the same effect when putting a volt meter across the battery terminals and hit the starter. With the load out of the circuit (the starter), you measure 12.6 volts. In this case it doesn’t matter much whether the meter is a DVM or a cheap 1kohm/volt analog meter because the batteries internal impedance is on the order of a fraction of an ohm. For example, a battery with a rating of 630 CCA would have an internal impedance of about 0.02 ohms. 1k, 20k or 1M ohms would not make a measurable difference. Any way, you hit the starter and the starter motor has an impedance of less than an ohm, the batteries internal impedance will be significant and will drop a couple of volts. In these high current applications, again a 1k, 20k or 1M ohm meter won’t make a measurable difference. but in very low current, high impedance circuits like the O2 sensor, the impedance of the meter can have a measurable difference. The cheap meter may only be off by a tenth of a volt or even less, but if an absolutely accurate value is important to you, you need the higher impedance. I don’t think that kind of accuracy is needed in this case and I tried to imply that in my response. If the voltages are in range and sweeping at the correct rate, the sensor is good to go. Who really cares if the voltage reading shows 0.66 volts when the book says “about” 0.7 volts. I do get a little anal myself now and then, but not that anal. --- _[View the full topic](https://community.cartalk.com/t/easiest-method-to-test-o2-sensor/81889)._