Gunson tester calibration
 

Got my gunson tester, per instructions and posts ive read i hooked it up to a battery other than the one in the car im testing. let it do its thing for 5-10 min. set it at 2.0% let it sit a while longer to be sure its stable,then i drove my warmed up but not to warm 80 sc up to the tester, the o2 sensor disconected and stuck my modified [used a steel brake line because it fits] in the side port of the CAT and set the o2 at 3.5%. I waited a bit and it stayed stable at 3.5%[great] took the probe out moved the car away and waited for the gunson to return to 2.0%or close to it. after 20-30-40 min. it got no lower than 2.7%, is there that much margin for error on this thing, the manuel says .5% is to be expected. my question is if it only returned to 2.7%my o2 is really .7% higher than 3.5% actually 4.2%is this correct?Ive used this tester 4 times now and it never returns to its original setting. I did discover that if i set the duty cycle between 45-55% unplug the o2 sensor then check the o2 with the gunson it falls right in at .3 to.4% which is were i want to be to pass California smog. it runs much better without to o2 set at 3.5%.

I found the Gunson to be a bit skitish. Until I learned to warm it up for 1 whole hour before calibration, I did not get consistant results. I turn it on and then go for a long drive, testing when I get back to the garage. It also takes nearly an hour to return to the 2% baseline after testing is done. As long as I follow these steps, the settings I get are similar to my friends professional (Sun) tester.

BTW, I have my 3.0 set at around 2.5% CO, and it runs very strong at that setting.