According to my books, code 538 is from the computer not seeing an rpm change during the key on, engine running test. If you got this code while running the KOER test, did you yank the throttle when the test prompted you to? If not, re-run the test and when you get 1 flash (or needle sweep if using a voltmeter) pull on the throttle. This is where the computer checks the throttle position sensor. If you do the test properly and still get the code, your throttle position sensor may be failing.