AN APPARENTLY MISPROGRAMMED GPS CONTRIBUTES TO AN UNWITTING TRACK DEVIATION FOLLOWED BY A REPRIMAND FROM ATC FOR A SMA PLT.

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DEPARTED ONT AND PROCEEDED ON ASSIGNED HDG OF 255 DEGS AND 4000 FT. UPON REACHING 4000 FT; I WAS ASSIGNED A HDG OF 210 DEGS FOR TFC. I WAS THEN ASSIGNED A HDG OF 230 DEGS TO JOIN V186 AS PER OUR ORIGINAL CLRNC. ABOUT 30-60 SECONDS LATER; I WAS QUERIED BY ATC AS TO WHY I WAS DEVIATING FROM THE ORIGINAL ASSIGNED HDG OF 210 DEGS. I STATED TO ATC THAT I WAS ASSIGNED HDG 230 DEGS TO INTERCEPT V186 AND THAT I REPEATED THE ASSIGNMENT AND WAS NOT CORRECTED. I WAS THEN TOLD THAT I WAS NOT SUPPOSED TO HAVE DEVIATED FROM THE ORIGINAL 210 DEG HDG. AT THIS POINT I WAS CONFUSED AS TO WHAT WAS HAPPENING SINCE I FULLY BELIEVED I WAS COMPLYING WITH ATC INSTRUCTIONS. I DID TURN BACK TO A HDG OF 210 DEGS AS DIRECTED. THE CTLR ALSO ASKED WHY I WOULD TURN TO THE R WHEN A L TURN WOULD BE REQUIRED TO INTERCEPT V186. I OBSERVED THE 'ACTIVE' LEG ON THE ONBOARD GARMIN 530 GPS WHICH DIRECTED ME TO TURN R TO INTERCEPT THE AIRWAY. THUS; THE CTLR WAS SAYING A L TURN WAS REQUIRED TO JOIN AND ACCORDING TO THE ONBOARD GPS; A R TURN SEEMED APPROPRIATE. BASED ON THIS CONFLICT; I DECIDED TO REVERT TO MY #2 NAV TO ESTABLISH POS ON THE AIRWAY. I THEN ESTABLISHED MYSELF ON V186 AND WAS THEN ISSUED A PHONE NUMBER TO CALL UPON LNDG TO THE ATC AREA MGR. I BELIEVE THAT I MADE EVERY EFFORT TO COMPLY WITH ATC INSTRUCTIONS. I REPEATED VIRTUALLY EVERY ASSIGNMENT GIVEN AND WAS NOT CORRECTED. IF A DEV OCCURRED; IT WAS BY NO MEANS INTENTIONAL. I DID NOT OBSERVE A TFC CONFLICT. HOWEVER; UPON TALKING TO ATC BY PHONE; THEY ADVISED ME THAT THEY HAD LOST SEPARATION WITH ANOTHER ACFT. NO OTHER CONFLICTS OR DEVS OCCURRED FOR THE REMAINDER OF THE FLT. EVERY EFFORT WAS MADE TO COMPLY AND/OR RECTIFY THE SITUATION. HOWEVER; I CANNOT SAY FOR SURE WHAT CAUSED THE INCIDENT. AT THIS POINT; I CAN ONLY BE EVEN MORE CAUTIOUS ABOUT ATC ASSIGNMENTS AND POSITIONAL AWARENESS. CONTRIBUTING FACTORS MIGHT INCLUDE: HIGH PLT WORKLOAD (DEP PROCESS); PLT FAMILIARITY TO AREA; POSSIBLE EQUIP PROGRAMMING ERROR (GNS530); ATC NOT CLARIFYING/CORRECTING READBACK OF ASSIGNMENTS. USE OF SECONDARY NAV SYS COULD BE HELPFUL IN SITUATIONAL AWARENESS AND WILL BE UTILIZED HENCEFORTH. COMPANY WAS NOTIFIED AND AN IRREGULARITY RPT FILED.