Car Doctor Q&A: OBD Codes Are Only A Starting Point

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Diagnostic codes from the OBD-II system are a valuable tool for determining what may be wrong with your car, but they’re just a starting point, says AAA’s Car Doctor, John Paul.

Q. I have a Ford F-150 and whenever I completely fill the gas tank and restart it, there is a loud clicking sound under the hood. If I only fill the tank partway there is no noise.

If I fill the tank and I get the sound, the check engine light can come on. I took the truck to a local parts store that does free diagnostic checks and they listed at least 3 different engine codes. I’m not sure where to start, other than don’t fill the tank to full and I don’t have a problem.

I’m cheap like you, but I would like to get this fixed. Can you offer any suggestions?

A. Engine codes can be sometimes misleading and can take even the best technician down the wrong path. When I diagnose a vehicle I use a combination of diagnostic codes, technical databases, technical forums and a little old-fashioned detective work.

I start with the easiest and cheapest things possible. The first thing that I would do is find the source of the clicking sound. A long rubber hose can make a pretty good mechanic’s stethoscope (hold one end to your ear and search with the other end). When you hear the clicking noise, use the hose to narrow in on the source sound. If you find the noise coming from the evaporative solenoid purge valve, unplug it. If the noise stops, replace the evaporative purge-valve or purge-valve and canister.

Alldata is the technical database that I use. It does list a technical service bulletin that references various codes and the noise.

By the way I prefer “frugal.”

John Paul is senior manager of public affairs for AAA Northeast. A certified mechanic, Paul tests dozens of new cars each year and also hosts a radio show on AM 950