Question: I have a 1996 Chevy Tahoe 4x4 with 261,000 miles on the original 350 Vortec engine. The check-engine light comes on sometimes with code PO172: right side too rich. I swapped the left O2 sensor with the right one, but there was no change. I also tried a new airflow meter, and after there was no change I took it back. I did a tuneup about 10,000 miles ago, replacing the plugs, wires, distributor cap, and rotor. I'm about to give up and go to the Chevy people. Please help me--I've never been there for service.

Answer: Swapping oxygen sensors was a good move. That there was no change indicates the cause is not an improper oxygen content signal from a bad O2 sensor (unless there's a poor electrical connection), which narrows it down to a true rich condition. Just be sure you swapped the two sensors in the exhaust manifolds, and not the one or two sensors downstream of the catalytic converter(s). The problem with a rich condition trouble code is that there are a lot of possible causes that have to be eliminated one by one. For example, the fuel pressure regulator may be leaking fuel through its internal rubber vacuum diaphragm. Pull off the vacuum hose at the regulator and check for the sight or smell of gas. The list continues with an obstructed air intake duct, clogged air filter, low or unsteady idle due to carbon buildup on the throttle plate, sensors (including the throttle position sensor) sending incorrect data, poor fuel pressure, evaporative emissions system fault, leaking fuel injectors, and a misfire. On a job like this, experience in using a scan tool and accurately evaluating incoming data is crucial. Check out the items you might be familiar with, and then get it to Chevy before you drive yourself crazy.


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