Diagnostic Aids

WARNING: This page is about a different car, the 2002 GMC Sonoma and 2002 Chevrolet S10 Pickup. However, it is still accessible from the selected car via links, so may be relevant.

Diagnostic Aids only talks about conditions causing a rich O2S. A lean O2S can also cause this Diagnostic Trouble Code (DTC) for the same reason as rich.

An O2S performance or slow response DTC is most likely caused by one of the following items:

Fuel pressure. The engine may run rich if the fuel pressure is too high. The PCM can compensate for some increase. However, if the fuel pressure is too high, a DTC P0172 may set.
Leaking injector. A leaking or malfunctioning injector can cause the system to go rich.
MAP sensor. An output that causes the PCM to sense a higher than normal manifold pressure, low vacuum can cause the system to go rich.
Pressure regulator or restricted fuel filter. Inspect for a restricted fuel pressure regulator return hose by checking for a damaged or kinked hose between the fuel filter and the fuel tank.
A restricted exhaust system.
TP sensor. An intermittent TP sensor output can cause the system to go rich due to a false indication of the engine accelerating.
O2S 1 contamination include leaking exhaust before O2S causing low reading, lean. Inspect the O2S 1 for silicone contamination from fuel or the use of improper RTV sealant.