Diagnostic Aids

WARNING: This page is about a different car, the 2003 GMC Sonoma and 2003 Chevrolet S10 Pickup. However, it is still accessible from the selected car via links, so may be relevant.

Fuel pressure - System will run lean if fuel pressure is too low. Monitor the fuel pressure while driving the vehicle at various road speeds and loads in order to confirm. Diagnose fuel system. See FUEL SYSTEM DIAGNOSIS under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 2.2L SONOMA & S10 PICKUP article.
MAP sensor - An output that causes the PCM to sense a lower than normal manifold pressure or high vacuum, can cause the system to go lean. Disconnecting MAP sensor will allow PCM to substitute a fixed or defaulted value for the MAP sensor. When the lean condition is gone when the sensor is disconnected, substitute with a known-good sensor and recheck.
Fuel contamination - Water, even in small amounts near the fuel pump inlet, can be delivered to the fuel injector. Water causes a lean condition.
O2S harness - The O2S 1 pigtail may be mis-positioned and contacting the exhaust manifold.
An engine misfire will result in unburned oxygen in the exhaust, causing a lean condition.
A plugged fuel filter can cause a lean condition.