Possible Causes

WARNING: This page is about a different car, the 2003 Toyota 4Runner. However, it is still accessible from the selected car via links, so may be relevant.

Possible causes are:

Open or short in engine wiring harness.
Defective connections at ECM or component.
Disconnected, restricted or damaged vacuum hose(s).
Ignition system malfunction.
Defective fuel injector(s).
Improper fuel pressure.
Defective Mass Airflow (MAF) meter.
Defective Engine Coolant Temperature (ECT) sensor.
Improper engine compression.
Improper valve clearance.
Improper valve timing.
Defective Positive Crankcase Ventilation (PCV) valve hose or connection.
Defective ECM.