Any ideas what it could be?

What codes are you getting?
Have you checked the spark plugs and related components?
Check fuel pressure as well.

I need the codes, not the decription.

Will wait for them.

Here are the diagnostic aids.

Diagnostic Aids

Check for the following conditions:

Check HO2S pigtail wire for breaks, contamination or grounding on exhaust manifold. Check for intermittent ground in signal wire between sensor connector and sensor.

Check PCM grounds.

Check fuel pressure.

Perform injector balance test.

Vacuum leaks: check for disconnected or damaged vacuum hoses and for
vacuum leaks at the intake manifold, throttle body, EGR system, and PCV system.

Exhaust leaks: an exhaust leak may cause outside air to be pulled into the exhaust gas stream past the HO2S, causing the system to appear lean. Check for exhaust leaks that may cause a false lean condition to be indicated.

MAF sensor: the system can go rich if MAF sensor signal indicates an incorrect engine airflow measurement that is not correct. Disconnect the MAF sensor to see if the lean condition is corrected. If so, replace the MAF sensor.

Fuel contamination: water, even in small amounts, can be delivered to the fuel injectors. The water can cause a lean exhaust to be indicated. Excessive alcohol in the fuel can also cause this condition. Check for fuel contamination.

If none of the above conditions are present, replace the suspect H02S.