P0170
P0171
P0172
Theses codes are usually caused by a lean condition that the PCM can no longer adjust for. The condition is caused by the deterioration of a fuel metering component or system that has occured over time. Over time the PCM has been making adjustments to fuel trim to compensate for the deterioration of the fuel metering system until now when the need to adjust (compensate) has exceeded the PCM's ability to do so.
the cause could be a deteriorated O2 sensor but that will usually cause a code P0136 for the O2 sensor range or response fault. More likely it is a mechanical problem such as low fuel pressure, a restricted fuel filter or a worn fuel pump.
Misadjsuted valves can cause this condition as well. On the 1997 the valve adjustment is critical and at 190k you shoutld start with the valve adjustment if you can't remember when you had it done last.
Once the valves are adjusted use the car to see if the code sets again. If it does, then have a fuel pressure test done; if the pressure is low check the filter and lines before condemning the pump.
You need a digital oscilloscope or scan tool to properly test the O2 sensor. I think that's a possibility but its much further down the list than you would think.
here's the possible causes in most likely order
Valve Clearance needs to be adjusted
Fuel Pump insufficient flow/pressure
Fuel Filter clogged
Fuel Injector clogged,
Fuel Feed Line clogged, leaking or bent
Fuel Pressure Regulator stuck open
Primary HO2S (Sensor 1) deteriorated
EGR System malfunction (too much flow) (egr hung open)
Exhaust leak before the O2 sensor (exhaust manifold cracked or leaking manifold gasket)
