For the P0174 be on the lookout for an exhaust leak on the left hand side (driver's). Check the EGR components as a possible leak source. Check the vacuum lines to/from the DPFE sensor.
For the P0136 check for the possibility that the wires running to the rear O2 sensor have touched the exhaust and melted. This could also be from a bad O2 sensor or an EXHAUST leak.
Since the P0136 calls out the rear O2 sensors, the driveability will not be directly affected. However, it will not pass inspection.
To all reading this, today's cars will not run right with vacuum leaks and/or exhaust leaks. The PCM needs accurate metering of the air in/out of the motor. Unmetered air entering after the MAF will make the PCM run LEAN.
Unmetered air in the exhaust will make the PCM THINK the motor is lean when it's not.
Make sure the basics are covered and then start digging deeper. Your wallet will thank-you.
P0174 - System to Lean (Bank 2)
P0136 - HO2S Sensor Circuit Malfunction (HO2S-12)
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.