A code P0121 may mean that one or more of the following has happened:
- TPS has intermittent open or short internally
- Harness is rubbing and causing an open or short in the wiring
- Bad connection at the TPS
- Bad PCM (less likely)
- Water or corrosion in connector or sensor
(TPS = throttle position sensor - mounted on side of throttle body and connected to accelerator linkage.)
A code P0171 may mean that one or more of the following has happened:
- The MAF (Mass Air Flow) Sensor is dirty or faulty. Note: The use of "oiled" air filters may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.
- There could be a vacuum leak downstream of the MAF sensor.
Possible solutions include:
- In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
- Inspect all vacuum and PCV hoses, replace if necessary
- Check for a dirty fuel filter and proper fuel pressure
Hope this helps.