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According to the Factory Service Manual for the 2002 model year (you don't specify year),
P0108 - BARO/MAP Sensor High Voltage Detected. Likely causes for this code can be:
- VREF shorted to VWPR
- BARO/MAP signal shorted to VPWR
- Damaged BARO/MAP sensor
- Damaged PCM
P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
Likely causes for this code are:
- Use of leaded fuel
- Damaged HO2S
- Malfunctioning ECT
- High fuel pressure
- Damaged exhaust manifold
- Damaged catalytic converter
- Oil contamination
- Cylinder misfiring
- Downstream HO2S wires improperly connected
- Damaged exhaust system pipe
- Damaged muffler/tailpipe assembly
- Retarded spark timing
I'd suggest you start by making sure the MAP sensor is correctly connected and that the air intake tubing is connected securely and correctly. Using your Matco scan tool, see if you can determine how many engine starts since either of the codes were set. Also, check for freeze frame data, specifically what the MAP voltage was when the code was set. Enable the datastream mode and record a run when the car is acting up. Monitor the MAP, misfire counts, and oxygen sensor voltages to see if something stands out as abnormal.
-Rod