99 Mercury Mystique LS - 3 CEL Codes - Ford Forums

CVPI · 07-16-2008, 16:48

This post is in regards to my fathers 1999 Mercury Mystique LS (2.5L Engine). I got the following CEL (Check Engine Light) codes:

P0420 - Catalyst Efficiency Below Threshold (Bank 1)
P0171 - System Too Lean (Bank 1)
P0174 - System Too Lean (Bank 2)

What could the problem(s) be? Thanks!

jglen490 · 07-22-2008, 11:01

According to obdcodes.com:

Quote:

A code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for
An oxygen sensor is not reading (functioning) properly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings

This probably relates to the both the other OBD code outputs you are getting. Both the P0171 and P0174 indicate:

Quote:

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