Check here for your codes: http://www.contour.org/FAQ/FAQ.php?s=probs&display=checkengineThe following is from the Ford TSP (Technical Service Publication): Diagnostic Trouble Codes (DTC) P0420 and P0421 indicate that bank 1 catalyst system efficiency is below the acceptable threshold.
Diagnostic Trouble Codes (DTC) P0430 and P0431 indicate that bank 2 catalyst system efficiency is below the acceptable threshold.
NOTE 1: Complete the spark timing check in «Quick Test» before proceeding with this test step. Spark timing retarded below specification may increase exhaust gas temperature and decrease catalyst efficiency over time.
NOTE 2: Be sure customer has not:
(1) Refueled vehicle with leaded gasoline.
(2) Experienced high vehicle oil consumption.
NOTE 3: If entering this Pinpoint Test for symptoms only, go immediately to «HF5».
NOTE 4: Internal deterioration of a catalytic converter is usually caused by abnormal engine operation upstream of the catalyst. Events that may produce higher than normal temperatures in the catalyst are particularly suspect. For example, misfiring can cause higher than normal catalyst operating temperatures.
Possible causes:
-- Use of leaded fuel.
-- Oil contamination.
-- Cylinder misfiring.
-- Damaged HO2S.
-- Damaged ECT sensor.
-- Downstream HO2S wires improperly connected.
-- Fuel pressure too high.
-- Damaged exhaust system pipe.
-- Damaged exhaust manifold.
-- Damaged muffler/tail-pipe assembly.
-- Damaged catalytic converter.
96 Contour SE
Duratec V6 24-valve 2.5L ATX
108,000 mi.
Replaced: crankshaft w/ DMD, EGR valve (clogged), EVR, DPFE, PCV valve, evap emissions hose & tube (cracked), window regulator (broken), LH & RH PCV tubes, UIM gasket (leak), ignition coil
| 
