The two codes point at two different systems but there one thing in common
Trouble Code: P1296
Cooling System Malfunction
Engine coolant temperature sensor has failed
An open circuit or a short to B+ is present
Sensor circuit is short to ground
ECM has failed
Trouble Code: P0420
Catalyst System Efficiency (Bank 1) Below Threshold
Air leaks at the exhaust manifold or in the exhaust pipes
Catalytic converter is damaged, contaminated or it has failed
ECT/CHT sensor has lost its calibration (the signal is incorrect)
Engine cylinders misfiring, or the ignition timing is over retarded
Engine oil is contaminated
Front HO2S or rear HO2S is contaminated with fuel or moisture
Front HO2S and/or the rear HO2S is loose in the mounting hole
Front HO2S much older than the rear HO2S (HO2S-11 is lazy)
Fuel system pressure is too high (check the pressure regulator)
Rear HO2S wires improperly connected or the HO2S has failed
Note: When looking carefully at the two codes, there is one thing in common and that is the coolant sensor. Computer rely on the coolant sensor to adjust fuel mixture. When computer add to much fuel when it should not because of a lazy coolant sensor, it could lead to catalytic converter and oxygen sensor damage.A damage or fouled oxygen sensor will probably give wrong information to the computer too.
What you need to do is to check the wiring to the coolant sensor for open or short. If there is no wiring problem, test or replace the coolant sensor.
Next, have the catalytic converter replaced along with the oxygen sensor.
Monday, February 15th, 2010 AT 9:41 AM