1999 Dodge Dakota Trouble code P0420

99DODGE

MEMBER
1999 DODGE DAKOTA
6 CYL
2WD
AUTOMATIC
10,700 MILES

After operation @ highway speed, If O have to immediately drive slow (e.G. If I get caught at a red light at the end of an exit ramp) my 1999 Dakota 3.9l check engine light comes on. A scanner reveals trouble code P0420 catalyst efficiency below threshold. If I get off the highway and am able to continue travelling at nominal speed (35-40mph)no light occurs. Likewise once the code is reset, the light may stay out for a month or more until the highway to slow speed condition is repeated. Catalytic converter (entire exhaust) was recently replaced due to rust out, but the intermittent trouble light remains. I'm guessing a bad O2 sensor, but would prefer to get some collaboration before I start randomly changing parts. Otherwise the vehicle performance and economy are unaffected by the presence of the check engine light.

1 Reply

RASMATAZ

MECHANIC
75,992 POSTS

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
Possible SolutionsSome suggested steps for troubleshooting a P0420 code include:

Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
Inspect the downstream heated