O2 sensor / catalytic converter issues
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Registered User
Joined: May 2008
Posts: 9
Likes: 0
From: Tall Timbers MD
Unfortunately, there are no others in my area. I would have to drive an hour and a half. Im taking it in this weekend. I hope to come out of this with no check engine light. Cross your fingers for me 
Thanks again for all of your advice. It was a big help.
Thanks again for all of your advice. It was a big help.
Registered User
Joined: Oct 2008
Posts: 11
Likes: 0
From: Kamloops, BC
This is what it says in the service manual with regards to P0420:
P0420-CATALYST 1/1 EFFICIENCY
For a complete wiring diagram Refer to Section 8W.
² When Monitored:
After engine warm up, at medium throttle with vehicle speed between 35 and 60 mph.
² Set Condition:
If the switch rate of the downstream O2 sensor is within the calibrated fail threshold during monitoring. Two trip
fault. Three good trips to turn off the MIL.
Possible Causes
EXHAUST LEAK
ENGINE MECHANICAL CONDITION
AGING O2 SENSOR
CATALYTIC CONVERTER
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE -DIAGNOSIS AND TESTING).
Diagnostic Test
1. EXHAUST LEAK
NOTE: A thorough exhaust leak test must be completed before removing an O2 Sensor or any other part
from the exhaust system. Do not remove any parts before an exhaust leak check has been completed.
NOTE: An exhaust leak may cause downstream fuel adaptives to rail out, resulting in a false O2 Sensor
failure
Turn the ignition off.
Inspect the exhaust system for leaks between the engine and the 1/1 Oxygen Sensor.
Inspect the exhaust system for leaks between the engine and within 6 inches downstream of the 1/2 Oxygen Sensor.
Perform a leak test with Miller Tool #8404A Evaporative Emissions Leak Detector (EELD), even if an audible leak is
not heard. Pay particular attention to the exhaust manifold to head surface, exhaust pipe to manifold connection,
and the O2 sensor boss for leaks.
Connect the SMOKE supply tip (black hose) to the exhaust cone adapter (if equipped) and place it into the tail pipe.
Set the smoke/air control switch to SMOKE.
Press the remote smoke/air start button.
While still holding the remote smoke/air start button, use the white light (#8404-CLL) to follow the exhaust system
path and look for a leak, indicated by exiting smoke.
If a leak is concealed from view, release the remote smoke/air start button, and use the ultraviolet (UV) black light
#8404-UVL and the yellow goggles 8404-20 to look for residual traces of dye that is left behind by the smoke.
The exiting smoke deposits a residual fluid that is either bright green or bright yellow in color when viewed with a
UV light.
Were any exhaust leaks found?
Yes >> Repair as necessary.
Perform the PCM Verification Test Ver. 1 (Refer to 9 - ENGINE - DIAGNOSIS AND TESTING).
No >> Go to 2
P0420-CATALYST 1/1 EFFICIENCY
For a complete wiring diagram Refer to Section 8W.
² When Monitored:
After engine warm up, at medium throttle with vehicle speed between 35 and 60 mph.
² Set Condition:
If the switch rate of the downstream O2 sensor is within the calibrated fail threshold during monitoring. Two trip
fault. Three good trips to turn off the MIL.
Possible Causes
EXHAUST LEAK
ENGINE MECHANICAL CONDITION
AGING O2 SENSOR
CATALYTIC CONVERTER
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE -DIAGNOSIS AND TESTING).
Diagnostic Test
1. EXHAUST LEAK
NOTE: A thorough exhaust leak test must be completed before removing an O2 Sensor or any other part
from the exhaust system. Do not remove any parts before an exhaust leak check has been completed.
NOTE: An exhaust leak may cause downstream fuel adaptives to rail out, resulting in a false O2 Sensor
failure
Turn the ignition off.
Inspect the exhaust system for leaks between the engine and the 1/1 Oxygen Sensor.
Inspect the exhaust system for leaks between the engine and within 6 inches downstream of the 1/2 Oxygen Sensor.
Perform a leak test with Miller Tool #8404A Evaporative Emissions Leak Detector (EELD), even if an audible leak is
not heard. Pay particular attention to the exhaust manifold to head surface, exhaust pipe to manifold connection,
and the O2 sensor boss for leaks.
Connect the SMOKE supply tip (black hose) to the exhaust cone adapter (if equipped) and place it into the tail pipe.
Set the smoke/air control switch to SMOKE.
Press the remote smoke/air start button.
While still holding the remote smoke/air start button, use the white light (#8404-CLL) to follow the exhaust system
path and look for a leak, indicated by exiting smoke.
If a leak is concealed from view, release the remote smoke/air start button, and use the ultraviolet (UV) black light
#8404-UVL and the yellow goggles 8404-20 to look for residual traces of dye that is left behind by the smoke.
The exiting smoke deposits a residual fluid that is either bright green or bright yellow in color when viewed with a
UV light.
Were any exhaust leaks found?
Yes >> Repair as necessary.
Perform the PCM Verification Test Ver. 1 (Refer to 9 - ENGINE - DIAGNOSIS AND TESTING).
No >> Go to 2
I have the P0420 Code. I have done the reading on the code and well, I'm confused most people report a loss in power with the DTC Displayed in my case the truck has more power but does flood a bit on cold start.. but other than that It seems fine, what do you make of this???
Registered User
Joined: Oct 2008
Posts: 11
Likes: 0
From: Kamloops, BC
P0420-catalyst 1/1 efficiency
Hey Thanks For the warm welcome.
My Fuel Milage got better and more power too. Thats without reseting the DTC's . I Also noticed that my CAT Gets hotter than usual.
and yes it does flood but only at cold start if you suddenly depress the gas pedal.
My Fuel Milage got better and more power too. Thats without reseting the DTC's . I Also noticed that my CAT Gets hotter than usual.
and yes it does flood but only at cold start if you suddenly depress the gas pedal.
Registered User
Joined: Oct 2008
Posts: 11
Likes: 0
From: Kamloops, BC
P0420-catalyst 1/1 efficiency
Hey
So its been a week or so and I have now reset the DTC's and the fuel milage is horrid.. can almost watch the needel drop.
Could this still be a catilyst Efficency Issue??
The code stayed off.
So its been a week or so and I have now reset the DTC's and the fuel milage is horrid.. can almost watch the needel drop.
Could this still be a catilyst Efficency Issue??
The code stayed off.
Site Moderator
Joined: May 2008
Posts: 9,657
Likes: 24
From: Louisville, Ky
Not usually. That code just means that the exhaust isn't clean enough after the converter. Usually a bad sensor. Have you rescanned for codes anyway? may be a new one hidden in there but not lighting the CEL.
