Oxygen Sens Mon inc

Hi all,
I got Envoy XL 03. It keeps giving me the "Service Engine Soon" light. I reset it using the Auto scanner plus CP9180. The error code I receive is P0420, and the error reads "Catalyst Efficiency Below Threshold Bank 1". I know it is easy to understand that tha cat is defective, but I noticed that there is something else in the report I got from the scanner. It says "Oxygen Sens Mon inc." So, what's wrong here? Is it really the cat or is it something else? Is the "Sens Mon" the oxygen sensor itself, or is it another part that monitors the sensor? Some info that might help, I replaced the 2 oxygen sensors 8 months ago but this problem is recent. When I first installed the sensor on the exhaust manifold I mislinked the wires and the engine stalled 4 times before I recognised my mistake. But after fixing the order of the wires, the car ran smoothly for nearly 5 months. Then one day I drove it without warming it and it gave me the nice service engine light. This has become more frequent. I noticed that whenever I drive it cold, it gives off the service engine light. Today I hooked up the scanner and got a freeze recording of data in addition to dignostic monitors tests, and here are the results:

Freeze Data:

Global OBDII

Non OBD Freeze Frame

Frame 1 Mod $10
TROUB CODE P0420
ENG SPEED RPM 616
CALC LOAD % 31.7
MAP KPA 33
COOLANT �F 185
ST FTRM1 % -2.2
LT FTRM1 % 0.8
ST FTRM3 % -51.0
LT FTRM3 % 8.5
VEH SPEED MPH 0
FUEL SYS 1 OPEN
FUEL SYS 2 N/A

********************************************
Global OBDII

Non OBDIM Monitors


Since DTCs Cleared

Misfire Monitor ok
Fuel System Mon ok
Comp Component ok
Catalyst Mon ok
Htd Catalyst n/a
Evap System Mon ok
Sec Air System n/a
A/C Refrig Mon n/a
Oxygen Sens Mon inc
Oxygen Sens Htr ok
EGR System Mon n/a

********************************************
Diagnostic Monitors


Test $02

ID: 04
MAX: ----
MEAS: 0
MIN: 0
STS: OK
MOD: $10


Test $05

ID: 2E
MAX: 803c
MEAS: 87b5
MIN: ----
STS: HI
MOD: $10


Test $0c

ID: 60
MAX: 807d
MEAS: 808d
MIN: ----
STS: HI
MOD: $10


Test $0e

ID: 11
MAX: c6
MEAS: 0
MIN: ----
STS: OK
MOD: $10
**********************************************
I have also recorded Data but it's too big to post here. I searched for the "Attachment" button here but I couldn't locate it.
Can the experts help me in this issue? I really appreciate the help you offer.

Welcome to AF The engine was in open loop so the o2 monitor was incomplete. at 185 degrees temp, unless you were just idleing for a period of time, you should have been in closed loop. Retest test at 1500 rpms and make sure the engine goes to closed loop. The 420 is set when the pre cat sensor and post cat sensor inputs are skeweed to the point the comparison of the 2 show excess 02 in the exhaust stream which would indicate short of a misfire issue, that the cat was failing. To properly check the post and precat sensors, you would need a graphing scanner to do a comparison, google 420 there is a ton of info out there, also how many miles on this truck. Another way to check the cat is to compare the temp at the input pipe of the cat to the output pipe, there should be 125 degrees difference minimum , higher at the outlet, that would say the cat is working.

Welcome to AF The engine was in open loop so the o2 monitor was incomplete. at 185 degrees temp, unless you were just idleing for a period of time, you should have been in closed loop. Retest test at 1500 rpms and make sure the engine goes to closed loop. The 420 is set when the pre cat sensor and post cat sensor inputs are skeweed to the point the comparison of the 2 show excess 02 in the exhaust stream which would indicate short of a misfire issue, that the cat was failing. To properly check the post and precat sensors, you would need a graphing scanner to do a comparison, google 420 there is a ton of info out there, also how many miles on this truck. Another wat to check the cat is to compare the temp at the input pipe of the cat to the output pipe, there should be 125 degrees difference minimum , higher at the outlet, that would say the cat is working.



Hi,
Thanks for such a quick response. the truck has done 135000 miles. How can I make sure the engine goes to closed loop. I really appreciate it if you explain what you mean by open loop and closed loop. I wasn't idling the engine. It was running at around 650 rpm when I took the readings.
After resetting the computer, the light disappears unless I drive it without enough warming up. Weather temperature is between 4 and 10 degrees celsius in the morning, and around 14 when I leave work. Testing temperature before and after the cat in not possible at present as I don't have the needed tools.
Thanks a lot.

650 is idle, the scanner says fuel sys 1 open it should be closed, then the pcm monitors the o2 sensor.

Thanx a lot for your replies. I will drive the truck today for 20 minutes and make the test. I'll be back with the test result but late tonight. Later...

OK

I drove for nearly an hour,but the service engine light didn't come up. I hooked up the scanner but didn't find freezing data. I initialized a "Monitors Test". The results were OK except for two:

1- Catalyst Mon Inc
2- Oxygen Sens Mon Inc

Does this mean the cat is faulty?

I really appreciate your efffort trying to solve my problem.
Thanx a lot

No that means the pcm monitors are not online, the drive cycle was incomplete, google GM drive cylcle for the info how what it consists of, plenty of info on this.

Thanx a lot for your help and support.