View Full Version : Poor MPG related to old O2 sensors?
LT 03
07-25-2007, 06:26 AM
The catyltic converter clogged on my Trailblazer at 60,000 miles. The dealer replaced it saying that the pcm had run the mixture to rich and caused the cat to meltdown. A week later the check engine light came on,so back to the dealer. The rich code was found and so the pcm was reprogramed with GM's latest fix. Last night I filled up and found that my mileage has gotten progressively worse(same senario as just before the cat completly clogged leaveing me stranded on the side of the hwy 500 miles from home) . mileage on the last 3 tanks of fuel has gone from 18.1 mpg to 17.3mpg to 15.03mpg. All in basicly the same driving conditions and weather . I suspect that the o2 sensors are not working properly and may have caused the cat meltdown in the first place. Anyone have any ideas? I guess its back to the dealer before the cat fries again. Any info /input is apprieciated. Thanks ,Jim B.
Blazernut
07-25-2007, 07:26 AM
That is really strange because the rich code should give you a flashing check engine light to let you know to shut it down?
If it is the PCM that is the culprit it is part of the emmision system and will be covered but there must be more to it. The PCM is not a common failure. The 02 sensor on the exhaust should also throw a code if it is not working?
You should post the codes here and it may give some insight for the guys that see them all the time. :m2:
LT 03
07-25-2007, 08:43 AM
The code was PO172, which I believe translates to "system too rich, left bank". Thanks ,Jim B.
PROSPHOTO
07-25-2007, 10:31 AM
Bulletin related to that code (another one on top of the PCM reprogram):
Due to various factors, the fuel injectors may become restricted. Extensive testing has demonstrated that fuel related issues are the cause of clogged injectors. At this point, no specific fuel, fuel constituent, or engine condition has been identified as causing the restriction. The restriction causes the engine to operate at a lean air fuel ratio. This may either trigger the MIL to illuminate or the engine to develop various driveability symptoms.
Your next step is to clean the fuel injectors and re-check your gas mileage/check engine light symptoms.
If you have this code, the PCM is trying to lean out the mixture; usually indicating that the O2 sensors are working properly/doing ther job :yes: Especially if the truck is still running rich...you have a fuel concern that is affectin all of your cylinders.
The light will only flash if a rich misfire condition exists that can damage the cat. If the vehicle is running rich, but there is no misfire detected, the light will not flash. Could also be a blocked regulator that is causing high fuel pressure :undecided
Also make sure that you do not have a restricted air intake (air filter, plenum, tube)
BTW...check out this website for info on gasolines: Top Tier Gasolines (http://www.toptiergas.com/index.html)
johnt000
08-02-2007, 01:48 PM
I read on another board that the, "The code was PO172, system too rich, left bank" was directly related to the 02 sensor(s) being bad. It might be a cheaper fix...
PROSPHOTO
08-02-2007, 01:54 PM
I read on another board that the, "The code was PO172, system too rich, left bank" was directly related to the 02 sensor(s) being bad. It might be a cheaper fix...
The common misconception (even in most garages) that the code sensor is the problem, when in fact that response usually indicates that the person does not know how OBDII vehicles operate & therefore cannot properly diagnose the concern :duh:
The description itself is telling you that a rich condition exists :undecided