2002 Trailblazer Lt 12 MPG Code P0420

TByuri

Original poster
Member
Jun 17, 2012
57
Getting terrible gas mileage for at least half a year or more (didnt check till now).

Have had the code P0420 for months as well. Finally decided to try to fix this issue and purchased two new delco pre and post oxygen sensors.

Installed and still get 12 mpg and the code pops up. Also has the regular tune up done (filters,oil,plugs).

Thing is the truck does not drive badly and so have not put it at a priority to fix the light. However, the gas mileage i have researched i should be getting 15mpg? Am i right?

If so how do i proceed?

Ive read it can maybe be my coolant temp sensor, should i replace that less expensive part along with the thermostat? Temp reads slightly left, the next click to the left of 210 and sometimes goes straight up so i doubt its that.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
P0420 is low cat efficiency. May not be a sensor problem - might be an internally broken/cracked/dead cat. Get a scan tool or mechanic and look at the sensor data instead of guessing it's the sensors. If the post-cat sensor wiggles up and down at the same rate as the pre-cat sensor, then the cat's dead. The front sensor should wiggle, and the rear one should be steady. Low coolant temp may or may not throw a P0128 code, but deferring that repair can often be the thing that kills the cat. And that's false economy.
 

TByuri

Original poster
Member
Jun 17, 2012
57
Sorry if this is dumb but by wiggle do you mean grabbing it at the point where you screw/lock the sensor?

For example the front pre cat sensor was tightened so it should not wiggle right? so where do i wiggle it.

Also would a bad cat give me the 12mpg im getting. Im getting 205 miles from full tank to last empty line on gauge at 16.4 gallons fill up
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
By wiggle I mean the voltage output of the sensor goes up and down about once per second. Lots of Youtube videos showing how to look at the sensor data with a capable scan tool. Sorry - not a physical wiggle.
 

TByuri

Original poster
Member
Jun 17, 2012
57
I will most likely take it to a cat converter shop.

Will the cat drop me to 12 mpg on a truck thats supposed to do plus 15?

Also i have not cleaned the throttle body ever(got this truck about 2 years ago, would this help with mpg? Also my plugs are not delco branded.
 

jrSS

Member
Dec 4, 2011
3,950
Yes clogged cats will hurt your mpg's and no throttle body cleaning will not make your mpg's any better. Just make it idle run better.
 

BRomanJr

Member
Dec 9, 2011
371
TByuri said:
I will most likely take it to a cat converter shop.

Will the cat drop me to 12 mpg on a truck thats supposed to do plus 15?

Also i have not cleaned the throttle body ever(got this truck about 2 years ago, would this help with mpg? Also my plugs are not delco branded.

IMHO, Non Iridium plugs (Delco preferred) can cause low mileage and less than smooth running.

What Brand / Model plug is in there?

Cleaning the Throttle Body improves air delivery control at low throttle settings, it at least causes driveability if not economy issues.
 

TByuri

Original poster
Member
Jun 17, 2012
57
Yes this truck does not have original ac delco plugs and im due soon to change the plugs and will order them on amazon soon.

As far as the cat converter, i still have the p0420 now a few months later. The reason i have not went to check the converter at a shop is that the truck still continues to run pretty well, yet i still do get pretty bad mileage.

I am getting like 12-13mpg but i have a heavy leg, so is that good?

Also if i was to have a moms n pops shop check and change the cat, do those aftermarket cheaper cats still be able to fix my check engine light?

Thanks.
 

TByuri

Original poster
Member
Jun 17, 2012
57
Also when i go to pump gas and turn the gas cap to open it, there is absolutely no pressure sound at all.

I heard that most cars are supposed to make an air pressure sound when opening the gas cap, is the trailblazer supposed to do this?
 

dfc739

Member
Jul 29, 2012
170
Des Moines, IA
TByuri said:
Also when i go to pump gas and turn the gas cap to open it, there is absolutely no pressure sound at all.

I heard that most cars are supposed to make an air pressure sound when opening the gas cap, is the trailblazer supposed to do this?

I don't know about other cars, but ours are designed to pull vacuum on the fuel tank after a little bit of driving to burn the vapors. There is also a charcoal cannister that catches vapors as it sits. So it doesn't surprise me there isn't any pressure build up after you drive to the gas station. I don't typically have pressure either.

You're in Los Angeles? Sucks to be you. California has VERY strict emissions laws, or at least far more strict than other places in the country. You are even required to get a California-approved catalytic converter put on your vehicle. So no, I don't think you'll be able to put the cheap aftermarket converters on your vehicle.
 

nopaybob

Member
Apr 3, 2013
97
TByuri said:
I am getting like 12-13mpg but i have a heavy leg, so is that good?
I don't exactly drive like a granny, But I know its not a hotrod I get 17-18
 

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