P0420 and bad MPG, what to look at?

[cwilliamsws6]

I recently became the owner of a 2005 GMC Envoy XL. It runs and drives just fine however it does have a P0420 that comes and goes, and my fuel mileage even on the highway won't surpass 11.5 mpg.

Is it possible it has a clogged cat? How would I check?

 

[Trav]

4.2 or 5.3?

 

[WobblyElvis]

P0420 does not necessarily indicate the CAT is plugged. Poor acceleration and lack of power is a better indicator for a plugged CAT. Are you sure the mileage on the highway is only 11.5? Perhaps you have not re-set the lifetime fuel mileage indicator. You want to get on the highway and reset the fuel computer so it shows only mileage during the current trip.

 

[cwilliamsws6]

4.2

I did reset it for a drive from Lewiston to Moscow and back and it topped out at 11.5

I will say it does feel like I'm getting gas fairly often considering the little bit of in town driving I do.

I don't personally feel there's a lack of acceleration or power. It'll squeal the tires from a dead stop with little effort lol.

 

[Mechanician]

P0420 is for cat efficiency, the rear O2 sensor is seeing a bouncing voltage, and it should be steady.
Possibly a bad sensor - but not likely. Either the cat is not fully operational, or there is a leak in the exhaust system between the sensors.

 

[irad]

Vacuum test. Connect gauge and slowly accelerate. If the gauge begins to decline after 10 seconds, this could be a clogged cat. Clean the iat sensor on the air intake regardless, it is a 2005.

 

[irad]

Sorry I meant maf not iat.

 

[LazyDog]

Some at some point, you also need to do a Crank and Cam relearned. Not sure if it applies to your 05.

 

[zzyzzx]

How many miles are on the vehicle?

 

[Chris142]

Ambient temp sensor, coolant temp sensor, Oxygen sensor

 

[nascarnation]

I'd recommend buying a cheap code reader if you don't have one already and looking at your fuel trims.
That fuel usage (assuming you're not cruising at 110 mph or pulling a 7000lb trailer) is way above normal for this vehicle.

 

[D60]

I'd recommend buying a cheap code reader if you don't have one already and looking at your fuel trims.
That fuel usage (assuming you're not cruising at 110 mph or pulling a 7000lb trailer) is way above normal for this vehicle.

This is excellent advice. Look at fuel trims. Look at MAF flow rate and ideally check it before cleaning just so you know if cleaning seems to make any difference if it seems amiss. Look at coolant temp, intake air temp, and O2 switching (unless these are AFRs, not sure?).

I'd wonder about a vacuum leak if trims are high.

Basically, collect some data before deciding on a course of action.

WWW has a good vid of a clogged cat here

 

[Bladecutter]

You might also want to check to see if the fuel injectors are OEM GM parts.
Lots of people buy junk from eBay, and it ruins their fuel economy while they are trying to solve some other issue.

 

[panthermike]

Is this computer only, or hand calc? Like some of the suggestions above. As for easy stuff, tried a good fuel cleaner? I ended up putting a fouler on the Corolla to get rid of the light, don't think mpg was effected though.

 

[Hirev]

I would also be looking at your fuel trims. They are a good indicator of issues.

 

[Intelligent Molecule]

Exhaust system leak is a real possibility for that code, but 11.5 mpg? Those are 1974 type numbers.

 

[Cmobile]

If you want to check for exhaust leaks, one option is to use a shop vac in blower mode, (clean vav with new filter of course) with the hose on the discharge, or exhaust port. (Some vacs have it as an option to use the unit as a blower)

On a cold engine, engine off, use painters tape to seal the hose to exhaust tip ( you may have to plug the other tip of dual exhaust)
Turn on shop vav to blow air into exhaust,

Then get under vehicle with a spray bottle of soapy water and spray the exhaust, looking for bubbles. Focus on exhaust manifold, near 02 sensor, and upstream of cats.

My p0420 turned out to be bad downstream sensor. But that might not explain poor MPG as downstream usually is only monitoring cat efficiency and not controlling fuel trim.

 

[cwilliamsws6]

If you want to check for exhaust leaks, one option is to use a shop vac in blower mode, (clean vav with new filter of course) with the hose on the discharge, or exhaust port. (Some vacs have it as an option to use the unit as a blower)

On a cold engine, engine off, use painters tape to seal the hose to exhaust tip ( you may have to plug the other tip of dual exhaust)
Turn on shop vav to blow air into exhaust,

Then get under vehicle with a spray bottle of soapy water and spray the exhaust, looking for bubbles. Focus on exhaust manifold, near 02 sensor, and upstream of cats.

My p0420 turned out to be bad downstream sensor. But that might not explain poor MPG as downstream usually is only monitoring cat efficiency and not controlling fuel trim.

One of my friends has a Tahoe which was having the same symptoms and he told me it turned out to be a dying upstream O2 sensor.

I do have a shop vac so I'll give that a try after work and report back with my findings.