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2003 chevy trailblazer_ltz
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cat

hi guys
i'm new to this site and it ROCKS! so much helpful info
i'm haveing the same lack of power issues
plan on doing a cat test and probably replacement asap
is there a way to test the o2 sensor or should i just replace it? is it hard to remove w/o a slotted socket?and what about the downstream sensor? test? replace also?
someone asked me if there are 2 cats on the 03' tbl, i don't think so, i haven't had a chance to look
 

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How many miles on your vehicle? Lots of us recommend changing the front sensor with every plug change, if nothing else than to prevent it seizing to the manifold. If you are under 100,000 miles, then you can see the performance of the front sensor easily with a good OBDII scan tool with a live readout function.

Front sensors may fail suddenly but they are far more likely to just deteriorate in their performance because the critical sensor part is right in the exhaust stream.

Rear sensors rarely fail and will likely last the life of the vehicle. They don't contribute anything to the drivability; they only measure the performance of the cat.

Replacing the front sensor can be easy if it is not seized to the manifold. You cut off the wire and use a deep socket with a long breaker bar to remove it. You will only need the special slotted socket to replace the new one. (Don't use the slotted socket to remove the old one. It WILL slip.)

Don't bother replacing the downstream sensor. And, yes there is only one cat. (Plus a muffler, plus a resonator.)
 

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Don't bother replacing the downstream sensor. And, yes there is only one cat. (Plus a muffler, plus a resonator.)
I will disagree with this only in the event that you are changing the CAT. Then you might as well replace the downstream sensor. Otherwise, leave it alone. Of course if you're using a universal cat, then maybe you don't want to replace it, but for all the work it takes to put in a universal cat, IMO, your better off doing the OEM pipe.
 

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Chickenhawk and Tequila if you use antiseize on the O2 threads it won't seize. If your cat clogs up you must make sure the stat is OK before replacing the cat. I had a code which turned out to be an undertemp condition caused by an open stat. The ECM(computer runs in open loop mode not monitoring any sensors) and doesn't go to closed loop until the engine comes to temp. The rich condition clogs the cat. As far as GM's warranty to replace the cat and stat under warranty it is 80,000miles or 5 years whichever comes first.
 

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I had a similar issue couple years ago pulling a trailer. Light came on engine power loss or something to that extent. 45-50mph max. Cat was cherry red, cut it out welded a pipe in. Check engine light stays on but I get better gas milage now. 2003 with 86k at the time.
 

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gaadaly in NJ we have motor vehicle inspection stations. They used too check everything on the car,truck or motorcycle but now they only plug in a computor in the OBD port. If codes are found or the light is on you fail inspection. They also check your gas cap to make sure it seals properly. You have 2 months too get the vehicle fixed and return for another inspection(window sticker tells if pass or fail). If not fixed and a cop pulls you over it is a hefty fine and they pull your registration. Brand new cars are inspected after 4 years and every 2 after that. The same for trucks and motorcyles. Actually bikes are inspected for lights,brakes,loud exhaust and proper helmet(we have manditory helmet laws in Jersey)
 

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thanx for the feedback everyone
CH.......it has 145k . i bought it used and don't know the serv history.....am i due for a cat?.....how can i test it? or are you saying that it might just be the #1 bank sensor (code P0171). my scaner doesn't have a live reader...any other way to test it? or do i just start by replacing the sensor( if i can even get it off....i"ll spray the [email protected]#t out of it in the A.M. and hope for the best when i get home from work)if it's seized any tips to getting it off? i don't have a slotted socket, do i need to get one to install? or can i use an open end wrench?
Doc....... i agree, if i have to replace cat i will certainly replace both sensors and be sure to use antiseize grease on them
Stu.........i also had the undertemp code p0128 but the temp gauge was reading 210 which i thought was alittle high cause when i got the car temp was running at 190-200 (or was this the under temp temp condition?) w/ no ch eng lite on. could it be that it's just the temp sensor gone bad? any way to test it? if i need to replace i will do the stat and coolant too.
sorry for so many questions...... been awhile since i had to work on my car (things have gotten so much more complicated) but getting tired of getting soaked by repair shops...........
confused?
where do i start???
::ugh:
 

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Replace the front sensor; it's time.

They come with anti-seize already applied to the threads if you buy AC Delco brand (and you should.) To get the old one off, snip the wire and use a deep socket and a long breaker bar. To put the new one on, you will need the special slotted socket but they are cheap and can be bought at any good auto parts store. (Don't use the slotted socket to remove the old sensor; it will slip.)

If your plugs have never been changed, it is past time to do that too. Use AC Delco 41-103 plugs.

You also need to look at the CAUSE of a possible plugged cat before you consider replacing it, or you will just plug the new one up too. The most likely cause is that the thermostat has failed and is allowing the engine to run too cool, thus causing it to run too rich. (The computer thinks the engine is cold and runs a very rich mixture, which causes cats to fail after a while.)

The other possible cause is your coolant temperature sensor is going bad, and the engine only THINKS it is too cold and runs too rich a mixture. The reality is that one can't really tell if it's the thermostat and it's running too cool or the sensor and it only thinks it's running too cool, but it doesn't matter; change them both. They are not expensive and are located side-by-side, so once you have the alternator off, you might as well do both.

So ... the order you want to do this is:
#1 - If your plugs have never been changed, change them for AC Delco 41-103 plugs. Easy fix and there are good threads devoted to this in here.
#2 - Replace the front oxygen sensor with a new AC Delco sensor.
#3 - Replace your thermostat and coolant temperature sensor.
#4 - Have your cat checked out at any good muffler shop. They will remove the front oxygen sensor and replace it with a backpressure gauge. They test the pressure at idle and again at 2500 RPM. Pressure should be near 0 at idle and no more than 3 or 4 PSI at 2500 RPM.

(Here is a tip to save time and money. If you are going to have your exhaust backpressure tested anyway, buy the new oxygen sensor and take it to the muffler shop. Then the muffler shop can remove the old one for you and replace it with the new one when done the test. It will save you having to buy the slotted socket.)
 

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thanx for the quick response!
i had the sp replaced in june by a mechanic. don't know what brand he used but he did charge me $10/ea + an hour labor @ $90.
because of the problems i'm having should i check the plugs for fouling and/or replace them / delco brand?
will also use your tip about the o2 sensor and the muffler shop (that tip was great)
is the delco brand from the dealer only? so far the parts stores here don't use ac delco brand.
are there problems w/ other brands?
i got the stat (stant brand) and sensor from pepboys. should i try to get the delco brand for these too? as well as any other parts i need?
oh and what about the coolant? can i use a brand that says good for all makes and models (color looks green) or go w/ the DEX cool (orange in color)?
 

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Tequila plugs last about 80000 to 100000 miles so replace them now. Go to Ehow and type in replace sparkplugs for Chevy Trailblazer. You won't find the plugs if you don't know where to look as they are buried. I purchased the stat at Autozone for $27 and it doesn't matter what brand as long as it is a known brand. It cost me $140 dollars to have it replaced by my mechanic as the stat is buried halfway down the engineblock on drivers side and took almost 2 hours. Maybe someone on this post has a better way. When you replace the cat replace both O2 sensors(very expensive unfortunately) as they last about 100000 miles and degrade slowly. Just installed a K&N FIP77 cold air kit and am getting about 1and a half mpg with a pronounced intake honk(my wife and I don't mind the noise). Next is Gibson super exhaust catback exhaust system(stainless not aluminm coated). Stainless is twice price of aluminum but lasts 10 years while aluminum lasts 3 to 4 years.
 

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Tequila don't replace what you have already replaced as they are not the problem. You must use Dexcool antifreeze in your GMC product. Again use a well known brand and it should say change every 5 years(don't use the green stuff that says change every 5 years use the orange/red antifreeze).
 

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stu thanx i'll return the green af and get the good stuff
anybody have any answers for the other questions i posed in my last threads?
you've all been so helpful.....i realllllly appreaciate all the input i'm getting.....i can't thank you all enough!.......thankyou, thank you, thank you all !!! :tiphat
 

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Two problem items that repeatedly pop up on these forums according to long-term members are non-AC Delco spark plugs and oxygen sensors. While a few people have successfully replaced them with other brands, there are MANY people who have reported problems. This is why experienced members of this forum advise people to ALWAYS use AC Delco 41-103 spark plugs and AC Delco oxygen sensors.

Why use any other brand when the AC Delco ones are not much more expensive and you KNOW they work instead of throwing the dice and hoping that your experience will be different from hundreds of others? :)

Not all auto parts stores carry the AC Delco plugs and sensor, but many do. Plus you can always get them at the dealership. I got mine from Rock Auto. They are worth the few extra dollars!

So, that being said, I would suggest you pull one of your plugs. (It is easy to do; just follow the instructions in this forum, and be SURE the coil is reseated properly when you reinstall it. And be SURE to only pull the plug on a COLD engine.) Take a look at it. If it is not AC Delco 41-103, I would highly suggest you change them, even if they are not due. The 41-103 iridium plug will easily last 100,000 miles before they need to be replaced again. If it is a 41-103 plug, then your mechanic knew what he was doing. If you are not sure about wear or if the engine is running rich, take a photo of the end of the plug and post it here; we can tell you.

As for the thermostat and the coolant temp sensor, just get a good name brand and you should be fine. Plus, that is solid advice you got on the coolant. Use only Dex-Cool (orange) coolant. If your mechanic or parts store recommends switching out the orange for the green, they don't know what they are talking about. Years ago, Dex-Cool had a bad reputation for building up sludge in the cooling system but that does not apply today unless you have a air leakage in your cooling system.

So, grab a wrench, pull a plug and let us know what it is.

(The interesting thing is that I am getting far more enjoyment and doing far more of my own work on my Trailblazer than any other previous vehicle I have owned - and I have owned a LOT. This is mostly because of the help of this forum, plus an investment in some diagnostic tools such as OBDII scan tool with live readout, ABS code reader, fuel pressure tester and exhaust backpressure tester, and some inexpensive specialized tools such as oxygen sensor socket, coolant temp sensor socket and a good torque wrench. This forum is also one of the reasons why we just came home last week with a SECOND Trailblazer in our family! My wife's Blazer was getting on in years and she liked driving my TB, so we decided to purchase a very clean 07 TB to match my 04.)
 

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CH, i too get somekind of enjoyment of accomplishment when i can do my own repairs (especially when i'm saving big buck$$)
today i will attempt to check the plugs. do the other brands cause the 171 code/loss of power problems?
so far i have:
cleaned the throttlebody (fairly dirty) = no change in situation. i did disconnect batt gnd which cleared the codes (128, 105 & 171). drove the car and at 50mi code 171 came back and the engine was now loboring hard but the temp was at one mark below 210. pulled over for a bit (15min or so) and it ran alittle better and got it home.
so i took out the o2 sen and drove around the neighborhood upto 40mph noticed a change in accel. seemed like it was shifting alittle high (3200rpm) temp was the same. let it cool (1hr), changed the upstream o2 sensor (bosh was the only one i could find in stock) and drove 10mi. seamed the same as when it was out exept when i took it out on the hwy it was still laboring, hard to get speed, press on accel to 3k picked up speed but would not shift unless i backed off ,then it didn't want to hold that speed especially up inclines , going down was easier
so now i'm thinking cat? or could something else be causing this problem? the recent non ac delco plugs? injectors? fuel pressure/pump? is the stat/EC still an issue if it's not the cat?(the temp seems normal to me or is it?)
i'm trying to narrow it down so i'm not throwing too many unnessesary parts at it. i'm on a tight budget! but i will do what's needed.
 

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hi guys
i'm new to this site and it ROCKS! so much helpful info
i'm haveing the same lack of power issues
plan on doing a cat test and probably replacement asap
is there a way to test the o2 sensor or should i just replace it? is it hard to remove w/o a slotted socket?and what about the downstream sensor? test? replace also?
someone asked me if there are 2 cats on the 03' tbl, i don't think so, i haven't had a chance to look
It is hard to check a O2 sensor. A plugged cat will cause a rich condition on the upstream O2 sensor. Try drilling a hole upstream of the cat and reset the computer. Worse case you can have it welded back up. See if it fixes your problem. Greg
 

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Tequila, don't drill any holes! It is not necessary!

It is almost certain your cat is blocked, based on your symptoms. Just follow the steps that we detailed for you and don't throw any unnecessary money away on parts or repairs you don't need.

1- Check your plugs. See what shape they are and what brand. If they are not 41-103, then change them.

2- Pick up an AC Delco oxygen sensor. If there are none in your area, visit a dealer or order online from places like Rock Auto.

3- Change your thermostat and your coolant temp sensor. You can't tell which one is defective anyway, so you might as well change them both. This is likely what caused the cat to be plugged. One tick below straight up and down indicates a defective thermostat or coolant temp sensor. The normal needle is straight up or a tick to the right. (While one tick down doesn't seem like much, I have found through watching the gauge and comparing it to a live readout from the PCM that the gauge is NOT linear; one little tick is quite a bit cooler than it would seem.) Just trust us; this is a common issue and if you are one tick to the left, it is too low.

4- You are likely going to need a new cat. If you want to confirm first, then don't self-diagnose and don't drill any holes. Get a cheap (or even free) exhaust backpressure test done at any good muffler shop.

5- Start reading up in these forums on cat replacement. (Personally, I would avoid trying to save a few dollars by using "universal" type cats, but it's your money.)

And if you live in a location that doesn't require regular emission testing or inspections, please replace your cat and don't just cut it out like some people do. Don't do more harm to our environment for other people.

In answer to your questions, changing plugs and oxygen sensor will not have much effect on drivability (unless they were in real bad shape.) It will simply mean long-term smoothness and lack of problems in these two areas for the next 100,000 miles. Changing the coolant sensor and thermostat will result in a bit of improvement in drivability, but they tend to cause long-term problems more than intermediate power loss.

BUT putting in a new cat will make it seem like a brand-new truck. The acceleration and power will astound you.

By the way, if you pull a plug, take a photo and post it here. And, just for fun, if you get an exhaust backpressure test done, post the results. I am willing to bet you are up about 2 PSI at idle and 8 or 10 PSI at 2500 RPM.

Keep us informed!
 

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CH....yeah i sure your right. i will order one online (Walker direct fit $158 free s/h) and then try to find a shop to install and do a B P test.
is there a dounut and clamps or bolts i order also?
i'm on my way to the autoparts store for a slotted socket to install the new temp sensor so i can complete the whole coolant project incdl a good flush.
and when i check the sp i will try to get you some pix.
are you saying that i should replace the new bosh o2 sensor i just bought and replace it (along w/ a new downstream sensor as opposed to reusing the old one) w/ ac delco's when i get the new cat installed?
MAY...tanx for the sp vid!
 

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MAY....will order AC DEL brand sensors (both) and replace them when i get the cat put on. ( looks like alot of trouble for a DYI) . are there any brands of direct fit replacement cats i should avoid or to look for? what is the OEM brand?
i wanted to replace stat/ECT/coolant together but i'm having a hard time finding the 18mm slotted socket for the ECT sensor install. i got an off set box wrench but the connector won't fit throuh it so it loos like i need that socket! DAMM!
also the ECT is not AC DEL it's a BWD from pepboys is that a problem? i only want to do this once and get it right the first time.
 
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