I OWN 03 EXT TRAILBLAZER AND THE SERVICE ENGINE IS ON. I HAD IT RESTED ON A SIMPLE HAND SCANNER AND IT SAID THE O2 SENSOR OR CAT IS BAD. I DO NOT SMELL ANY ROTTEN EGG SMELLS SO COULD THE PROBLEM BE THE SENSOR. IS THIS SENSOR AN EXTERIOR OR AN INTERNAL ONE?
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Re: SERVICE ENGINE LIGHT
The sensor will be external and cats don't always smell like rotten eggs when they go. Also check as you will probably have a post converter sensor as well that sends a signal back to the main computer.
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Wich o2 sensor did you replace? If it's the upstream one you may have a clogged Cat converter, this makes the engine run badly due to restricted exhaust flow. When you first turn the ignition to the on position make sure the "check engine " light comes on momentarily, if you notice it come on the the above is likely your problem. Scan the car for codes anyway (you still might have codes in memory!)
there should be 4 sensors in the system two are normal O2 sensors that are located after the cat converter and two are heated sensors HO2S sensors located before the cat converter. Use the special sensor removal/installer tool so that the wires are not damaged . If you only replace d 2 sensors then you may have missed the faulty unit. The check engine light will stay on until you reset it with a scanner
Neither have anything to do with each other. The cam sensor is used to fire the injectors. The O2 sensors is for fuel mileage and to monitor the catalytic converter. If the O2 sensor is bad it will throw O2 codes and maybe a cat code (420). If the cat is bad, then only P0420. If the cam is bad it will set a cam code, but the ECM will go in default mode and still run the engine. Bad gas mileage, yes, But will still run. Best way to check a cat is remove the O2 sensor closest to the engine or loosen the exhaust flange. If it runs better, the cat is clogged up. Have the codes checked..
If the engine is a V6 (2.7 or 3.5 lit), bank 1 is next to the firewall (right hand) when facing the back of the engine.
If the engine is a 4 cly (I4), there is only one bank and is designated Bank 1 (exhaust manifold).
The catalyst to which the code refers is the UPSTREAM, and there is only ONE upstream cat for any bank.
Clue: Two upstream O2 sensor and two downstream O2 sensors - one before the CAT and one after the CAT. the downstream O2 sensors are identical to their upstream sisters. The only reason for the DOWNstream sensor is to monitor the CAT efficiency (threshold).
The catalyst on either bank is essentially the exhaust manifold which houses the CAT... it is the only CAT to which the code P0421 applies.
This code means the downstrean (behind) O2 sensor determines a reduced level of effectiveness in the cat. converter-it may not be 'lighting off' (600+ degrees) to burn any hydrocarbons left over from the combustion process. I can be that the cat is bad, or there is an exhaust leak before the cat, or the sensor itself is bad. You can do a visual inspection to determine if there is a leak, and use a scanner to see if the O2 sensor is working properly. It should read a fairly steady voltage and not switch nearly as fast as the upstream O2 sensor. The downstream O2 sensors only purpose is to monitor the cat-you might try clearing the Check Engine Light first to see if it comes back. Then you should consider contacting your dealer since you may be covered under an extended warranty that they have to put on emission devices.
Our engines only have 2 O2 sensors. This one would be after the cat. I would replace both, but it's odd that this particular went out first. Usually it's the one upstream right next to the exhaust manifold. The O2 sensor that is after the cat is much more difficult to change due to the wire connector. It's located darn near on top of the transmission, PIA, but doable. The first sensor is the one that will give you better gas mileage, plus it's super easy to do, so replace both while you're at it. Try and stay away from Bosch if possible, both Trailblazers, and Envoys don't like these. However, I had to buy Bosch due to availability of local autoparts stores, and I'm one of the few who haven't had a problem with them.
these O2 sensors are cat convertor check sensors. id be checking cat convertors and hopefully you should try replacing if not all ready is the front bank 2, O2 sensor
if you have scanner can you see fuel trims in data display to confirm high fuel trim only on bank 2 either +-5 or more indicates problem and if bank 2 sensor 2 has fixed high 700mv reading cat no good(warm idling)