How do you replace the rear 02 sensor on a 2004 trailblazer? I can't find any diagrams on the web. Maybe I haven't looked hard enough yet.

On the I6 engine, the precat sensor is located on the passenger side of the engine right below the exhaust manifold. Easty to get to, unplug the sensor and use an 02 sensor socket to remove. Post cat sensor is underneath after the cat. The connector to unplug is located in a difficult position, near the top of the transmission. I find that moving yourself toward the rear of the car and reach in the easiest. As opposed to reaching in right above the 02 sensor.

www.GMTNation.com for lots of Trailblazer into and help

The code is indicating that the reading the computer is picking from the rear O2 sensor is off. This can be caused by many things in the engine, rich fuel mixture, lean fuel mixture. A high-end scan tool needs to be hooked up and a diagnosis done.

Attached is a drawing of 6 cyl 4.2L engine, crankshaft position sensor is item #709 on driver's side of engine toward rear.

you can go to auto zone and they will print u a diagram on how to do it an u can maybe go to there web site an look at it at home

P0172 02 sensor is detecting that the system is running too rich. An 02 sensor that is starting to go out will pull this code, but the following will cause the 02 sensor to throw this code as well:

A few things to look at. 1st thing I'd do is clean your throttle body with the battery disconnected. Use sensor safe throttle body cleaner, about $4.

From there, I'd look at your upstream 02 sensor, located on the exhaust manifold on passenger side of engine, very easy. Try and stay away from Bosch if possible.

New fuel filter, clean or replace your MAF sensor located on top of engine next to firewall. Also, probably time to change your spark plugs. I recommend A/C Delco 41-103, will idle better than other brands.

To replace spark plugs, the plastic intake that goes over top the engine needs to be removed, and all electrical connectors for sensors that are attached to it. After that just remove the ignition coils and there are the spark plugs.

All those codes and such an easy fix. ALL of these codes point to your upstream 02 sensor. It's the one on the passenger side of the engine coming off the exhaust, very very easy to get to. Soak that puppy with PB Blaster then remove the electrical connector and remove and replace sensor. TRY to stay away from Bosch 02 sensors, have been known and reported problems with these. In my case, I didn't have a choice but to buy the Bosch, but I haven't had any problems with mine.

Hope this helps, look to get a tad more miles per gallon too.

check if the speed sensor melted again. Then you have a problem of figuring out where the short is.
You'll need a mechanic to help with that cuz the variables are a lot.
but double ck the wiring going into the speed sensor for fraying/broken wires., look in the connector to see if there is any gunk.
hope this helps a little.

Look for a vaccum leaks, check 02 sensors, check Map Sensor, make sure the pcv hose to the resonator is hooked back up. These are the simple things to check that would cause this code (running rich).