A damaged or cracked exhaust manifold in your 2002-2005 Chevy Trailblazer can cause an exhaust gas leak under the hood. The manifold on your 4.2-liter engine is fairly accessible and you can perform this repair job in your garage with the right tools.
Drive your Trailblazer on the highway for about 20 minutes to raise the engine and exhaust manifold temperatures. This will help when you remove the oxygen sensor from the exhaust manifold.
Return home and open the hood. Locate the oxygen sensor, which is above the exhaust manifold flange where it joins the front exhaust pipe that contains the catalytic converter. The sensor has a cylindrical body, about the size of a spark plug, with an electrical wire on the end. Unplug the sensor's electrical connector.
Unfasten and remove the oxygen sensor with a J 39194-C oxygen sensor wrench and ratchet on the 2002 and 2003 Trailblazer models; use the J 39194-B oxygen sensor wrench and a ratchet on the 2004 and 2005 Trailblazer models. Keep the sensor in a safe place so that dirt and grease do not come in contact with the tip and electrical connector.
Unfasten the front exhaust pipe, which contains the catalytic converter, from the exhaust manifold with a breaker bar and socket. If necessary, use penetrating oil on the mounting bolts.
Block the rear wheels with wood blocks and set the parking brake.
Raise the front of your Trailblazer with a floor jack and support with a jack stand on each side.
Put on your goggles, get under the SUV and disconnect the rear of the front exhaust pipe from the exhaust system with a breaker bar and socket. Detach the front exhaust pipe hook from the rubber hanger using a standard screwdriver as a pry tool.
Lower your Trailblazer off the jack stands with the floor jack.
Remove the heat shield from the top of the exhaust manifold with a ratchet and socket. With the help of an assistant, unfasten the exhaust manifold mounting bolts with a ratchet, short ratchet extension and socket, and remove the manifold and gasket from the engine.
Position the new exhaust manifold in place along with a new gasket with the help of an assistant. Coat the threads of the mounting bolts with the threadlock and hand-tighten the bolts.
Tighten the exhaust manifold mounting bolts to 15 foot-lbs. with a torque wrench and socket. Start with the bolt on the center of the manifold and then proceed with the one on the left and then the one on the right. Follow this alternating sequence until you torque all the bolts.
Torque the exhaust manifold bolts two more times following the same sequence to ensure all the bolts are properly installed. Reattach the heat shield.
Raise your Trailblazer with the floor jack and install the jack stands.
Put on your goggles and connect the rear of the front exhaust pipe to the exhaust system with the breaker bar and socket and insert the pipe hook into the rubber hanger.
Lower your Trailblazer and remove the wood blocks from behind the rear wheels.
Connect the front exhaust pipe to the exhaust manifold with the breaker bar and socket.
Coat the threads of the oxygen sensor with antiseize compound and install the sensor hand tight. Tighten the sensor on the 2002 and 2003 Trailblazer models with the J 39194-C oxygen sensor wrench and ratchet. Use the J 39194-B on the 2004 and 2005 Trailblazer models.
Plug in the oxygen sensor electrical connector and close the hood.
