Fix for Po171 on a 2002 Trailblazer

Ring blowby overcoming vacuum inside the engine, so worn out engine, may also be the problem with low oil pressure due to excessive clearances inside the engine. Run a cylinder leak down test or a dry then wet compression test before throwing too much money in it.

P0171 has nothing to do with the catalytic converter. It's telling us there's an overly lean air to fuel mixture on bank one. Bank one is the side of the engine where cylinder number one is. This code can be triggered by too much air or not enough fuel. Since the exhaust system has been worked on check for an exhaust leak between the engine and the upstream oxygen sensor. If you have an exhaust leak in that area of the exhaust system fresh air can be pulled into the exhaust, past the upstream oxygen sensor, and trigger a false P0171. Next, check for obvious vacuum leaks. Broken, misrouted, or disconnected vacuum hoses. Spray some carburetor cleaner around the base of the throttle body, where it bolts onto the intake manifold. Spray carburetor cleaner around the edges of the intake manifold where it bolts to the head, especially on the side of the engine where the number one cylinder is. If the engine speeds up and or smooths out, you've found your vacuum leak. Only spray the carburetor cleaner in short bursts, it is flammable and you don't want it going everywhere and creating a fire hazard! Lastly, the upstream oxygen sensor, on that side, could be bad and reporting a false lean condition to the computer. If all this checks out ok, you most likely have a fuel delivery problem. Check the fuel system pressure at the fuel rail and check the fuel injectors for proper operation. Hope that helps! Jim

Cheap things to check....Low oil pressure, check the pressure switch right above the oil filter. If you see oil on it or around it, replace it, cheap and easy fix. Pretty common problem on our platform. P0171, can be a ton of things, best thing to do is smoke test it. Tip, to rule a hose out. Under the plastic "Vortec" resonator is a curly hose near cylinder 1 that connects to valve cover. Put 2 hose clamps on this hose. Dealt with P0171 for months, 2 smoke tests, no smoke present. That little curly hose was sucking in just enough air to trigger the CEL. Even replaced the hose, same problem, hose clamps solved it.