Workshop Manuals Factory service & repair GMC

GMC common faults and how to fix them

Faults that are well known on GMC models we hold manuals for: what the owner notices, what causes it, and the pages in our manuals that cover the repair.

GMC P0300: rough idle and misfire on the 4.8, 5.3 and 6.0 V8 (2002–2004)

Symptoms

A rough idle and misfires, with the check engine light on and P0300 (random misfire) stored. It is worst on the 5.3 flex-fuel engine, but the same fault can occur on regular-fuel 4.8, 5.3 and 6.0 Vortec V8s in the Sierra, Yukon, Yukon XL and Envoy.

Cause

GM bulletin 05-06-04-029A traces it to the upper intake manifold gaskets. On the flex-fuel 5.3 (L59), the engine computer delivers more fuel for ethanol, and the fuel can pool on the gasket material between the manifold and the cylinder heads. The fix is a revised gasket material that copes with it.

Fix

Start with the normal misfire checks: plugs, coils and leads, and a smoke test for intake leaks. If the intake gaskets are leaking, the GMC P0300 bulletin replaces them with the teal green gasket material, part number 89017589, and warns not to refit the original orange gaskets (17113557). The same repair applies across the 2002–2004 Sierra and Yukon. Clear the codes and road test afterwards to confirm the misfire has gone.

Repair instructions on this site

Sources: GM service bulletin 05-06-04-029A.

GMC P0171 and P0174: lean codes on the 5.3 and 6.0 V8 (2005–2008)

Symptoms

The check engine light comes on with P0171 (fuel trim lean, bank 1) and often P0174 (bank 2), sometimes with P0300. Usually the engine runs normally, though some idle unevenly.

Cause

GM bulletins give two causes on the trucks we hold manuals for. On 5.3 V8s with Active Fuel Management in the 2005–2006 Envoy, and on 2007 Yukon and Yukon XL 5.3 and 6.0 V8s, injector connectors can be crossed, for example cylinder 6's plug on injector 8 (bulletin 06-06-04-001A). On 2007–2008 Sierra and Yukon trucks with vacuum brakes, a damaged or missing master cylinder seal at the brake booster lets in unmetered air.

Fix

Smoke-test for vacuum leaks and check the mass airflow sensor first. On the AFM V8s, the GMC P0171 bulletin says to check every injector connector is on the right injector, whatever the mileage. On 2007–2008 trucks with vacuum brakes, inspect the master cylinder seal at the booster, as the Sierra P0174 bulletin describes, whenever the master cylinder has been off. Clear the codes and recheck the fuel trims.

Repair instructions on this site

Sources: GM service bulletin 06-06-04-001A; GM bulletin on P0171/P0174 and the brake master cylinder seal.

GMC P0446 and P0455: EVAP vent valve on the Yukon and Yukon XL (2007–2010)

Symptoms

The check engine light comes on with EVAP codes P0442, P0446, P0449 or P0455, and the fuel tank may be hard to fill, with the pump clicking off early.

Cause

GM bulletin 09-06-04-028C covers the 2007–2010 GMC Yukon, Yukon XL and Yukon Denali (and their Chevrolet and Cadillac twins), excluding the hybrids. The EVAP canister vent solenoid draws fresh air through a vent attached to the fuel filler pipe, and water gets into the valve, so it sticks or fails.

Fix

Read the codes and check the vent valve for water. The GMC P0446 bulletin fits a revised canister vent solenoid valve, which comes with two tie-straps to fix its vent pipe to the fuel filler pipe as the bulletin shows. A loose fuel cap gives similar codes, so check that too. Clear the codes and confirm the EVAP tests pass on a later drive.

Repair instructions on this site

Sources: GM service bulletin 09-06-04-028C.

GMC knock sensor: P0332 and spark knock on the 4.8, 5.3 and 6.0 (1999–2002)

Symptoms

A mild to severe pinging (spark knock), usually worse under acceleration, and the check engine light on with P0332 (knock sensor 2 circuit).

Cause

GM bulletin 02-06-04-023A says water gets into the cavity under the intake manifold where the rear knock sensor sits, and corrodes the sensor. It is more common on trucks whose engine bay is often washed. It covers the 1999–2002 Sierra, Yukon and Yukon XL with the 4.8, 5.3 and 6.0 V8s.

Fix

The knock sensors sit under the intake manifold, so the manifold has to come off. The GMC knock sensor bulletin replaces the rear sensor and builds a dam of RTV sealant around it to keep water away. Check the wiring to both sensors while the manifold is off, and fit new intake gaskets when it goes back.

Repair instructions on this site

Sources: GM service bulletin 02-06-04-023A.

Repair instructions for every GMC model: GMC workshop manuals