DTC P0411 - keywords AIR DTC flow injection low O2 pump SAI secondary

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Some L81 equipped vehicles may set DTC P0411 for low flow detected from the Secondary Air Injection (SAI) system (air pump). The ECM tests this DTC by calculating total exhaust air flow, based on wide-band oxygen sensor (O2) feedback along with other relevant engine data and comparing that to calculated air flow of cylinder combustion volume only. MAF value along with commanded fuel rate allows for combustion volume to be calculated. Combustion volume is then subtracted from the total exhaust volume to provide SAI system volume. P0411 sets if the calculated SAI volume for either or both banks is lower than expected values.

Published SI diagnostics provides tests to ensure the air pump and related components are functional. Certain exhaust system conditions can also cause DTC P0411 to set.

Thoroughly inspect all SAI components for damage and/or leaks. The SAI pump delivers air through a check valve to both banks via the SAI pipes. These pipes are joined by special heat resistant rubber hoses. Deteriorated hoses can contaminate the check valve. GMSPO hoses (P/N 90448816) offer improved durability and should be used with an updated check valve (P/N 09199513) which is identified by a white dot on the valve housing. Restricted or malfunctioning check valves can further be identified by either debris in the valve reeds or by verifying the valve does not freely pass air from the pump side to the exhaust side. If all components functionally test OK using published diagnostics then several potential faults may exist including:

1. Slow responding O2 sensors
2. Erroneous MAF sensor feedback
3. Restricted exhaust flow
4. Restricted SAI air passages in the exhaust manifold or head exhaust ports.

Note: Certain weather conditions at very high altitude locations only may cause intermittent false P0411. If a DTC is truly intermittent under these conditions it may be normal at this time. If a high altitude DTC is repeatable then proceed with the following steps that apply to cars at all elevations.

The ECM stores calculated SAI air flow under freeze frame when the DTC sets. This value is also stored when performing service bay SAI system tests. The value is stored when the DTC is run and is not updated until the DTC runs again on a subsequent key cycle. Expected normal air flow value is approximately 17-21 KG/H (kilograms per hour). Volumes less than 17 KG/H may cause P0411 to set.

Slightly contaminated/lazy O2 sensors may cause P0411 with no O2 related DTCs set. If only one bank shows low SAI flow then swap O2 sensors bank to bank and retest. If the problem follows the sensor then replacement is required. Check the failed sensor for signs of contamination and diagnose/repair as needed to prevent repeat O2 sensor failure. If both banks report low SAI flow then check for erroneous MAF readings (often indicated by unexpected short/long term fuel trim values). Wide-band O2 sensor voltage should increase when a scan tool is used to command the SAI system on. Normal running O2 voltage is about 1000mv. Wide-band O2 voltage should rise to about 3000mv under forced SAI operation. If both rise then an erroneous MAF sensor is possible.

A restricted exhaust system (catalytic converter, muffler or resonator) may alter both O2 values and calculated SAI volume. Verify normal backpressure (less than 1.5 PSI at 3000 RPM in drive with brake torque load). High backpressure in both converters is unlikely. Test for and correct contributing conditions prior to converter replacement

If all tests check OK then restrictions in the SAI passages of the exhaust manifold or head may exist. Published diagnostics test air flow to the pipes and checks for pipe restrictions and leaks. Step 19 (SI document 626981) further indicates exhaust manifold restrictions should be checked. Technicians may be reluctant to check this as manifold removal is difficult. SI step #1 for left manifold removal indicates engine removal is required, however this is not accurate and the manifold can be removed with the engine in the car. Access to forward bolt positions is difficult but the bolts can be removed. An engine coolant pipe makes access to the right manifold difficult but again engine removal is not required.

The SAI pipe injects air into two positions between the three cylinders on each bank. The air follows a passage in the manifold to a head passage located below each cylinder. Air flows into each exhaust port via these passages. Carbon restricted passages in either the manifold or head will reduce actual SAI flow and lead to DTC P0411. Remove the exhaust manifold(s) to inspect for and remove carbon restrictions. Use scan tool SAI volume data to determine if removal of one or both heads is needed.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.