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2004 VW Touareg V8 4.2L - Error Codes P0491 and P0492: Secondary Air Injection System Diagnosis

Model: 2004 VW Touareg V8 4.2 Fault Code: P0491 Posted: 2015-11-03 12:41

Hi everyone, I'm new to this forum and seeking expert advice on a recurring issue with my 2004 Volkswagen Touareg V8 4.2L engine. I've been troubleshooting the secondary air injection system for approximately three weeks and am still unable to resolve error codes P0491 and P0492. I understand these codes relate to the secondary air injection system performance monitoring. Here's what I've already checked: - The air pump, solenoid, and relay have all been tested and confirmed as functional. - At the solenoid connector pins 1 and 2, I measured 8.36 VDC power — this indicates proper voltage supply. - When I directly connected these two wires to ground, the solenoid activated and the air pump relay pulled in successfully, causing the air pump to operate. The key observation is that whenever I disconnect the solenoid or disable the relay, the ECU immediately detects a fault and throws error codes P049-1 and P0492. This confirms the ECU is actively monitoring the system. Despite these tests, I'm still unable to identify the root cause. The issue persists even after verifying all components are working correctly. Any insights or suggestions on potential wiring issues, sensor faults, or ECU-related problems would be greatly appreciated. Thank you in advance for your help!

Related fault codes
P0491P0492
Comments (4)
Anonymous 2015-11-03 20:28

I resolved this issue by identifying a damaged wire between the air pump relay and the ECU. After replacing the wiring harness, the error codes P0491 and P0492 cleared when I restarted the vehicle.

Anonymous 2015-11-07 17:04

Great solution — thanks for sharing! This is a common issue on older Touaregs with aging wiring in the air injection system.

Anonymous 2015-12-07 07:49

I recently replaced both fuel pumps and spark plugs, but after warm-up, my car shuts off completely and won't restart until 30 to 60 minutes later. Anyone else experiencing this? Could it be related to the ECU or a sensor issue?

Anonymous 2015-12-07 09:42

leonard872 — you might consider starting a new topic for your specific issue, as it could help more technicians see and respond to it. Did you replace the fuel pumps and spark plugs before or after the problem started? Was there any change in behavior following these replacements? For reference: https://www.obd-codes.com/ask