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P0491 & P0492. O2 problem?

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2004 986S.

I've been getting CEL P0491 & 492 ever since I replaced the AOS last July. I was hoping the problem was just some oil in the lines and it would go away, but it wouldn't. The P0032 code is new; this is the first time I've seen this code, but this was the first time I've been able to check my engine actually cold, the other times have been after driving to the auto parts store to borrow their scanner.

CEL codes:

P0491 Secondary Air Injection System (bank 1)

P0492 Secondary Air Injection System (bank 2)

P0032 HO2S Heater Control Circuit High (Bank 1 Sensor 1)

Quoth Bentley:

"The secondary air system is monitoring using the precatalyst oxygen sensors. Once the air pump is active and air is injected into the system, the signal at the oxygen sensor reflects a lean condition. If the oxygen sensor signal does not change, a fault is set identifying the fault bank(s). If after completing the next cold start a fault is again present, the MIL illuminates."

I put together a screen showing graphs of all the O2 sensors. The first snapshot is right after I turned the car on (not entirely cold). The 2nd is after it was idling about 5 minutes. I don't know what O2 data is supposed to look like, but I'm pretty sure it's not this. Which sensor is bad. All of them?


Also got these B codes. Not sure what they're about, but they're clearly unrelated.

B1104 Audio [-] output #8 (TBD) circuit open

B1200 Audio [-] output #18 (TBD) malfunction

Edited by grover
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P0032 H02S Heater Control Circuit High

Possible cause of fault

- Short circuit to B+ in triggering wire

P0491 Secondary Air Injection System, Bank 1 - Insufficient Flow

P0492 Secondary Air Injection System, Bank 2 - Insufficient Flow

Possible cause of fault

- Electrical fault in power supply or line between relay and secondary air injection pump

- Secondary air injection pump relay mechanically faulty

- Mechanical fault in electrical secondary air valve

- Pneumatic secondary air valve faulty/sluggish

- Electrical fault in secondary air injection pump

- Mechanical fault in secondary air injection pump or secondary air injection pump blocked

- Vacuum system leaking

- Air hose to secondary air injection pump slipped off or constricted

Looks like a bad O2 sensor (or wring) and the secondary air pump is not running.

Check the air pump fuse to start.

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Thanks Loren!

After a little more digging, I was surprised to learn that's what pre-cat O2 data is supposed to look like; it's when they stop moving that you know there's an issue. I can hear the secondary air injection pump kick on and off, so that much is working. I'm hoping the O2 readings will tell me if there's a problem between the pump and the exhaust, or if it's elsewhere. I'm going to wait until the engine is fully cold and see if I can log some good data; I should see the pre-cat sensor should go full lean while the pump is running, then return to /\/\/\/\/\ after that, if it's all running properly.

Anyone with a properly working 3.4L have any baseline data I can compare to?

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I logged the O2 data this morning - pretty clear from the 30-odd seconds of very rich exhaust when it should be very lean that the problem is somewhere in the secondary air injection. I don't have a durametric; is there a PID I can use with my OBDII scantool to remotely activate the vacuum switch for testing?


I found this awesomely detailed checklist for troubleshooting secondary air injection problems on Chevy S10 I plan on using as a troubleshooting guide, as the system components all appear to be identical, even if the actual parts are different. http://ww2.justanswe...23817_p0410.pdf

Suspecting a vacuum leak, I checked my fuel trim as well; was between -10% and +10%, and very close to 0 most of the time. Is that enough to eliminate a vacuum leak as a potential cause, or could the leak be enough to foul up secondary air injection, but not enough to mess up the engine? It does seem to idle a bit rough, but smooth out at higher rpms.

Edited by grover
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Found it! I apparently knocked the vacuum line off the vacuum reservoir when I changed the AOS. Boneheaded, but at least easy and cheap to fix. Runs great now! :clapping: Thanks Loren, thanks Mike!

Latest readings, presumed to be correct and how it's supposed to look for a 2004 Boxster S:


Q. I understand the .45V readings are a fake value used for cold (not yet functioning) O2 sensors ignored by the ECU. But the other numbers would be actual readings. shouldn't the downstream O2 sensors (1-2/magenta and 2-2/cyan) always read leaner then the pre-cat O2 sensors?

Edited by grover
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