(I'm desperate and am also posting on Rennlist - tired of this chase....)
CAR: 2002 Porsche Targa 3.6; about 160,000 miles; always regularly serviced at Porsche dealership prior to my ownership, with records at the dealership.
I bought this car over a year ago, and within a couple weeks, the cel came on. The car was running fine – no problems except the presence of the light.
We reset the light with our OBD-II tool – the code (P0491) had something to do with the secondary pump malfunctioning (actually “insufficient flow to Bank 1” – see at the bottom of this post for the various interpretations I have gathered). The light stayed off for a few more days – then re-appeared. The car continued to run beautifully.
Well, for more than a year now, the light continues to come on every few days to weeks – highly variable, but seems to be related to the car sitting in traffic or with extensive idling. Yes, the car runs through enough cycles for the light to have turned itself off after repairs or resets.
We reset it every time. In fact, my wife carries the code reader in the car and resets the light as if it were part of a usual routine.
I read many threads on Porsche forums about cel problems – and the thousands of dollars that members have spent trying to track down the causes, and a usual outcome being A TEMPORARY FIX, but inevitably a recurrence of the trouble light.
I finally decided to have it checked out by a local Porsche Indy (Lutzo at LMN Motorsports – one of those ace, and completely honest, independent mechanics in the area).
Lutzo has all the Porsche factory diagnostics, plus decades of Porsche experience. He went down the Porsche factory diagnostic list and chased the various possible causes, including MAF, piping, O2 sensor, SAI pump checked for operation, even smoke tested for leaks. He called the factory asking about other possible solutions. He swapped out parts on a trial basis (no charge) to see if the problem was solved.
I had the plugs and air filter replaced just for general principles.
The problem was localized by the diagnostic machines to the first bank cylinders only. The MAF and the secondary pump both checked out OK; the hose that runs under the manifold TO THE SECOND BANK seemed to be leaking (could not get it to seal where it attached), so was replaced. The O2 sensor was over 10 years old, so was replaced.
So, listing the chase so far: new plugs; new air filter; new O2 sensor; new SAI tube to Bank 2; MAF checked with a replacement; SAI pump operation checked; smoke test for leaks. In addition, there have been HOURS spent by an expert Porsche mechanic trying to track this down.
Every time we thought the problem was fixed, a few days/weeks later, boom, on pops the cel again. I finally gave up – the recurrent 60 mile drive to the mechanic was ending like the horror stories in the Porsche forums. I think most owners just do like we are doing – reset the light and drive on.
HOWEVER, HAVE THERE BEEN ANY NEW POSSIBLE SOLUTIONS TO THE DREADED cel repetitious and intermittent recurrence?
Below are the various definitions of the P0491 code, as well as a partial list of Porsche forum posts (Pelican and Rennlist) with similar “cel” problems, but highly varied causes and outcomes.
http://forums.pelicanparts.com/porsche-964-993-technical-forum/294749-ignore-check-engine-light.html
http://forums.pelicanparts.com/porsche-996-997-forum/442891-check-engine-light-popped.html
http://forums.pelicanparts.com/porsche-996-997-forum/826633-obd-error-codes-advice-please.html
http://forums.pelicanparts.com/porsche-996-997-forum/540035-p1123-1125-errors.html
http://forums.pelicanparts.com/porsche-996-997-forum/443140-error-code-po410.html
http://forums.pelicanparts.com/porsche-996-997-forum/347382-obd-code-0420-a.html
http://forums.pelicanparts.com/porsche-996-997-forum/560136-2000-996-recurring-check-engine-light.html
CEL after deceleration from highway speed to idle - Rennlist Discussion Forums
P0562 Low Voltage Fault - Rennlist Discussion Forums
CEL Error: P0491 - Secondary Air Injection System Bank 1 - Rennlist Discussion Forums
Day two of ownership and I already need help - Rennlist Discussion Forums
996 secondary air injection valve replacement DIY
P0491 and PO492 - **SOLVED!** - Rennlist Discussion Forums
02' 911 996 Smog Pump Issues - Rennlist Discussion Forums
CODE DEFINITIONS GATHERED FROM SEVERAL SOURCES:
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
P0491 Secondary Air Injection System, Bank 1 - Below Limit
Possible cause of fault
- Fuse F3 faulty
- Open circuit to secondary air injection pump
- Short circuit to ground in the lead to secondary air injection pump
- Secondary air injection pump relay mechanically faulty
- Secondary air valve sluggish
- Secondary air injection pump (air cleaner) blocked
- Air hose to secondary air injection pump slipped off or crushed
- Secondary air injection pump faulty/stuck
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