Diagnostic trouble codes, not always what they appear!

[Jeremy airjer W]

Every once and a while even I get thrown for a loop. Yesterday was one of those days. This is a perfect example of, things are not always what they seem and that a scan tool is just that, a tool, not a magic machine that tells all!

It started with a ’99 Chevrolet Malibu with a 3.1 liter. The check engine light would come on intermittently and its symptoms, high idle and shifting funny, would do the same. It had happened twice and both times it was during the initial drive in the morning. A quick scan of the computer revealed these trouble codes.

P0122 – Throttle position signal low voltage

P1122 – Throttle position signal intermittent low voltage.

There was also a code for incorrect gear ratio but since the throttle position plays a role in how the transmission operates I wasn’t real concerned about that code at this point.

I started the vehicle and took a look at the data stream. Everything seamed to be fine. Idle was around 800rpm’s and Throttle position was at .63volts. I popped the hood for a visual inspection and found the harness going to the TPS (Throttle Position Sensor) was misrouted, stretching the harness just a little. While watching the scan tool and listening to the engine run I wiggled the harness and connector to see if there was any possibility that the wiring was damaged or the pins in the connector where loose. Everything ran fine.

Next I consulted the Mitchell for some diagnostic info. I knew that because the symptom was not present all the diagnostic flow chart was going to lead me to was that the problem was intermittent at this time but some times it can help by helping you find something that is wrong that you may not have thought to check. This was not the case.

I checked for the 5 volt reference and it was fine. I jumped the 5 volt reference to the signal wire and the PCM (computer) showed 5 volts. Unplugged the PCM showed 0 volts. I did a TPS sweep with a lab scope and it was fine. The vehicle had a chance to cool of while I attended to other vehicles. This paid of well because the symptom finally showed itself.

Know the engine is idling at 1,700 rpm, the check engine light is on, and code P0122 is present. A quick scan of the data and the TPS is at .63volts. At this point I’m stumped and start trying to figure out why the Idle is high.

After about a half an hour I finally figure it out. The IAC (Idle Air Control valve) is stuck open. There where no codes for the IAC because it was a mechanical failure (an electrical failure would have flagged a code), and the diagnostic info never mentioned anything about an IAC.

Why the TPS codes? The only thing I can figure out is that with the IAC “stuck” (the PCM has no real way of knowing where the IAC position is) the MAF signals went up and the MAP signals went up. The reasonable deduction by the PCM is that the driver is trying to accelerate. It looks at the TPS and sees that the signal is at the closed throttle voltage, but it knows based on the other signals that it should be much higher. It thinks that mechanically everything is working so the “logic” that’s programmed into it says the TPS signal is low and the check engine light comes on.

[Scott K]

Its jobs like them that keeps us on our toes, and that puts a check in our pockets, if they were all easy ones people would do them at home on there own!

[BobT]

I'd like to make a comment. Airjer, you appear to be an example of what a model technician really is. You use the "tools" that you have to diagnose and solve the problem.

I can't count how often I hear of people taking their automobile to a mechanic only to have them throw parts at the problem until they figure it out. Heck, I can do that for a lot less cash out of pocket.

When I take a vehicle in to a technician I expect expert diagnosis of the problem. That's why I hire them. The labor isn't worth the dollar per hour rate. In most cases, I will not enter into an agreement with them unless they are willing to back up their diagnosis. In other words, you throw parts at it, it'd better be the right solution or the labor is yours.

Keep up the good work!

Bob

[MattWtech]

i know on the newer TAC systems there is no IAC anymore, so it narrows down the reasons for bad idle or high idle, thats why i like the new drive by wire systems so much, it makes it more simplistic..but at the same time more troublesom, not the case for this malibu though, thats cool you got the symptom though, customers hate the old ,"couldnt repeat complaint"

[Jeremy airjer W]

Quote:

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I'd like to make a comment. Airjer, you appear to be an example of what a model technician really is. You use the "tools" that you have to diagnose and solve the problem.

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Believe me, I can botch a diag just as good as the next guy. I won't be sharing those as they don't happen often and I like to forget about them!

Its fun to get a good example of what the scan tool says is not whats actually wrong. Gathering all the info and making a reasonable deduction is all anyone can ask for, and with any luck its the right one!!!