November Diagnostic Challenge (wrapped up)

[gquist]

Follow the flow chart for a code P0641

[Jeremy airjer W]

Follow the flow chart for a code P0641

That would be a good direction to head but.....

I get a little ADD at this point and I have to start digging. I grabbed the voltmeter and decided to check voltage at the MAF sensor since it was the easiest to get at. KOEO (key on engine off) I had 2.7 volts. Perfect, I know the problem is present and at this point the lightbulb goes off. Jeeps have a similar issue (without codes) when the crank sensors go bad. The crank sensor will drop the reference voltage to zero to a bunch of other sensors. Unplug the crank sensor and bingo, reference voltage returns.

We just eliminated the MAF sensor as a culprit, so what's the next step?

[Scott K]

Unplug the rest of the 5v ref sensors and see if you get the 5 ref back.

[Jeremy airjer W]

Unplug the rest of the 5v ref sensors and see if you get the 5 ref back.

Starting with the next easiest sensor, the oil pressure sending unit, which is just down and to the right of the MAP sensor. While monitoring the voltage at the MAP sensor I unplugged the oil pressure sending unit. Wouldn't you know, the 5 volt reference returned. Plug it back in and the voltage dropped back to 2.7 volts. I unplugged it again and decided to start the vehicle to see if it would run.

The vehicle started and stayed running but there was no throttle response? Uh oh, now what's going on?

What's the next step?

[Jeremy airjer W]

Time to wrap this one up.

Even though the 5v reference was back the computer still had the code in its system and was still in a "limp" mode. Clearing the code and then restarting the engine brought everything back to life.

Our diagnostic timeline just hit the 35-40 minute mark which isn't to bad.

The bad oil pressure sending unit was replaced and everything was back to normal.

Recap of the process

1. Confirm the complaint

2. Search for clues (Check codes, fuses, etc.)

3. Clear the codes (we found way to many codes that didn't make sense, and one code returned)

4. Research the problem (in this case we gathered more info for the code that returned)

5. Test the circuit (we found less than 5 volts)

6. Find source of the problem (unplugging sensors systematically until voltage returned)

7. Repair the problem (new oil pressure switch installed)

These steps can greatly differ depending on the problem but the approach largely remains the same. I hope that sharing this diagnostic process shed some insight on how we, automotive technicians, think through the problems we encounter and maybe help you figure out your next problem a little easier!

[Eric Wettschreck]

Now can we go to the bathroom?

[Jeremy airjer W]

Now can we go to the bathroom?

And get a snack!

[Macgyver55]

Strangely enough I just ran into this very problem a few weeks ago!

I really had to bite my tongue to not say what what I thought it was in this case, but I had the benefit of the owner being a friend. When he was telling me what happened he mentioned the oil pressure gauge had been acting up recently and fluctuating. On a hunch, I bypassed all the diagnostics since I figured we needed to put one in anyway and bingo... two birds with one stone..... and I looked like the hero!

(Some times even a blind squirrel finds a nut!)

[Jeremy airjer W]

Strangely enough I just ran into this very problem a few weeks ago!

I really had to bite my tongue to not say what what I thought it was in this case, but I had the benefit of the owner being a friend. When he was telling me what happened he mentioned the oil pressure gauge had been acting up recently and fluctuating. On a hunch, I bypassed all the diagnostics since I figured we needed to put one in anyway and bingo... two birds with one stone..... and I looked like the hero!

(Some times even a blind squirrel finds a nut!)

I swear sometimes it feels like I figure out more stuff by accident!