Mechanics

WILL A CEL CODE FOR A SPEED SENSOR COME ON IF I ALREADY REPLACED MY SPEED SENSOR, DOES IT MEAN I NEED TO REPLACE MY SPEEDOMETER?

2004 Nissan Xterra • 89,500 miles

Nissan dealer told me after replacing the speed sensor on the transmission that now I needed to replace my speedometer which is working perfectly fine and never skips/ jumps etc- they want to charge me $650 for this, they did not look at my speedometer, the mechanic just keeps saying that since the code for P0500 speed sensor came back on 2 weeks after replacing the speed sensor that it must be the speedometer. I feel like I'm getting swindled, something sounds off. I don't know what to do. The original problem was that the car was chugging and the check engine light was coming on for a speed sensor code po500. I have taken it to 3 different mechanics and no one knows what is going on. When the car chugs it feels like my car is running out of gas, but there's a full tank. I replaced fuel filter, battery, mass airflow sensor, speed sensor and new rotor/caps, oil change and still nothing is working. I am out tons of money. Every mechanic said I needed to reflash my ECU but when I took it to Nissan they said there was no computer update for my year to reflash it with and that the ECU looked fine. One mechanic said it looked like the current to the speed sensor was going in but nothing was coming out when he did a diagnostic. I need some type of help.
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Xterrluver
March 21, 2012.




Recheck the VSS and wiring back to the computer for short/open circuit

Rasmataz
Mar 21, 2012.
So your having chugging issue? Was that cured by new sensor until repeat code?

Tiny
Fixitmr
Mar 21, 2012.
Thanks for replying, I am at my wits end- the chugging stopped for 2 weeks after the speed sensor on the transmission was replaced, then at about 40 miles into the 2 weeks (you know after the diagnostic how the CEL resets after about 50 miles) it starts to chugg all over again and do it intermittently, like one day its fine the next it's chugging, I throw it into neutral then it drives fine for a mile or so then starts again. This has been going on for 2 months- I've had it diagnosed 3 times now for the speed sensor code and chugging.
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Tiny
Xterrluver
Mar 21, 2012.
This is what happens when mechanics speak a different language than car owners, (just like doctors, accountants, and airline pilots), and things get mixed up in translation.

First of all, while you may indeed have a fault code for the speed sensor, they have nothing to do with engine performance. That is going to be a separate problem. Sensors are responsible for setting a code only about 50 percent of the time. Other causes include a corroded splice in a wire, a broken wire, a stretched terminal in a connector, and things like that.

Second, one mechanic's explanation of "current going in but not out" is messed up. Either he doesn't understand electrical theory and shouldn't be working on that problem, or he isn't very good at explaining what he found. Very few mechanics have good customer communication skills. Current is always the same in a series circuit. A series circuit can be visualized as a river flowing through many cities. Whatever water flow, (current), flows into the city is the same as what's flowing out. A garden hose is maybe a better example. Whatever flows in one end is the same as what's flowing out the other end. If the mechanic "diagnosed" it and came up with that explanation, why didn't he follow through and fix the cause? Diagnosis of electrical problems is where 90 percent of the time is spent. Solving the problem is just the other ten percent.

Speed sensors produce a voltage signal. That's electrical pressure, not flow. That's arguing a point irrelevant to the issue but it reveals my suspicions that this mechanic might not be your best choice for this problem.

Reflashing the computer means reinstalling or updating the software. There's nothing you're going to "look at" to tell when that's necessary. When no updates are listed, it just means they didn't see the need to make changes to correct a common emissions or running problem affecting many cars of that model. The software could have become corrupted from some other cause, just like your Windows operating system can become corrupted and need to be reinstalled. While I'm skeptical that reflashing a computer will solve an intermittent problem, I have to admit it does happen.

Check Engine lights don't reset after 50 miles. They might turn off on their own but it has nothing to do with mileage. Some models erase fault codes that don't reoccur after 50 engine starts. As for the light, there are varying degrees of fault code severity that determine how the light acts. If the code is related to something that won't affect emissions, it likely won't even turn on at all. If it's a minor problem, the light will go off while you're driving. If it's a little more severe but the intermittent problem goes away, the light will stay on until you stop and restart the engine. If the problem is even more serious, even if it goes away, the light will turn on every time you start the engine. When the light is flashing, way too much raw fuel is entering the engine and will damage the catalytic converter if the engine isn't stopped right away.
Since no other codes are being set related to the running problem, that suggests it's not related to a failing sensor. I'm actually dealing with the same issue with my minivan right now, and based solely on observations when it stumbles or when it stalls and won't restart without cycling the ignition switch a few times, I'm suspecting a tired fuel pump. Fuel pressure is not monitored on my van, and I suspect on your vehicle either. Only the RESULTS of low pressure might show up if the problem occurs long enough. I'm going to prove my suspicions by driving around with a fuel pressure gauge clipped under my wiper arm so I can see what happens when the engine stalls, (as soon as I can find the darn thing).

The problem with this type of running issue with no related codes and being intermittent is you can't have a mechanic standing right by it when it occurs, but there are a few things you can look into. Most commonly, most of the scanners the mechanics use have a record feature that records about a five-second snap shot of sensor data while you're driving. Rather than borrow you their multi-thousand dollar scanner, there are some "flight recorder"-type units made for this purpose. Almost all new car dealers have something along those lines. They leave it in the car while you drive it, then when the problem occurs, you press the record button. Since the data travels through the unit's memory, the recording actually starts a few seconds before you pressed the button. Later, that can be played back frame-by-frame while watching how each sensor responds.

If you had to guess, the mass air flow sensor is a good suspect, but also look for any leak in the tube between it and the throttle body. If any air sneaks in that the sensor doesn't know about, the Engine Computer won't command enough fuel to go with it.

A cause of low fuel pressure can be a plugged pickup screen inside the gas tank. If your vehicle has a fuel pressure regulator on the fuel rail on the engine, a plugged screen will cause stalling when the largest volume of fuel is pumped, ... Which is during coasting. The engine will typically run fine under load or when cruising at a steady speed. I never stopped to think about it, but if your vehicle uses the newer "returnless" fuel supply system with the pressure regulator in the gas tank, I think the stumbling would occur on acceleration instead of coasting.

Those are just some other things to consider. Intermittent problems are always the most frustrating to find. You might also do a search for related service bulletins although they typically are produced after reviewing numerous warranty claims for relatively new vehicles seen at the dealerships. When common problems occur on older vehicles, the manufacturer typically doesn't write bulletins for them.

Hope to read soon that this is solved.

Caradiodoc
Mar 21, 2012.
Thank you for your informative reply. So the speed sensor won't cause my car to chugg or effect its driving? I'm soo confused. I'm sorry, you must excuse the way I explained things about my car- I must admit I have no car knowledge so I'm trying to educate myself by reading up on everything. My car has been at the dealership for 4 days now and they can't get it to replicate the chugging problem. They tell me they have been driving it around to check it and seem to think it's running fine, but the one thing they did say they found while looking over the car was that the distributor wiring needs replacing. They are now quoting a price of $800- I sure wish I knew more about cars. I don't know if that price sounds right or if they guarantee their work at dealerships to fix the chugging problem. My check engine light has not come on again yet. Good luck with your van! I'll keep you updated with what they find.

Tiny
Xterrluver
Mar 23, 2012.
Wiring harnesses are surprisingly expensive but 800 bucks is way too high. Normally the only way to get a new harness is when there's a broken or corroded wire AND the car is under warranty. Most manufacturers and aftermarket extended warranty companies will not pay for a repair to a wire. They insist the entire harness be replaced which in many cases takes a lot more time. Their reasoning is most mechanics can't be trusted to repair wiring properly to make it "like new". They're mechanics, not electricians.

When the car is out of warranty, that's a different story. We might install a used harness from a salvage yard but typically we repair the wire. The first thing my students learn is they have to locate the exact spot on the wire where it has a break or short. That's because they need to see what happened so it doesn't happen to the next wire. If the first one rubbed through against a sharp metal bracket, how long before the next one does that? Same if the harness fell down onto hot exhaust parts.

The exception to this has to do with import cars that got submerged in deep water. Many of them use wires that tend to soak up water under their insulation, and that leads to horrendous corrosion problems weeks and months later.

It's never a good idea to second guess someone's diagnosis when we aren't right there to see how they arrived at it, but I'm skeptical a new wiring harness is going to be the solution. If they found a stretched out connector terminal not making good contact, that can be fixed. All dealers have replacement terminal kits. If a wire is frayed, which commonly happens between door hinges, those sections can be replaced. We do it all the time. I guess I'd like to know why they feel a wiring harness must be replaced.

To put things in perspective, when I worked at a very nice Chrysler dealership, I had a few occasions to repair frayed wires to the driver's door. Up to 22 wires took about two to three hours to splice in new sections. When an extended warranty company was paying the bill, the entire harness had to be replaced. It went from inside that door, through the hinges, a two-inch plug had to be fed through a one-inch hole to the fuse box, the dash and steering column had to be removed since the harness was clipped just under the windshield, another two-inch plug on the right side, then part of the harness went into the right door, and part of it went to the sliding door pillar and under the carpet to the sliding drawer under the seat. That huge mess of wires cost around $200.00 and took about six hours to replace THAT could equate to around $800.00 total today, but not for any harness under the hood.

I wonder if they are planning on replacing something else such as the distributor AND it's wiring harness. That would still seem awfully expensive but they might be including the time they already spent driving and diagnosing.

One more comment about a plugged pickup screen in the gas tank. That usually shows up after driving at least 15 miles, then, if it stalls and makes you sit on the side of the road in a puddle of tears, it will typically start and run fine in five minutes, and get you another few miles before it acts up again. That five minutes gives the screen a chance to "uncollapse" and let fuel through again.

Caradiodoc
Mar 23, 2012.

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