Flatrater Diagnostics

Yes! Finally another customer pay job… This one has possibilities. 96 S10 with 79k miles on it. The only warrantable items would be the fuel injectors and since its running ok, those are out. Wait a minute… what is the fine, hard to read print on the bottom of the RO?

No!! This guy has an extended warranty policy. Worse yet, its not a factory warranty but one of those aftermarket deals. Its an old story, the customer is sure to believe that they cover everything and that they will not have to pay anything to get it checked out.

First step… go find the service writer and make SURE that everyone understands that regardless of who is paying for what, someone will pay me diagnostic time <g>. You know, I love my job but I sure HATE having to constantly fight to get paid for diagnostic work. Ok, that’s done… now lets look at this beast.

First stop, freeze frame and failure records.

Wow, this one looks like at the very least, it will be interesting. But interesting doesn’t pay the bills, so lets get cracking. There are 5 failure record slots, so we know we have all the DTCs that are stored in view now. We also know the order in which the DTCs set. Start from the bottom up. The P0141 set FIRST. Now as you have noticed, the second DTC to set (P0122), set the freeze frame and illuminated the MIL. Or did it? Could the P0141 have illuminated the MIL and set a freeze frame? Can a freeze frame EVER be

overwritten? Yes, they can… But only by a fuel trim or misfire DTC if the current freeze frame doesn’t already have a misfire or fuel trim DTC stored. Now keep all that in mind and we will look at the DTC info to see what these codes mean.

The first DTC to set was Bank 1 Sensor 2 heated oxygen sensor heater performance. This is the sensor just before the Cat is not used for fuel control (there is more to this to be covered later). B1S1 is mounted on the left or drivers side of the vehicle.

(for reference)

Moving UP the list, we have the next DTC to set. As you will remember, it also set a Freeze Frame. Looking at the code description, do you really need to see enable criteria? Hardly… Who the heck cares WHEN the PCM runs the diagnostic? We already know what it saw or thinks it saw. It saw abnormally low voltage. What is the normal lowest low? Oh… 500mv or so, right? So how about this DTC sets if the TPS voltage dips to around 200 mV for some specific time? Sounds close enough for me! So heck, scope it out if you want, you now know pretty much what to look for.

The next DTC to set was a P1406. EGR position sensor performance. This truck is equipped with a Linear EGR valve. The pintle position is monitored by a sensor that is part of the valve.

Again, do we need enable criteria? I don’t think so. We know that the EGR sensor is telling the PCM that the pintle position is outside of the expected range. Either this is true, or it is not. Keep that in mind…

Next up, a P0300… Something is strange here. I looked up history misfires and all were zero…

This was the last DTC info screen stored.

NO… I didn’t make a copy error! What do you see wrong or odd about this picture?

Ok, now that we know what codes are stored and what they mean, did the P0141 first set a freeze frame and then get overwritten by the P0122?

What should our next step be? I hope your not going to say “clear the codes and test drive the vehicle under the conditions found in the freeze frame and failure records”. Why waste your time and what would you hope to accomplish by doing that anyway? I could go on and on with that one… someday I would like to cover where that all leads to, but for now, lets move on… <g>

My next step was to turn the radio dial to my favorite station (www.aironeradio.com) Next, firmly grasp the Tech 2 in my hot little hands and start pressing buttons…

After letting the truck cool down for 10 minutes, here is a look at the O2 sensors with the Key-On-Engine-Off.

Does that tell you anything? Or would you rather hold the truck overnight to try and get the PCM to run the diagnostic?

A few key punches down the road and we are at the EGR output test. Heck, we can already see something noteworthy…


Being familiar with this truck, I know it is unfair to say I already pretty much know what is wrong… But I do <g>. Even at Chilton time, this is a 3 hour slam dunk. Oh… and lets not forget some sweet maintenance I up sold <g> As usual, follow-up will be on Wednesday. I will try to incorporate any questions into it.

The first DTC to set was the P0141 (due to the order of listing) so lets look at it. GM currently uses the TTA (time to activity) method to monitor the O2 Sensors heater performance. This diagnostic will only run on a cold start so it can be difficult to reproduce unless your customer is willing to leave the vehicle overnight. Once the engine is started and enable criteria is met, the PCM will time how long it takes for the oxygen sensor to become active. The time it takes to become active is compared to calibrated fail parameters in the PCM. “Activity” would be a sensor output above 600 mV or below 300 mV.

As you will remember from this snapshot plot, B2S1 HO2S came to life very quickly while our problem sensor (B1S2) did not. The fact that it did drop to 390 mV tells us that it is being powered up. Be careful here, although it is most likely we have a bad sensor, we might have added resistance to the heater circuits in the way of corroded wires etc… This would lower the amperage running through the sensor and lengthen the warm up time. Using a test harness I made, I was able to verify that the sensor had a good heater ground and 12v feed. So at this point, it was obvious the sensor was defective. Some technicians

will test the oxygen sensor heater circuits with an ohm meter. This is a very inaccurate test for reasons we won’t get into here (it is a long thread in itself). Case in point: I laid the old sensor and a new one on my bench and ohm’d both of them out. The new sensors heater circuit resistance was 4.3 ohms. The defective sensors heater circuit measured 4.5 ohms. Specs for the heater circuits (according to the manual) are 3 to 10 ohms. The actual resistance of the heater circuit will vary with temperature as well as other factors.

Lets back track for just a moment… Here is the Heated O2 Sensor schematic:

Moving right along we have the P0122 that set…

P0122 means that the TPS sensor output voltage dropped too low for a specific amount of time. You can count on this low voltage being lower than normal closed throttle voltage. We would not be looking for a bad ground as that would increase sensor output. It is possible that our 5 volt reference voltage took a dive so I carefully inspected the wiring to the sensor. To save weight, the wiring is smaller and is more prone to breaking inside the insulation. The telltale signs of internal breakage would be a “bubble” in the wiring insulation. This one came up clean. We could also have had a loss of 5 volt reference from the PCM. If so, we would have expected DTCs from other sensors using the same 5 volt reference. As it turns out, there was also an EGR DTC. As we will see though, the EGR DTC is not due to a loss of 5v reference so the odds are very high that our P0122 was set by a defective TPS. As you might be able to see from the above picture, these newer TPSs are not sealed as well as the older style sensors and it is not at all unusual for them to fail with intermittent opens (low output voltage). There was no need to sweep the sensor with a DSO; instead I used my GMM in peak detect mode looking for any dropouts (below closed throttle rest voltage) while moving the throttle. I really did not expect to find anything as these can be very difficult to duplicate. As I see it, its now time for a judgment call; I vote for a new TPS… <g>

The next DTC we will consider will be the P1406; EGR Position Sensor performance test. This DTC indicates that the pintle position is out of the normal range or is 10% greater or less than the commanded position.

Here again we have a choice. Do we clear the DTC and attempt to get the PCM to run a monitor? Or do we run the valve thru it paces and see what comes up?

I’m sure you can guess what my choice will be… <g>

First stop… lets check the EGR sensor voltage with the valve in its closed position. There’s a problem… the normal closed pintle sensor voltage is around 700 mV.

From here I commanded the valve to 10% with the Tech 2. The duty cycle jumped to 60% for a moment and then steadied at 51%. Another indication that there is a problem with the valve. A clean valve will take about a 47% duty cycle to open and then a duty cycle of 43% or less to hold steady at a 10% pintle command.

Lastly, I commanded the valve to 100%. As you can see, the actual position was only 90%. The valve is operating right on the border of what is considered good or bad. Is it no wonder then that this DTC (P1406) passed on its last test? Isn’t it safe to say that running a monitor would have proved nothing that we didn’t already know?

Possibilities… defective wiring. Sure that is possible, but I know we have a problem with the valve.

In the future I will cover the Linear EGR valve in more depth including the DTCs associated with it. For now however, I will show you the scan data while operating the NEW EGR valve that this truck received.

The next DTC to set was a P0300; random misfire. Why it did not set any history misfire counts, I’m not sure. I decided to ignore this DTC as it may have set due to failures in the above circuits. Now, what about that P1122? Now it certainly set due to the faulty TPS, but what is so strange is that it didn’t fill the last failure record slot as it should have. Considering how sophisticated the PCM programming is, it is no surprise that we don’t find more bugs than we do.

So in summary, I replaced the B1S2 HO2S, the TPS, and the EGR valve. All were common failures. I have had good success cleaning the EGR valves instead of replacing them on other vehicles. Once they are off of the vehicle, I simply invert them and fill them with GMs top engine cleaner and let them soak for a bit. Next exercise the pintle with a small screwdriver until most all of the carbon is cleaned out.