Not ready for smog test

billr

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Make
Chevy
Model
Express 1500
Year
1997
Miles
170K
Engine
5.7 VIN R
No codes set, runs fine. Last time I checked for pending codes was yesterday, none then. All the "readiness" checks have passed except the "HTR" monitor. This is on an Innova 3140, and I think that flashing "HTR" means the heater function of the O2 sensors has not passed yet.

The S1 sensors seem to be working fine, toggling above/below 450mV regularly, with FTs responding. FTs are all less than +/- 3%

The S1B2 sensor also seems like it may be OK. It stays low most of the time but rises above 450mV at times.

The S2B2 sensor seems stuck low, about 200mV, although i did see it go up to 700mV on one occasion. I quickly verified that anomaly using and old Snappy MT2500. The Innova and Snappy agree on all what I am observing.

So... I examined the two S2 sensor, visual and resistance. Both seem the same, so I swapped them. S2B2 remained as the "stuck" one, the problem didn't follow the sensor.

Next, I checked power from the S2 heater circuits. Both are the same, give 12V and maintain that with a 10 ohm load.

Lastly (for now!) I applied 200mV and 700mV signals to the harness. The scanner shows those correctly for both B1 and B2.

What next? Am I correct in thinking that "HTR" monitor relates to the O2 sensor heaters? Am I correct in thinking the S2 sensors should both be primarily low? At least I feel pretty confident the two S2 readings should be similar...
 

grcauto

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The best way to test the heater is to look at current. Similar to testing glow plugs on a diesel.
The code sets because the O2 sensor is not switching soon enough. I would replace it and test again.
In have the factory info if you need something.
 

billr

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With the 10 ohm load resistor attached, I am confident there is !A or more. That is not full heater current, but in the same ball-park. I wish I had a BOB to do this testing with. I tried a clamp-on DC ammeter, but got no reading on the "bad" one and didn't try the other yet because I would have to peel away the harness cover to get to individual wires.

If it was a simple problem with the heater, wouldn't the PCM set a code for low/high/out-of-range heater current? Or is this early OBD2 not likely to be able to set such codes?

How about the operation of the one I think is OK? Is "mostly low" with occasional highs what I should see for S2?

Are you familiar with Innova, does "HTR" mean monitoring of the O2 heaters, or am I off in the weeds?

Bonus question: do you suppose S1 heaters could be inop, but the sensors still give good readings once they warm up from exhaust heat? Yeah, I think I will check those heaters, too.
 

grcauto

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There's a big difference between 1 amp and 5 when it comes to heating elements. Look at it with your scan tool from a cold start and see how long it takes to have full swings.
 

billr

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That is my plan, to look how long it takes the S1s to start making "crossings" from a cold start. That will be done using the old Snappy; it has a far better display of the O2 stuff.

I realized later that you may have been advising me to pay attention to what current these sensors are drawing, along with ensuring the circuit could provide enough current, as I was focused on.

I have used an inline meter to read current to each of the S1s. They both seem about the same, 1.5A, but that doesn't correlate with the voltage. Both read 4.8 to 4.9 ohms resistance, and I am testing at 12.2V, so current should be about 2.5A I am doing this on the sensors alone, using a bench power supply. None of the truck wiring was involved. Point is, I don't know how the current could be low, or if it matters that much, but it seems like some kind of clue. Neither of those (S1) sensors seemed to be getting warm-to-touch with 12V applied, but I will cook them longer tomorrow; and will probably do similar testing of the S2s.

Do you have specs on what the sensor resistance and current should be?
 

grcauto

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That is my plan, to look how long it takes the S1s to start making "crossings" from a cold start. That will be done using the old Snappy; it has a far better display of the O2 stuff.

I realized later that you may have been advising me to pay attention to what current these sensors are drawing, along with ensuring the circuit could provide enough current, as I was focused on.

I have used an inline meter to read current to each of the S1s. They both seem about the same, 1.5A, but that doesn't correlate with the voltage. Both read 4.8 to 4.9 ohms resistance, and I am testing at 12.2V, so current should be about 2.5A I am doing this on the sensors alone, using a bench power supply. None of the truck wiring was involved. Point is, I don't know how the current could be low, or if it matters that much, but it seems like some kind of clue. Neither of those (S1) sensors seemed to be getting warm-to-touch with 12V applied, but I will cook them longer tomorrow; and will probably do similar testing of the S2s.

Do you have specs on what the sensor resistance and current should be?
Heating elements change resistance quickly as they heat up. It's far from linear.
 

grcauto

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Even when the heaters draw the correct current and everything else seems fine yet they still do not reach the threshold set. Many times it's just to tired to work correctly. Many times the only thing to correct it was a new OEM sensor.
 

billr

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I am not completely dismissing that "too tired" theory. Something is wrong, and I don't know what it is, so I want to keep an open mind. However, if voltage is dropped while current is flowing, then heat is produced someplace; the energy doesn't just disappear.

Moving on... no joy this morning

I bench-tested all four sensors. All are about 4.8 ohms. All heat up similarly with 13V applied. They don't heat up as fast as I would expect, it takes a couple of minutes before the tips get hot enough that you don't want to hold on to them. Yeah, I know the tips are acting as "heat shields" around the actual heated parts; but still, it seems to me exhaust heat would get things hot faster than these electric heaters.

Next, I connected all four sensors to the truck harness, but left them dangling free. After KOEO for a few minutes, all four sensors had heated up similarly. The only blessing I have found so far is that the engine does not have to be running to test this stuff.

I re-installed all sensors, connected a battery charger to keep system voltage up in the 13-14V range, then went KOEO for 5-10 minutes to get the sensor heaters all warmed up. Next, I went key-off for about 30 seconds, just in case the PCM was keeping track of how long the ignition had been off before cranking, then started the (cold) engine. The S1 sensors became active (crossings) simultaneously, in about 30 seconds or less.

After running the engine until CLT was 188F, the "HTR" monitor had not cleared. I did find on the Innova site that the "HTR" icon is, indeed, related to the O2 sensor heaters. Not specifically for my 3140, but I doubt they have different definitions for their various scanner products.

I also found a sub-menu on the scanner purporting to show recorded data for O2 sensors. It indicates the the S! sensor thresholds and rise/fall times are within limits; no surprise, since I can see that visually in live-data. The S2 sensors are "not supported", and there is no indication about heater time, of course.

I remain "bewitched, bothered, and bewildered". I have no clue what conditions are require to run that HTR monitor. Anybody have that data? I can't afford (both financially and emotionally) to just shot-gun it with new sensors, cats, battery, alternator, wiring harnesses, PCM, ignition switch, engine, etc until the problem goes away. Who knows, could this be a problem with my scanner, not the truck at all??? Should I get an ELM327 dongle for a "second opinion"? Will the Snap-On MT2500 shows those readiness monitors somehow?

I am still wondering what the S2s should read. It seems they both can be low or high, but change very, very slowly. Like, many minutes between changes.

PS: I reported wrong before on the fuel trims. One LT trim is up to 3.1% That is three-point-one, not thirty-one

Let's keep talking!
 

billr

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Front and rear have different connectors, I would have to make four adapter cables. I'll pass on that for now.
 

billr

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I'm grasping at straws here, but...

The only thing I see in live-data that seems odd is that MAF is 3grams/sec when the engine is not running. I would expect that to be near-zero. Also, one online discussion claims the "HTR" issue was cured by cleaning the MAF. They were puzzled why it worked, just reporting the apparent result. Should I try cleaning the MAF? I fear doing more harm than good. Engine runs fine, no codes set/history that I can find, and fuel trims are all low.
 

billr

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Cancel the panic. I did nothing more since last post except erase all the monitor settings and take it for another brief ride from cold-start. All the monitors passed and I quickly took it down for a successful smog test.
 

JackC

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Another case of, " I have no idea of what I did to fix it or what was really wrong, but--"

That will drive you crazy for awhile. Anyway , glad you got it fixed.
 
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