CL-Fault: 5.7 Vortec

Hey all, first post. Usually the ole google and forum search is all it takes but not this time.

I have a 98 w/ 5.7 Vortec. When I look at the OBD scan tool I see on cold start, as engine warms up, it goes from OL to CL-Fault for both banks. Is this normal? Anyone with a scan tool want to plug in and take a look at their truck?

The reason I ask if its normal is that perhaps early OBDII stuff works this way, though I'm aware newer cars do not.


Some background: Recently I had a CEL for P0154 [bank 2 sensor 1 low voltage] so I decided to swap in a pair of new Denso upstream O2 sensors for the old girl. Code fixed, trucks runs well things seem fine.

BUT, this got my watching things in the OBD scanner [wanted to make sure voltage was bad in the one sensor and checked with tool to confirm]. I also noticed my coolant temps were low so a new t stat went in [old one was clearly failing open] and i plugged in scan tool a number of times while driving to keep tabs.

My understanding of CL-Fault is that one bank will have an issue [say bad O2] and then steal data from known good bank. but that only produces cl-fault for one bank and the other runs in CL as it should. I get CL-Fault on both banks so what gives?

All my fuel trims, sensors voltages, etc look spot on. no anomalies.

any ideas?

When your scan tool hooked up do you have a graph setting to see the flows of both cats?

No, not normal to go into CL-Fault!

I've never had to mess with this problem, but I've read about it before. I'd be checking all of your data from the sensors.

Either there's a short somewhere or a sensor is reporting way out of wack numbers.

@east302 maybe can post the service manual procedure for diagnosing this problem?

SUBURBAN5 said:

When your scan tool hooked up do you have a graph setting to see the flows of both cats?

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I am not sure I know how this is done. I see o2 sensor voltages both up and downstream, and then a real high ~98% value that I think shows cat efficiency?

stutaeng said:

No, not normal to go into CL-Fault!

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Simple enough answer thanks.

looking at data in the scan tool nothing looks out of place for the stuff I understand. MAP, and MAF numbers aren’t something I’m familiar with but o2, temps, fuel trims, seem to make sense. And no codes.

I bet this ones going to be fun to figure out..

Ok I was gonna say theres all kinds of scanners that show different readings for exhaust. Mine done numbers and I've seen some with a graph. All I'm getting at is make sure your cats are good. I believe before so gonna fluctuate fast and all over. After the cats should be very close and as you press the gas it should stay within range. Hopefully it make sense

Are confirming the temperature is okay from the scanner? The guage in the cluster uses the sender by the head #1 cylinder, and some models in not even accurate to begin with.

Sounds like the waveform of the upstream O2 sensors is correct per your description. They look like a sine wave from 0.0 to 1.0V?

What are your fuel trims? I think if the ECU sees fuel trims out of range it can throw this fault code?

How long idling before it goes into the CL-fault? At around 60-70F it should take probably less than 5 minutes, maybe less? Much longer in cold weather.

SUBURBAN5 said:

Ok I was gonna say theres all kinds of scanners that show different readings for exhaust. Mine done numbers and I've seen some with a graph. All I'm getting at is make sure your cats are good. I believe before so gonna fluctuate fast and all over. After the cats should be very close and as you press the gas it should stay within range. Hopefully it make sense

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Understood. I get a mix of both numbers and graphs depending on parameter. its an autel al619.

Will do more research on confirming if cats are OK. I was thinking since I had that post cat value of high 9X% it was OK. I think that basically is showing cat efficiency but not clear on it.

Truck does have 300k and no signs that these were ever replaced.

stutaeng said:

Are confirming the temperature is okay from the scanner? The guage in the cluster uses the sender by the head #1 cylinder, and some models in not even accurate to begin with.

never trust a dash gauge, data from scanner. I run a very consistent 190*

Sounds like the waveform of the upstream O2 sensors is correct per your description. They look like a sine wave from 0.0 to 1.0V?

yep, they switch back and forth quickly, especially now that i have 2 new sensors. they were a bit sluggish and one had died.

What are your fuel trims? I think if the ECU sees fuel trims out of range it can throw this fault code?


going from memory a bit here but always under 5%. 1-2% ST and 3-4% LT typically.

How long idling before it goes into the CL-fault? At around 60-70F it should take probably less than 5 minutes, maybe less? Much longer in cold weather.

you nailed it time wise. somewhere around coolant temp 120* it seems to change over.

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I added some comments above in bold.

I hate to sound like an idiot, but this is the first time I've ever heard of "CL-Fault".

Is that specific to the scan tool you're using? Or is it a legitimate factory-specified deal?

Closed loop fault is an OBDII term. The engine should be in closed loop, the fault part is the issue.

So how is that different from, for example, a knock-sensor issue, where the vehicle remains in closed-loop, but still has some detectable fault?