High Elevation Error Codes (P0131, P0151) at 7K+ feet - 5.7L 350 Vortec.

[privateer]

1996 Tahoe, 5.7L 350 Vortec.

I'm getting O2 sensor P0131, and P0151 error codes when driving up mtn. passes. The error appears soon as I get close to 7000 ft. It happens from anywhere between 2500 rpm to 4000 rpm. On a consistent level. Soon as I clear it, it reappears.

I've replaced the O2 sensors a few years back, is it possible these have gone bad already? Or is there another issue causing the sensor codes?

I'm not getting any errors at lower elevations, just when driving 7000+ feet.

Any feedback is appreciated.

Thanks

 

[great white]

Well, that's an odd one. I'm not completely sure what's happening, but I'll spitball it a bit for discussion and see what comes out of it.

Going from your description it would seem to be density altitude related.

The low voltage fault code (151) indicates the engine is going lean (0 v condition), or the o2 sensor has pooched altogether.

Higher altitudes thin the air, which means the engine should go rich unless the PCM does it's magic and corrects properly. Rich condition is closer to 1v from the o2 sensor, which is opposite of the 151 code.

Unless; the ECM is pulling so much fuel in response to the MAF signal at altitude that the engine is going waaaay lean. But that doesn't make a lot of sense as most Oem fuel and spark tables I've dealt with have modifiers for these parameters and they do their job rather unobtrusively (ie: works good, lasts long time).

You would think if the 02's were going bad, the fault codes would not be related to altitude.

Your 131 code also jives with a lean condition or o2 sensors going bad.

Since you've got bank one and bank two both up and downstream, I would tend towards a lean condition vice o2 sensors going bad.

My suggestion would be to pull the freeze frame data from the PCM and see what's up.

Failing that, clear your codes and see if you can get them to reappear without going to altitude with some heavy throttle load conditions and see if the o2 sensors respond properly to the transient throttle conditions.


You wouldn't happen to have a "tuned" PCM in this truck would you?

If so, details about it might help......

 

[98slv]

if you got a torch your can test your O2s. their voltage will change from room temp to higher heat. If no torch, drive your truck then shut it off and unplug it then test the voltage. all the vortecs have a four wire O2 which has a heater in them to heat them up and help get your truck to closed loop operation.

Have you cleaned your maf with MAF cleaner? With you having altitude issues, wouldn't be a bad idea to clean your altitude/pressure sensor too. I believe its the little dude after the maf.

If you can do the freeze frame like Great white mentioned, look close attention to your MAP reading.

 

[privateer]

Hey Guys, thanks for some feedback. I'll go out for another test drive at my home elevation here in UT (4500-5K Ft) and up at higher elevation to get some Freeze Frame Data, then report back.

Its a stock a stock PCM, and driving around town ( Salt Lake ) I don't have any issues nor codes. Every time I travel on I-80 or I-15 and have to go over mtn passes of 7k+ feet is when I get the errors.

I'll let you know what happens with the freeze frame data.

 

[twomanymontes]

It sounds like your mass air flow sensor is dirty. The Baro sensor is in the Mass air flow sensor and if it's dirty it can't update the baro reading. The air is less dense at higher altitiude, if dirty, it will tell the computer it's running the same and the O2 sensors are reading lean. Try cleaning it with cleaner and blowing it dry before reinstalling.

 

[1997chevydriver]

It sounds like your mass air flow sensor is dirty. The Baro sensor is in the Mass air flow sensor and if it's dirty it can't update the baro reading. The air is less dense at higher altitiude, if dirty, it will tell the computer it's running the same and the O2 sensors are reading lean. Try cleaning it with cleaner and blowing it dry before reinstalling.

Actually if it thought it was reading the same (5k feet) at 7k feet it would be rich not lean.....

 

[great white]

Actually if it thought it was reading the same (5k feet) at 7k feet it would be rich not lean.....

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[twomanymontes]

Actually if it thought it was reading the same (5k feet) at 7k feet it would be rich not lean.....

True! But, The Mass air flow sensor could be reading a normal Grams per second rate, indicating no load, and not allowing the computer to compensate for the thinner air. The O2 sensors would then be reading lean.

 

[privateer]

Ok guys , sorry for the delay. I've been out of town and haven't been driving up high. Cleaned the MAF Sensor and the AIR sensor next to it. Seems to run better at 5000 feet and at 7-9k feet. Only made one test drive but didn't through any codes. Issue could be fixed.

Thanks again everyone for your input.

Nick