Our standard grade gas out here in Colorado is 85, I believe the manufacturer recommends 87 or higher for a 96 T. Will 85 octane cause any damage to the vehicle?

No, all you need is 85. At altitude you need less octane to prevent pre-ignition.

I thought 85 octane would be fine as well then I got a check engine light so I got it tested and it came up with a pre cat oxygen sensor failure. I replaced the sensor and the check engine light stayed on. I got it tested again and they said bad pre cat oxygen sensor again. When I told them I had just replaced that sensor they asked what gas I was running to which I replied 85 octane. They (Autozone) indicated that running with lower than recommended octane can cause a false reading from the oxygen sensor. They recommended running a can of fuel system cleaner and changing to 87 octane. I did that and after about 400 miles the check engine light went out for about another 400 miles and now it is back on again.

What would you all do at this point?

Is it possible that the T100 ECU can be reflashed for 'high altitude' calibration? Ask a knowledgeable dealer service rep - was your truck originally sold in CO or from out of state?

Are you using OEM brand (factory P/N) replacement O2 sensor? If not I highly suggest you try that first.

The O2 feedback sensor signal is the heart and soul of modern fuel injection control systems. These sensors can be 'biased' slightly rich or lean to meet the needs of specific OEM engine calibrations. Outside the Toyota powertrain enginnering center you will never know what the techies had in mind when they spec'd out that particular Denso O2 sensor.

So called 'universal' O2 sensors are 'universally' a bad idea [IMHO.]

I did install the OEM Denso oxygen sensor. The high altitude calibration idea is intriguing as this vehicle was originally sold in California. I wasn't aware that some type of high altitude calibration could be made. I'll have to check into that. Thanks.

Well I called 3 Toyota dealerships about recalibrating the ECU and they all said that wasn't applicable in my case. ECU automagically adjusts for elevation. Thanks for the idea though. Any other ideas?

Correct.. There's a barometric pressure sensor in the vapor recovery system... and also it would see the thinner air in terms of reduced lbs/min air flow through the MAF.

I've had zero problems running 85 octane in the last 10k miles or so (all 5k~10k' in elevation). Truck has 131k miles and I've driven from my parents ski cabin in MT to my home in N. AZ as well as to several skiing hills in CO (like Wolf Creek which is 10,300' at the parking lot).

85 is fine for the truck in good running condition (you might have something going on that's causing your problems, but it's not the gas).

Problem solved. Thanks for the ideas.

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