Diagnose this car problem

A few months ago the “check engine” light came on in my Infiniti I30. I took it in and they said the catalytic converter was triggering the light, and that meant it was probably an oxygen sensor. However, they said that all the oxygen sensors seemed to be working fine when they checked, so they recommended I watch my gas mileage closely but not start replacing senors until they could figure out exactly what was going on. They said they would keep checking it when I bring it back for oil changes.

So I’ve been driving around for a few months with the “check engine” light on, and have not noticed any change in mileage.

Last week I drove to Colorado Springs, which is about 6500 feet above sea level. The first time I started the car after arriving I noticed that the check engine light was not on. I drove all over Colorado for a week and the light never came on. Then, when I was driving home, the light came on again in Kansas and has stayed on ever since. What do you think?

Sounds like a gas cap issue.

The gas cap usually allows for a bit of a vacuum in the gas tank, but if it’s leaky that lower air pressure never happens. When you are in the mountains, the lower air pressure could make your gas cap seem to be working properly.

Get a new gas cap.

The error code stored in the computer should indicate where the problem is. A couple of years ago my car had a cracked evap cannister; the code indicated a leak somewhere in the fuel system (it does a vac check every time you start the engine), and it took a quick look by a tech to confirm exactly where the problem was.

I won’t hazard a guess as to why altitude affects the OP’s vehicle performance, but if the error code on the OP’s car is indicating trouble with the exhaust O2 content, it’s safe to say the problem is not going to be a vacuum leak in the fuel system.

This is far too much trust in a computer/human interface.
My experience with those diagnostic computers only has them accurate about 85% of the time.

My check engine light has been on for 6 years. Every time I take it in - for unrelated issues, they tell me the computer says the light is on because of:

Vacuum seal on gas cap
Catalytic converter
a broken sensor in the cooling system

10 repairs in 6 years. 5 blame the gas cap, 3 blame the catalytic converter…

Modern mechanics rely too much on the computers. I remember when a car mechanic had to diagnose cars based on gathering evidence such as sounds, drips…

I’m not sure which “diagnostic” computer you’re referring to. I’m talking about the car’s self-diagnostic capability. Any car built after 1996 can be read via an industry-standard OBDII interface. There are a great many trouble codes that can be read that will point (either specifically or generally) toward the problem. In my case, the error code pointed to a leak in the evaporative emissions recovery system, but yes, it took some snooping around by a tech to pinpoint exactly where the leak was.

In the OP’s case, if the error code is pointing toward a problem with the cat or O2 sensor, I wouldn’t suggest mucking around with the fuel system.

Don’t Call Me Shirley, does any of your paperwork from the mechanic state exactly what error code was being given by the car’s computer?

google this: gas cap check engine light elevation

You are not alone. Various makes and models have gas cap issues that are affected by elevation. In some brands/models, a change in elevation triggered the engine light and an ‘emmissions’ code. Most often, the cap was loose or needed to be replaced.

In one Toyota model, the tank filler tube (between gas cap and tank) was to blame and needed replacing.

I’d switch gas caps. Also, if you have a gas cap engine light issue, just fixing the problem won’t reset the light. You have to have it reset by a shop or drive a few times/days for the diagnostic system to see the various conditions and only then will it clear.

note: CEL is commonly used for “Check Engine Light” on various boards, if you go a googlin’ .


IIRC it was P0420 and PO430.

Those codes indicate not enough difference in readings between the pre-catalyst and post-catalyst oxygen sensors. Each code refers to one side of a V-type engine.

I don’t see how a gas cap/evaporative emission system issue could set those particular codes.

There are various things that can trigger those codes. Among them are faulty oxygen sensors, faulty catalysts, exhaust leaks, and faulty/leaking fuel injectors.

Offhand I don’t know which of these might be affected by high altitude or the concomitant reduction in oxygen so as to prevent the code from setting, but this could be a significant clue in figuring out the cause of the problem. I’d love to see the readings from all four oxygen sensors at altitude and at sea level.

Clicky here.

See “Possible Cause” list near top of first page. Your car apparently has two cats, one for each bank of cylinders; P0420 indicates a problem with the Bank1 cat, and P0430 indicates a problem with the Bank2 cat. It would be weird for both cats to actually be failing exactly simultaneously, so it’s probably a problem somewhere upstream that’s affecting the performance of both. Per the “Possible Cause” list, my guess is that you may have an intake manifold leak. Reduced atmospheric pressure at high altitude would reduce the leakage rate to below the problem threshold, leading the ECU to think “everything’s fine” while you’re in Colorado.

Did you visit a Nissan/Infiniti dealer, or a Midas kind of place? I’ll wager the Infiniti folks will do a more thorough job of diagnosing the exact nature of the problem, as they did for me. Otherwise, if you’re a DIY kind of person, you can follow the diagnostic procedures outlined at the above link.

That sounds reasonable, but wouldn’t an intake manifold leak cause other codes to come up, possibly in addition to the cat codes?

I would have thought so too, but who knows? Maybe depends on size of leak. The link offers a number of things to be checked out, and an IM leak is just one. They make the diagnosis sound simple - “Listen for an air leak after the mass air flow sensor” - so maybe this will be easy to spot.

Worst case, take it to the Infiniti dealer and pay them for an hour of diagnostic time.