Q: I live at 9,200 ft. in the mountains above Colorado Springs. As long as I drive my '09 Forrester X limited at this altitude all is

asked by on

I live at 9,200 ft. in the mountains above Colorado Springs. As long as I drive my '09 Forrester X limited at this altitude all is well, car runs fine, plenty of power, good mileage, etc. If I drive the car to Colorado Springs at about 6,500 ft., It almost always sets the P 0420 Catalytic insufficiency code. Had it drilled and pressure tested, no blockage. Did a injector/throttle body clean and ran premium fuel for several weeks with no change. I can remove the code and drive it as much as I want till I go down the hill again where it will reset the code. Took it to dealer who pulled the code and said it needs $1,200 new converter. As a side note the car was driven in the Springs for awhile before we got it with no problems till we brought it up here. So the problem seems definitely related to altitude change. Has anyone heard of this and what is the problem?

My car has 132000 miles.
My car has an automatic transmission.

Hi there. This issue is rather common with vehicles that deal with dramatic changes in density altitude, such as what you’ve described. The P0420 OBD-II trouble code indicates that the catalytic converter is not working as efficiently as it is intended. If the voltage of the downstream oxygen sensor decreases, and begins to fluctuate like the upstream oxygen sensor, it means that the oxygen levels are too high and the Power Control Module will then store the P0420 trouble code. This doesn’t always mean the catalytic converter is faulty. In fact, quite often it’s either the O2 sensors are damaged or the electrical harnesses to the these sensors is faulty. The connector issue is common in Mountain or frequent snow communities where Mag-Chloride or other de-icing chemicals are used on roads.

Was this answer helpful?
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
  1. Home
  2. Questions
  3. I live at 9,200 ft. in the mountains above Colorado Springs. As long as I drive my '09 Forrester X limited at this altitude all is

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: How long for mass airflow sensor to recalibrate with new cold air intake?

Your mass air sensor does not need time to see how much air is going in the engine. The sensor may be detecting an air leak between the throttle body and the mass air sensor. The engine should not be...

Q: When accelerating from stop, rear end hops.

Hi there. Check the suspension and shocks on the rear of the vehicle. If the suspension is loose or the shocks are worn, then the axle will move around or bounce and keep the tires from being in contact with...

Q: Q: Car wont start

This may be a variety of possible issues ranging from a weak battery to a bad starter relay or burnt fuses. This is not likely related to the fluids as they generally do not have anything to do with the...

Related articles

Rules of the Road For Iowa Drivers
Driving on the roads requires knowledge of the rules, many of which are based on common sense and courtesy. However, even though you know the rules in...
P2428 OBD-II Trouble Code: Exhaust Gas Temperature Too High Bank 1
P2428 code definition A P2428 trouble code signifies that the PCM has detected a problem in the exhaust gas temperature sensor circuit in bank 1, which subsequently contains the number one...
P2103 OBD-II Trouble Code: Throttle Actuator Control Motor Circuit High
P2103 means there is a fault with the throttle actuator control motor circuit, likely due to a defective electrical component or part.