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Matt, Mechanical Engineer
Category: Toyota
Satisfied Customers: 21993
Experience:  BEng, Repaired my own cars and others for 20 years, worked in Formula 1 for 10 years
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on a 2001 toyota celica gt, i got CEL PO171, when i changed

Customer Question

on a 2001 toyota celica gt, i got CEL PO171, when i changed the air filter it was almost blackish, and when i ran a light test, no light passed through it. i changed that air filter, and then cel was still on, i did a full tune up, changed sparks plugs to OEM NGK Iridium, then i cleaned the Mass Air Flow sensor...with MAF sensor cleaner from autozone....that didn't work...i kept resetting cel each time, came back on within 2-15 miles of driving each time. listened for a vacumm leak and heard nothing...(not 100% sure...)...maybe change the fuel filter? or fuel pump? any ideas?
Submitted: 4 years ago.
Category: Toyota
Expert:  Matt replied 4 years ago.


Check for exhaust leaks before the O2 sensor as if this has been experiencing an exhaust leak then it will have been running very rich as the sensor will see the extra air pulled in as fresh / lean fuelling and be adding fuel to compensate.

this can lead to catalyst failure as the extra fuel can re-light inside and melt the catalyst

The best way to find an exhaust leak is to run the engine from cold and feel around the exhaust joints with your bare hands - you'll have about 30 seconds before it all gets too hot and you should feel any escaping gas blowing over your fingers

I'd also check for air leaks the best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.


Also replace the spark plugs if any of them fail this checklist: