I have a 2000 Corolla with 195K miles on it. Check engine light is on since 2 years ago. A local mechanic recommended to replace a part (he charged me about $280, parts and labor) and had the car pass emissions. 7-10 days later the CEL came on, but since by that time the car was registered for 2 years, I thought I'll catch up with the mechanic later on and then it never happened. Anyway, over the last year or so, I have been troubleshooting with OBD codes and systematically replacing parts to eliminate the CEL. Following the OBD codes, first I replaced the Idle Speed Control Valve, then comes codes for O2 sensors, both front and rear were replaced, next I replaced the front part of the exhaust system (Big Two Toyota service found a bolt missing and thought that is causing the CEL), but did not help. Then code 0171 pops up. I replaced the Mass Airflow Sensor (MAF, also replaced the car engine air filter at the same time). The same code 0171 shows up again and now it is the Idle Speed Control Valve again! I re-set the computer every time by disconnecting the battery, but the CEL comes back on after 50/60 miles of driving. At this time, I am just at a loss, feels like I am back to where I started with replacing the Idle Speed Control Valve almost a year ago. And now the code says
Idle Speed Control Valve again!

The emission testing time is coming up and I am becoming really concerned. Any suggestions/recommendations would be greatly appreciated. Thank you. Hasan Mushtaq.

And most recently I am observing some stalling of the engine while at idle when stopped at red lights. The car actually stopped once yesterday while I was stopped at an intersection, starting the car was not a problem though!

 

First things first you need better tools. Goto eBay and get an obd2 reader one with live data is best but anything to reset your code without removing the battery. Next up STOP THROWING PARTS TO FIX THE CAR. You must use the engine light as a tool to find a problem. Everything can be tested before you replace it. If your mechanic says code says you need _ and that's the only reason they give it's time for a new mechanic. Finally go back over your paperwork and look into exactly what parts were used on these repairs. You mentioned maf sensor I know there are countless posts of Bosch sensors doing more harm than good it's a part that must be Denso brand. Other parts are similar especially electrical.

Your stall at idle sounds like a dirty idle air valve. You can clean it yourself lots of videos online show how just be careful the 4 screws strip easy so take your time.

Welcome to toyotanation. Add your location to your profile maybe a member nearby you can get you a better mechanic

 

First things first you need better tools. Goto eBay and get an obd2 reader one with live data is best but anything to reset your code without removing the battery. Next up STOP THROWING PARTS TO FIX THE CAR. You must use the engine light as a tool to find a problem. Everything can be tested before you replace it. If your mechanic says code says you need _ and that's the only reason they give it's time for a new mechanic. Finally go back over your paperwork and look into exactly what parts were used on these repairs. You mentioned maf sensor I know there are countless posts of Bosch sensors doing more harm than good it's a part that must be Denso brand. Other parts are similar especially electrical.

Your stall at idle sounds like a dirty idle air valve. You can clean it yourself lots of videos online show how just be careful the 4 screws strip easy so take your time.

Welcome to toyotanation. Add your location to your profile maybe a member nearby you can get you a better mechanic

First things first you need better tools. Goto eBay and get an obd2 reader one with live data is best but anything to reset your code without removing the battery. Next up STOP THROWING PARTS TO FIX THE CAR. You must use the engine light as a tool to find a problem. Everything can be tested before you replace it. If your mechanic says code says you need _ and that's the only reason they give it's time for a new mechanic. Finally go back over your paperwork and look into exactly what parts were used on these repairs. You mentioned maf sensor I know there are countless posts of Bosch sensors doing more harm than good it's a part that must be Denso brand. Other parts are similar especially electrical.

Your stall at idle sounds like a dirty idle air valve. You can clean it yourself lots of videos online show how just be careful the 4 screws strip easy so take your time.

Welcome to toyotanation. Add your location to your profile maybe a member nearby you can get you a better mechanic

Since my last post and reply, I have driven the car for about 20K miles. Check engine light is still on. Recently, I have further replaced the Fuel Pump and the Fuel Filter. Replaced the spark plugs as they turned out to be extremely worn out (do no remember when I did my last tune up!). Bought Micro Mechanic from a local store, reset the CEL code. The CEL was off for a couple of days and now it has returned. Use of Micro Mechanic results "No problems Found - Car Safe to Drive". So, back to square one! I checked the MAF sensor, it Is clean, but is not Denso product. Planning to replace it with a Denso product as a last resort to see what happens. In November 2019, prior to the emissions check, I filled up the car with high Octane gasoline, drove it around for a few days for more that 100 miles, the CEL did not come back, took it to test, and it passed. Few days later, the CEL came back again. Any further ideas to try would be helpful. Perhaps, I will buy a obd2 reader one with live data and go from there. Thanks!

 

I have been troubleshooting with OBD codes and systematically replacing parts to eliminate the CEL.

Each code has a corresponding troubleshooting procedure to test the entire circuit. Parts should only be replaced after they are determined to be broken, not just based on the OBD code. Perhaps you did all the troubleshooting before replacing the parts.

Things other than the MAF can cause the P0171 such as vacuum leaks.