Diagnostic Code 720
Lead Driver
Joined: Sep 2006
Posts: 7,907
Likes: 143
It's a code telling you the trans output shaft speed sensor (oss) signal back to the PCM has an issue.
Short, open, cross to battery or sensor has no output.
This sensor is part of the logic to check on the gear ratio, tire size etc by compairing to a data table in software.
If you made a change in rear gear ratio and or tire size, it could generate the code.
Should have nothing to do with idle.
The IAC controlled by the PCM, controls all idle conditions cold and hot starts and under decell and A/C load conditions.
Good luck.
Short, open, cross to battery or sensor has no output.
This sensor is part of the logic to check on the gear ratio, tire size etc by compairing to a data table in software.
If you made a change in rear gear ratio and or tire size, it could generate the code.
Should have nothing to do with idle.
The IAC controlled by the PCM, controls all idle conditions cold and hot starts and under decell and A/C load conditions.
Good luck.
Lead Driver
Joined: Sep 2006
Posts: 7,907
Likes: 143
The IAC amost never sets a code but the effects of a fault with it can.
Normally, with no fault present; if the connector were removed from the IAC while the engine is at idle would cause the engine idle speed to drop down to about 500 rpm +/- which would be normal and an indication the IAC was operating 'at that time' but not tell you about any tendency to stick when cold etc.
.
The reason the idle drops is the 'base' idle is set by a stop setting in the throttle body to keep the plate from going fully closed. Never change that stop setting.
.
This does two things. Keeps the plate from sticking closed in the bore after the motor cools and allows for a dynamic operating idle range so the
IAC can close off enough air to maintain idle range within +/- 50 rpm of about 650 hot idle, as set in the computer program.
.
The crank sensor tells the computer what the RPM is so constant corrections can be made by varying the IAC while running.
If you suspect the IAC of sticking, remove it and spray clean the inside making sure the pintel valve can slide freely under it's spring tension.
.
Clean the throttle plate and bore so the small amount of air that is supposed to get by is not reduced.
.
If you find an issue with dirt and gum, I would reboot the computer after reassembly so the cold start, hot start and running idle tables are rebuilt by doing a cold start and drive cycle under the max load conditions the motor runs under.
To do that, remove the neg battery lead for about 2 minutes then reconnect, cold start with the lights on, fan running on high as the most simulated load you can put on.
.
Let the motor run to full warm up so the alternator is loading the motor as well as the other loads you turn on along with battery charging.
After full warm up, run about 5 miles and do several hot restarts.
After this procedure the tables are about fully built so the idle should be good under any conditions as long and there are no other issues.
.
BTW, doing this procdeure of rebooting cause the computer (PCM) to run all dignostics for all systems. That may take several drive cycles to complete so you could see a code 1000 if a reader can read it but won't turn on the CEL becaue it's not a direct trouble code only an indicatior the diagnostics are not all complete and passing.
Hope it solves your idle issue but there could be other causes such as an air leak after the mass air meter from bad hoses and rubber parts..
Good luck.
Normally, with no fault present; if the connector were removed from the IAC while the engine is at idle would cause the engine idle speed to drop down to about 500 rpm +/- which would be normal and an indication the IAC was operating 'at that time' but not tell you about any tendency to stick when cold etc.
.
The reason the idle drops is the 'base' idle is set by a stop setting in the throttle body to keep the plate from going fully closed. Never change that stop setting.
.
This does two things. Keeps the plate from sticking closed in the bore after the motor cools and allows for a dynamic operating idle range so the
IAC can close off enough air to maintain idle range within +/- 50 rpm of about 650 hot idle, as set in the computer program.
.
The crank sensor tells the computer what the RPM is so constant corrections can be made by varying the IAC while running.
If you suspect the IAC of sticking, remove it and spray clean the inside making sure the pintel valve can slide freely under it's spring tension.
.
Clean the throttle plate and bore so the small amount of air that is supposed to get by is not reduced.
.
If you find an issue with dirt and gum, I would reboot the computer after reassembly so the cold start, hot start and running idle tables are rebuilt by doing a cold start and drive cycle under the max load conditions the motor runs under.
To do that, remove the neg battery lead for about 2 minutes then reconnect, cold start with the lights on, fan running on high as the most simulated load you can put on.
.
Let the motor run to full warm up so the alternator is loading the motor as well as the other loads you turn on along with battery charging.
After full warm up, run about 5 miles and do several hot restarts.
After this procedure the tables are about fully built so the idle should be good under any conditions as long and there are no other issues.
.
BTW, doing this procdeure of rebooting cause the computer (PCM) to run all dignostics for all systems. That may take several drive cycles to complete so you could see a code 1000 if a reader can read it but won't turn on the CEL becaue it's not a direct trouble code only an indicatior the diagnostics are not all complete and passing.
Hope it solves your idle issue but there could be other causes such as an air leak after the mass air meter from bad hoses and rubber parts..
Good luck.
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