SES light on but diagnostics says engine fine

**Gale8706** · 10-22-2011, 10:41 AM

recently put on a flowmasters 40 series on my 02 Z28. a few days after the SES light came on auto zone and the muffler shop i was at both had the code P0420 which is bank1 o2 sensor. well gm says that my MV should be between 650-850 and im running high 700s. my fuel/air is 14.6 and optimal is 14.7, now i had my computer resest and my PCM reflashed so why is my light still coming on?

**My00Z28** · 10-22-2011, 01:13 PM

could be a couple of things start by checking your cats see if they are loose i had one of my cats housing break

look for exhaust leaks by the o2 sensors
also your o2 sensor could be failing
and possible bad coolant temp sensor
also it could be a bad converter

**redbird555** · 10-22-2011, 02:59 PM

which o2 sensor are you looking at? you need to look at bank 1 sensor 2 and monitor it. however most of the time when p0420 is thrown it means you have a bad cat gm has a 10yr 120k warranty on 01-02 cars so you maybe in luck however if yur car isnt an 01-02 it might just be time for headers lol

**My00Z28** · 10-22-2011, 03:15 PM

a p0420 code is bank 1 it can be thrown for different reasons the big one is both bank 1 sensor 1 and sensor 2 are reading the same voltage... it could mean the cat isnt doing anything or what i stated in my above post....

**Gale8706** · 10-22-2011, 08:49 PM

I had the whole system checked and the muff shop ssaid no so idk hope for the warrenty or it will go away when I awitch the o2 sensor

**My00Z28** · 10-22-2011, 09:58 PM

do you know how they tested the system?

**Gale8706** · 10-23-2011, 09:29 AM

They showed me the smoke and water when it was in the shop

**My00Z28** · 10-23-2011, 10:20 AM

Smoke and water?

**Gale8706** · 10-23-2011, 08:50 PM

Smoke underneath to see if it gets blown away from the car and the water by the intake to see if it was sucked in and shuts off

**My00Z28** · 10-23-2011, 09:00 PM

ok so they didnt test the sensors with a scope or obdII brick? or check the temp. of the cats to see if they are the same temp?

these are some things you may want to check if the cats are different temps but a good amount then you probably have a bad cat... and the scope/brick will show you wave forms of the o2 sensors to make sure they are working properly...

**Gale8706** · 10-26-2011, 02:16 PM

Im not sure bout temp but I did see them check the O2 sensor the line was really straight and he shot my rpm up and the did one straight upward climb the went back to horizontal

**My00Z28** · 10-26-2011, 02:31 PM

O2 sensors should not scope out in s straight line they switch from rich to lean over and over which would make a good wave form if thats how they tested it and that was the result then theres something wrong with your O2 sensors... also the test must be done in closed loop.

**Gale8706** · 10-26-2011, 02:52 PM

F oh well looks like im going to have spend the money thanks for the help

**My00Z28** · 10-26-2011, 03:15 PM

before you go nickel and dimming your car see if you can get a good test done, a easy and quick test see if you have a buddy with a temp gun shoot it at your cars see if the temp is close to the same to rule out bad cats.... most advance/autozones have scan tools that can read live data, see if you can get someone to scan your car using the live data feature and see if the voltage of your "front" o2 sensors are switching from 0.002ish-5 volts decently quick(if they are switching slow or not at all they are more than likely bad)... also look at the "rear" o2 sensors and see if they stay consistently around the same voltage(for example sit at 2.45v for like 2-3 seconds)(if they are switching to fast then they are bad or if they are reading close to the same as your front o2's then your cats are bad...

**BIGGDADDYWIGGS** · 10-26-2011, 06:10 PM

If the cap is loose it lets air into the fuel system and it can't pressurized. Usually taking thaw cap of and putting it back on better will turn the light off

**My00Z28** · 10-26-2011, 06:47 PM

That would be an evap code then!!!!!!

**0rion** · 10-26-2011, 07:08 PM

you either have a bad rear drivers side sensor or a bad drivers side cat...... easy (free) way to test is to swap your rear sensors side to side. If you then start throwing p0430 then you know it's the sensor.

**Gale8706** · 11-10-2011, 01:54 AM

Did the oil used Lucas fuel sys cleaner and went from 89 to 93 and two tanks later my light is off thanks for the help prob saved me an ass ton of money