I have had my mechanic replace multiple components in my EVAP system trying to resolve the following error codes which occur in no particular order within 24 hours after resetting my memory and cold starting.

P1133 - HO2S Insufficient Switching Bank 1 Sensor 1
P0133 - O2 Sensor Circuit Slow Response Bank 1 Sensor 1
P0130 - O2 Sensor Circuit Bank 1 Sensor 1
P0442 - EVAP System Leak Detected (small leak)
P0455 - EVAP System Leak Detected (large leak)

No mechanical solution has worked so far. My only option is to replace the ECM which sounds pretty desperate and is expensive.

Any idea which component might be triggering these error codes? Maybe there is a component that my mechanic has overlooked.

My model is a Saturn Ion 2003

  • 3
    You say you've "replaced multiple components" ... which exact components were replaced? Dec 4, 2016 at 3:17
  • If you're suspecting the ECM, try a junkyard one. Cheaper and probably still works.
    – tlhIngan
    Dec 4, 2016 at 3:52
  • 1
    One he might overlook is the filler pipe. I had a leak under the cap gasket. Take the pipe off and blown on the open end to test it.
    – George
    Dec 4, 2016 at 4:27
  • 1
    When diagnosing something like this don't just replace components, diagnose the problem. First check if the ECM has control of all the solenoids, check if the solenoids are actually working. Command the ECM to seal the system and pressure check it. If there is actually a leak fix it. If there is no leak something is causing the ECM to flag it. Check the tank pressure sensor. Do a visual inspection of everything. Cracked, broken or disconnected hoses, broken or damaged wiring. If all you mechanic has done is replace parts then it's time to find a new mechanic.
    – vini_i
    Dec 4, 2016 at 16:53

1 Answer 1


The EVAP system needs to be smoke tested. This requires special equipment, not only a smoke generator but also a scan tool sophisticated enough to control the EVAP solenoids manually.

Likely, the problem is a rubber hose or a cheap OEM squeeze clamp, not a solenoid, carbon canister, or gas cap. A smoke test will evaluate the entire system and have the capability to find the smallest leaks. Throwing parts (aka "shotgunning) at an EVAP leak, especially a small P0442 is risky business. However, many shops do not have the expensive smoke tester.

The first three O2 sensor codes are probably not related. Consider replacing the front "upstream" heated O2 sensor before the catalytic converter. I'm pretty sure it's just behind the radiator fan in the collector section of the exhaust manifold. It's not a hard job (on your car, anyway) and a good specific bespoke connector aftermarket sensor (Walker, Denso) is less than $50.

  • Why do you think I should start by replacing the up-stream O2 sensor?
    – the_prole
    Dec 7, 2016 at 9:06
  • To address the first three OBD codes.
    – SteveRacer
    Dec 9, 2016 at 4:34
  • That's what I figured. But why do you say they are not related in your answer?
    – the_prole
    Dec 9, 2016 at 6:56
  • Just replaced my O2 sensor. Still getting P1133. Where do I proceed next?
    – the_prole
    Dec 9, 2016 at 21:59
  • Because EVAP system leaks do not cause Heated O2 sensors to switch infrequently. I would verify wiring and connector integrity on the H2OS if the P1133 persists. Also, the smoke test of the EVAP system is sepearte, so at least you could eliminate those P0442/PO455 codes.
    – SteveRacer
    Dec 10, 2016 at 0:59

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