Last night the CEL came on, and I got the codes read on the way home.

  • P0122: Throttle Position sensor "A" circuit low voltage
  • P0139: Secondary HO2S12 (heated oxygen sensor - bank 1 sensor 2) circuit slow response
  • P0420: Catalyst system efficiency below threshold bank 1
  • P1456: EVAP emission ctrl sys leak detected (fuel tank system)

Then this morning the battery was very low and door locks barely worked. Checked voltage at the battery terminals and only got 8-9v. This makes me suspect that there is an electrical problem, possibly a bad alternator or battery. I'll check the age of the battery later and update this question.

I checked the obvious stuff (the gas cap was on) but can also check the throttle sensor once I get a jump start again.

I accidentally let the battery go flat about a month ago after leaving the lights on too long working in the dark. Lights got dim and car needed jumping. Could it just be a weak/old battery causing all this?

  • Were you able to resolve the bad battery and these CEL codes?
    – Zaid
    Dec 19, 2015 at 18:00
  • Yes, it appears to be low voltage that caused the CEL. The codes cleared and didn't return after charging the battery, and the battery was later replaced and no CEL returned. It also passed inspection a week or so later.
    – Nilloc
    Dec 21, 2015 at 15:48

1 Answer 1


Start the engine and turn on the head lights and rear screen heater then check the battery voltage, it should be 13-14 volts. If it is less than 13 volts, then your alternator is faulty. If the CEL came on while starting the engine it could be the low battery voltage that confused the ECU.

It is possible that something is drawing power from the battery when the ignition is off, which is flattening the battery, or the battery may just need replacing.

Lead acid batteries don't like being totally discharged, so your incident a month ago could sentenced your battery to failure.

  • I actually found out that the person who I'm borrowing it from already knew it needed a new battery, also after a full charging the check engine light went out, so I think it is charging well.
    – Nilloc
    Nov 10, 2015 at 3:58

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