O2 Sensor Code Help
 

Last week my 1999 4 cylinder Honda Accord threw (x2) engine codes. P0155 (Oxygen Sensor Heater Circuit Bank 2 Sensor 1) and P0135 (Oxygen Sensor Heater Bank 1 Sensor 1).

I tested the oxygen sensor in the exhaust manifold and the heating element was broken (ohm's reading was 0). Ordered a new O2 sensor, installed it and thought that would take care of my check engine light. Light stayed on, retested and the P0135 code was still showing (P0155 was fixed).

I'm confused what could be causing the P0135 code. I understand the bank 1/2 sensor 1/2 when you have 2 sensors before the CAT and 2 sensors after the CAT for a V6 or V8 engine. This is a 4 cylinder, so I'm pretty sure there is only 1 sensor pre-CAT and 1 sensor post-CAT.

Any ideas on what I should check/do for the P0135 code? Thank you for your time and help!

With a 4-cyl engine, P0155 shouldn't exist because there isn't a "bank-2". But that code is gone now & P0135 remains?

Check fuse #6 in the fusebox in the driver's end of the dashboard.

Check resistance between the heater pins of the new sensor - maybe it's bad right from the box??

With ignition ON, check voltage across the 2 terminals as shown on the wire that plugs into the sensor. Should be battery voltage when the ignition is turned on.

Thanks for the reply.

I did check the resistance for the new o2 sensor heater element and it tested fine before I put it in. I honestly didn't even think about checking the line going to the o2 sensor to give it power (oops). I get back into town later this week and will update with results from testing that.

I'm glad to hear you say that about no "bank 1 vs bank 2" on 4 cylinder. That was my understanding as well so those codes have me all confused.

Could anything else trigger the P0135? Bad gas cap, letting air in, etc? I'm wondering if putting in a good o2 sensor fixed the P0155 but something goofy like a gas cap is causing the P0135?

Another theory is maybe P0135 + P0155 codes triggered simultaneously due to the 1 bad O2 sensor. My understanding is the my 99 accord has an ECU that works for the 4 cylinder and 6 cylinder model. Maybe it triggered "both banks" bc of the compatibility with the 6 cylinder option. Maybe that explains both codes showing up but for some reason I need to reset the P0135 since the new o2 sensor didn't shut it off. See if it comes back ?

Okay - I'm done theorizing...just thinking out loud. I'll get back to you ASAP when I do the additional electrical test. Thanks again!

I can't imagine the same ECU for both engines. Wrong number of sparkplugs, wrong number of injectors, all kinds of fundamental things besides just the fuel & spark maps.

Loose gas cap sets a different code (P1456).

I read your post-1 again; did you reset it after installing the new sensor? Reset by pulling the #13 fuse in the passenger-end of the dashboard (near the door hinges). Otherwise it'll "remember" a code from the past.

Welp - I pulled the fuse just to make sure it was okay. That reset the ECU of course and the code disappeared and hasn't shown back up. Thank you. You are a genius.

Don't say that just yet...
Sometimes the codes take a day or 2 before they come back.