Skip the auto shop - Our mechanics make house calls

Q: Trying to determine if a Honda 2005 vtec (D172A) sohc, 127 hp has an O2 upstream sensor problem or the ECU

asked by on

I've taken voltage reading of both O2 sensors from the ECU connector(s) behind the glove box with digital multi-meter. Seems both initiate at 0.290v bias volts before the Htr circuit effect. Once heated, I observed the downstream sensor avg 0.4 v at idle and acceleration (+/- spikes). The upstream sensor avg 0.295v at idle and accl. With both sensors connected the engine idles fine but struggles on accl, power loss or hunting and when accl released, it gasp and sometime stalls out. When I disconn the upstrm O2 and leave the dwnstrm connected, the engine runs fine on idle and accl. When just upstream O2 connected, the engine runs bad as previously describe. I observed the readings on upstrm O2, it starts out at 0.290v - bias voltage then jumps up to 0.5 - 0.7 v (Htr ckt kicks in?) for a few seconds then drops and avg 0.290v. I notice before the Htr ckts effect (10s) with both sensors in, the engine had full pow at idle and accl. Took upstrm O2 rdg in open air, 0.75v - 0.9v, stable.

My car has 167920 miles.
My car has an automatic transmission.

Hello. From what you describe it would appear that your upstream oxygen sensors are causing the issue. If you disconnect them and the vehicle runs better then it may very well be possible that the signal they are sending or generating may be a bad one, and confusing the computer, causing your performance issues. Keep in mind that the heater circuit and the signal circuit will usually be on separate wires, and should not kick on and off and change the signal of the sensor. Most oxygen sensors are designed to operate in cycles, however, if they are giving off erratic readings that may be a sign that the sensor is bad. I would try to get the specifics on what the sensor voltages should be for that vehicle and compare them with your findings to see what you may conclude. If you need a diagnostic, an expert from YourMechanic can come to your car’s location to inspect the loss of power issue to help guide you through repairs.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
  • Home
  • Questions
  • Trying to determine if a Honda 2005 vtec (D172A) sohc, 127 hp has an O2 upstream sensor problem or the ECU

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Car hesitating upon acceleration.

The Civic 1.6L uses a speed density system to set up ignition and fuel injection. This system monitors intake air temperature, throttle position, manifold pressure, and a few other inputs. Generally a hesitation indicates one or a combination of these...

Q: Car still overheating after parts replacement

Hello. The head gasket is a common failure on this engine. I see them fail all of the time. To verify this I typically do a block test first. This will usually identify a leak in the head gasket. Sometimes...

Q: My car brakes are spongy

When replacing a master cylinder, it should be "bench bled" before installing it in the vehicle. This involves filling the reservoir with fluid, connecting rubber hoses to the brake line ports and placing the other end in the fluid reservoir,...

Related articles

What Causes Hoses to Leak?
While the largest part of your engine is mechanical, hydraulics plays a significant role. You’ll find fluids at work in a number of different areas. Your car's fluids include: Engine oil Transmission...
How Long Does a Heater Control Valve Last?
Keeping the right amount of coolant in a car is essential in keeping the engine at the right temperature. Failing to have the right amount of coolant or even bad elements...
P0240 OBD-II Trouble Code: Turbocharger Boost Sensor B Circuit Range/Performance
P0240 code definition Turbocharger Boost Sensor B Circuit Range/Performance What the P0240 code means P0240 is an OBD-II generic code triggered when the Engine Control Module (ECM) detects the intake boost...