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Marc, Technician
Category: Car
Satisfied Customers: 1888
Experience:  33 years exp. automotive repair NYS emisisions repair cert.
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Check engine light..two codes: P0420 and P1739..oxygen sensor..3rd

Customer Question

I had my check engine light on and went to procare to get the diagnosis and got two codes: P0420 and P1739. P0420 they said was for oxygen sensor replacement for both at $428. They said the P1739 was for the 3rd switch in the transmission and that they recommended I go to Aamco for that. I called Aamco, they said I should go through my dealer for this because they would have to get the part from them anyway and they could not get to me for two weeks. Went to the dealer and they say that since Procare cleared the codes and check engine light, they could not do anything so they said to drive it until the check engine light comes back on again so they can get the codes (which I gave them anyway). My the dealer playing some game or what? Any advice would be greatly appreciated! Do you know what the codes mean? Thanks
Submitted: 11 years ago.
Category: Car
Expert:  joe allen replied 11 years ago.

 It is no very common that you will have to replace both o2 sensors at the same time. The o2 sensors job in a car is to read the amount of actual oxygen coming fron the exhaust to see how well the emission devices in the car are working. If you have a code coming from the sensors there are a number of things that could be wrong. I would test the sensors before paying that amount of money. Do that as follows....


Oxygen sensors produce a voltage based on the amount of oxygen in the exhaust. Large amounts of oxygen result from lean mixtures and result in low voltage output from the O2 sensor. Rich mixtures release lower amounts of oxygen into the exhaust, therefore, the O2 sensor voltage is high. The engine must be at normal operating temperature before the oxygen sensor is tested. Always refer to the specifications supplied by the manufacturer.

Before testing an O2 sensor, refer to the correct wiring diagram to identify the terminals at the sensor. Most late-model engines use heated oxygen sensors (HO2S). These sensors have an internal heater that helps to stabilize the output signals. Most heated oxygen sensors have four wires connected to them. Two are for the heater and the other two are for the sensor.

Use a wiring diagram to identify the terminals on a heated oxygen sensor. Courtesy of American Honda Motor Co., Inc.


An oxygen sensor must be tested with a digital voltmeter. If an analog meter is used for this purpose, the sensor may be damaged.

Testing With A Digital Mulitmeter

  • Connect the voltmeter between the O2 sensor wire and ground.
  • Backprobe the connector near the O2 sensor to connect the voltmeter to the sensor signal wire. If possible, avoid probing through the insulation to connect a meter to the wire.
  • With the engine idling, the sensor voltage should be cycling from low voltage to high voltage. The signal from most O2 sensors varies between 0 and 1 volt.
  • If the voltage is continually high, the air/fuel ratio may be rich or the sensor may be contaminated by RTV sealant, antifreeze, or lead from leaded gasoline.
  • When the O2 sensor voltage is continually low, the air/fuel ratio may be lean, the sensor may be defective, or the wire between the sensor and the computer may have a high-resistance problem.
  • If the O2 sensor voltage signal remains in a mid-range position, the computer may be in open loop or the sensor may be defective.
  • The sensor can also be tested after it is removed from the exhaust manifold.
  • Connect the voltmeter between the sensor wire and the case of the sensor.
  • Using a propane torch, heat the sensor element. The propane flame keeps the oxygen in the air away from the sensor element, causing the sensor to produce voltage.
  • While the sensor element is in the flame, the voltage should be nearly 1 volt.
  • The voltage should drop to zero immediately when the flame is removed from the sensor.
  • If the sensor does not produce the specified voltage or if the sensor does not quickly respond to the change, it should be replaced.
  • If a defect in the O2 sensor signal wire is suspected, backprobe the sensor signal wire at the computer and connect a digital voltmeter from the signal wire to ground with the engine idling.
  • The difference between the voltage readings at the sensor and at the computer should not exceed the vehicle manufacturer's specifications. A typical specification for voltage drop across the average sensor wire is 0.02 volt.
  • Now check the sensor's ground.
  • With the engine idling, connect the voltmeter from the sensor case to the sensor ground wire on the computer. Typically, the maximum allowable voltage drop across the sensor ground circuit is 0.02 volt.
  • Always use the vehicle manufacturer's specifications. If the voltage drop across the sensor ground exceeds specifications, repair the ground wire or the sensor ground in the exhaust manifold.
  • Most late-model engines are fitted with heated O2 sensors.
  • If the O2 sensor heater is not working, the sensor warm-up time is extended and the computer stays in open loop longer. In this mode, the computer supplies a richer air/fuel ratio. As a result, the engine's emissions are high and its fuel economy is reduced.
  • To test the heater circuit, disconnect the O2 sensor connector and connect a voltmeter between the heater voltage supply wire and ground.
  • With the ignition switch on, 12 volts should be supplied on this wire. If the voltage is less than 12 volts, repair the fuse in this voltage supply wire or the wire itself.
  • With the O2 sensor wire disconnected, connect an ohmmeter across the heater terminals in the sensor connector.
  • If the heater does not have the specified resistance, replace the sensor.

This will tell you if it is worth the money. Secondly the dealer is not playing games the just usually insist on seeing the codes themselves to avoid the he says she says. as for the code definitions what you have is correct. Also sometimes if both sensors are acting funny a good culprit can be the converter.

Hope this helps if you have any questions let me know.

Expert:  Tech501 replied 11 years ago.
 Procare is dead wrong. A PO420 is a code for a bad Catalytic Converter, not an O/2 sensor. Your better off sticking with the dealer on this one. They were right, erasing the codes and freeze frame data took all the info away. When the light comes back on, let the dealer diagnose it. The Cat. has to come from them anyway on that car. The other code is a trans code. Use the dealer for that too.
Customer: replied 11 years ago.
Am I hurting my car by driving it? what can I expect to pay on these and is it worth it on a 98 accord with 134,000miles? thanks so much!
Expert:  joe allen replied 11 years ago.
 well untill the dealer checks the codes we wont know for sure. But it's never a good thing to run the car with a code present.
Expert:  Marc replied 11 years ago.
At 134K that Honda just finished breaking it's self in.  

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