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Discussion Starter #1
What is the difference between a heater circuit malfunction and a o2 sensor malfunction.

The reason I ask is that I replaced both front sensors and I am still getting a check engine light that says P0155 Heater circuit malfunction. I reset the computer then I start the car. No check engine. But the very next time I start the car, the check engine light comes back on.
 

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I found this in the 1994 PC/ED Manual. The problem may not be with the sensors, but with the harness or PCM:

H35 CHECK FOR HO2S HEATER SIGNAL SHORTED TO VPWR

Upstream DTCs 0135 and 0155 and downstream DTCs 0141 and 0161 are received in pairs and indicate the HO2S heater signal is shorted to a voltage source greater than 2.0 volts.

Possible Causes:

-- HO2S HTR SIG shorted to VPWR in the sensor.

-- Short in the harness.

-- Short in the PCM.

-- Both upstream sensors are disconnected.

-- Both downstream sensors are disconnected.

Use codes and inspect the upstream or downstream HO2S connectors for proper connection.

Use codes and inspect the upstream or downstream HO2S connectors for proper connection.

Were the HO2S sensors connected during Self-Test?

Yes
GO to «H36».

No
RECONNECT sensors.
RERUN «Quick Test».

H36 CHECK HO2S HEATER RESISTANCE

Suspect sensors disconnected.

Inspect sensor end of the connector for damaged or pushed out pins, moisture, contamination, etc. Service as necessary.

Measure the resistance between VPWR circuit and HO2S heater signal at the HO2S connectors.

Are both heater resistance readings between 2.0 and 30.0 ohms?

Yes
GO to «H37».

No
Resistance out of specification.
REPLACE HO2S(s)
RECONNECT all components.
RERUN «Quick Test».

H37 CHECK FOR SHORTS IN THE HO2S HEATER HARNESS CIRCUIT

Key off.

Suspect sensors disconnected.

Disconnect PCM. Inspect for damaged or pushed out pins, corrosion, loose wires, etc. Service as necessary.

Install breakout box, leave PCM disconnected.

Use the following chart to measure the resistance between the appropriate test pins at the breakout box.

-- DTC 0135 = HO2S HTR-11
Test Pin 93 and Test Pin 71 and 97.

-- DTC 0141 = HO2S HTR-12
Test Pin 95 and Test Pin 71 and 97.

-- DTC 0155 = HO2S HTR-21
Test Pin 94 and Test Pin 71 and 97.

-- DTC 0161 = HO2S HTR-22
Test Pin 91 and Test Pin 71 and 97.

Is the resistance greater than 10,000 ohms?

Yes
REPLACE PCM.
REMOVE breakout box.
RECONNECT all components.
RERUN «Quick Test».

No
REPAIR shorted wiring in harness.
REMOVE breakout box.
RECONNECT all components.
RERUN «Quick Test».
 

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Discussion Starter #3
That sounds nice and complicated. So is that saying that either I have a short, a bad connection, or a bad PCM. Is this going to be hard to figure out? Holy bugers.

What is a break out box?
 

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From Ye Olde 1994 PC/ED Manual Glossary:

Breakout Box: A service tool that "tees" between the PCM and the 104-pin harness connector. The breakout box contains test pins that can be probed for EEC system testing.
 

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Discussion Starter #5
Since I don't have access to a breakout box does this mean that I have to take it to a dealership? I have to have my car in for emissions no later then sept 20.

Not to mention that the light is really beginning to make me mad. I just want my car healthy!
 

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Discussion Starter #6
Should I put the car to as stock as possible when i bring it in to the dealership. Is the chip going to matter? etc.
 

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Midnight said:
Since I don't have access to a breakout box does this mean that I have to take it to a dealership? I have to have my car in for emissions no later then sept 20.
....
Honestly, I couldn't tell you, one way or the other, since I've never tested the PCM to that extent (I'm far from being electrical-savvy). A local private shop did a test for me once and determined that the PCM was keeping an injector on, so I figure the breakout box is not a dealer-only thing.

Regarding the 'returning to stock' question: I suppose it wouldn't hurt, since the diagnostics, on occasion, have the technician checking for aftermarket add-ons.

Good luck to you.
 
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