OBD1 Diagnostic Tips
Lean or Rich Diagnostic Codes
If a lean or rich diagnostic code is experienced, observe the behavior of the engine to verify that is in fact running that condition. There are scenarios in which a malfunction can result in a misleading diagnostic code.
As an example, an exhaust leak before the oxygen sensor will introduce outside air. In between exhaust pulses, there’s a vacuum inside the exhaust that will suck in air. The constant supply of oxygen is interpreted by the ECM as a constant lean condition. The ECM responds by continually increasing the fuel supply to resolve the lean condition. A lean diagnostic trouble code will be thrown when the engine is actually running rich.
Intermittent Issues
The ability to read sensor and voltage values while driving can be useful to diagnose intermittent issues. When the problem occurs, examine the OBD1 and DLC values for anomalies.
Diagnostic Trouble Codes
Default ECM behavior is that an engine fault will not be displayed unless the condition is met more than one time. Enabling TE1 and TE2 at the same time puts the ECM into technician troubleshooting mode. Faults will be displayed when the condition is only met once.
Non-critical diagnostic trouble code flash while the condition is met but the code is not stored by the ECM. The app will capture these codes as they occur.
The app will capture codes when the screen is off. All other app functionality is paused.
Code 51, switch condition signal, will be displayed (but not be stored in memory) if one of the following conditions is met:
- Air Conditioner switch is on.
- Accelerator pedal is depressed.
- Automatic transmission is in a gear other than (N)eutral.
Coolant Temperature Sensor
Coolant temperature plays an important role in the EFI behavior. If you’re having a problem that correlates with temperature, first ensure that there isn’t any air in the cooling system. The coolant temperature sensor, wiring and connection can be validated by simply reading the value that the ECM is using. If a discrepancy isn’t obvious, use an infrared thermometer check the temperature near the coolant sensor. If the engine coolant temperature sensor circuit is open or shorted, the ECM typically assumes an engine coolant temperature value of 80°C (176° F).
Throttle Position Sensor
The most common failure point of a Throttle Position Sensor (TPS) is that they stick then don’t close all of the way when you take your foot off of the throttle. The ECM thinks that the throttle is open but it’s closed. Testing the TPS normally necessitates a multi-meter and it’s often difficult to access to the connection. The TPS can also be out of adjustment or just have a bad connection. The throttle position data from the ECM can be used to validate the sensor, wiring and connections. The throttle position should read 0 or 0.5 when closed. The idle (IDL) switch is part of the TPS and its functionality can also be validated.
Oxygen Sensor Feedback
Closed loop mode is when upstream oxygen sensor readings are used by the ECM to adjust fuel delivery levels. It only operates when the engine is hot and speeds are well over 2,000 RPM. The actual activation speed depends on the vehicle and driving conditions. The throttle position sensor idle switch is often the activator.
If the engine runs poorly in closed loop mode, the oxygen sensor or wiring may be faulty. The oxygen sensor plays no role when the engine is cold or RPMs are low.
If the coolant temperature sensor provides an erroneous high temperature reading, the engine will run poorly because the ECM will be using oxygen sensor values when the sensor isn’t in a temperature range that will provide an accurate measurement.
Fuel Mixture Charts
The oxygen sensor measurement is taken directly from the sensor at a high rate. When the engine is hot and RPMs are high, the mixture should cycle back and forth between lean and rich. If the measurement stays in one state that’s a clue as to the cause of an engine running poorly.
The fuel mixture value is a digital version of the oxygen sensor reading. When the engine is hot and RPMs are high, the oxygen sensor and fuel mixture behavior should correlate.
The feedback correction is what the ECM is attempting to do. If the engine is running lean, then the correction will be to enrich. The engine should cycle between the two states.
Fuel Pump Voltage
The fuel pump (FP) voltage is provided to diagnose fuel delivery problems. The voltage reading can be used to validate the Circuit Opening Relay (COR), the signal that activates it and the wiring in between. To prevent feeding a fire, the fuel pump will only operate when the starter is engaged or the engine is running. The COR activation signal indicates that the engine is running.
Vane type Air Flow Meters (AFM) vehicles activate the COR using a fuel pump switch. The fuel pump should run if you push on the AFM door while the ignition key is in the on position.
Optical Karman Vortex AFMs, hot-wire type AFMs and Manifold Absolute Pressure (MAP) sensor vehicles activate the COR using a signal from the ECM that results from the ECM receiving the NE signal from the distributor, cam position sensor or pickup coil. If the COR isn’t being activated then the cause may be that the NE signal isn’t being received from the sensor.
Some vehicles have a two speed fuel pump that implement the COR after the FP terminal (instead of before it). On these vehicles, the FP voltage will validate the ECM signal but not the COR.
Other vehicles have a two speed fuel pump that is implemented using a separate Fuel Pump ECM. When trouble is detected by the FP ECM, it sends a signal to the engine ECM. The FP voltage is the output of the FP ECM.
Vehicle Speed
The speed (SPD) signal is used to control the Idle Speed Control (ISC) system and to control the air fuel ratio during acceleration and deceleration. Vehicles with a cable driven speedometer have a speed sensor in the dash cluster (combination meter) that sends the SPD signal to the ECM. Vehicles with an electronic speedometer, repeat the signal then send it to the ECM.
ECM Reference Values
Lexus & Toyota factory service manuals for 1994 & 1995 vehicles have a table titled "REFERENCE VALUE OF ECM DATA" that contains expected values, states and behaviors. This table is a summary of the data.
| TOYOTA |
| Years | Model | Engine | Injector
@ Idle | IAC
@ Idle | Air
@ Idle |
| 1993-1995 | 4Runner | 22R-E | 2 ms | NA | 6 m/h | VAF |
| 1993-1995 | 4Runner | 3VZ-E | 2.2-2.7 ms | NA | 2.2-3.4 v | VAF |
| 1992-1993 | Camry | 3VZ-FE | | yes | | VAF |
| 1992-1995 | Camry | 5S-FE | 3.5 ms | 30-60% | 180-280 mmHg | MAP |
| 1992 | Celica | 5S-FE | | | | MAP |
| 1992-1993 | Celica | 3S-GTE | | | | VAF |
| 1993-1995 | Celica | 5S-FE | 3.5 ms | 30-60% | 180-280 mmHg | MAP |
| 1994-1995 | Celica | 7A-FE | 3.7 ms | 5-25% | 250 mmHg | MAP |
| 1993-1995 | Corolla | 4A-FE | 3.7 ms | 5-25% | 250 mmHg | MAP |
| 1993-1995 | Corolla | 7A-FE | 3.7 ms | 5-25% | 250 mmHg | MAP |
| 1991-1992 | Land Cruiser | 3F-E | | yes | | VAF |
| 1993-1994 | Land Cruiser | 1FZ-FE | 3 ms | 29-35% | 1.2-2.4 v | VAF |
| 1993-1995 | MR2 | 3S-GTE | 2.2-2.7 ms | | 2.2-3.4 v | VAF |
| 1993-1995 | MR2 | 5S-FE | | | | |
| 1995 | Paseo (CA only) | 5E-FE | 1.66-2.9 ms | 22.5-43% | 150-360 mmHg | MAP |
| 1993-1995 | Supra | 2JZ-GE | 2 ms | 16-20% | 35 ms | VAF |
| 1993-1995 | Supra Turbo | 2JZ-GTE | 1.8 ms | 16-24% | 3.8 g/s | MAF |
| 1993 | T100 | 3VZ-E | 2.2-2.7 ms | NA | 2.2-3.4 v | VAF |
| 1994 | T100 | 3VZ-E | 2.2-2.7 ms | NA | 2.2-3.4 v | VAF |
| 1992-1995 | Truck | 3VZ-E | 2.2-2.7 ms | NA | 2.2-3.4 v | VAF |
| 1993-1995 | Truck | 22R-E | 2 ms | NA | 6 m/h | VAF |
| |
| LEXUS |
| Years | Model | Engine | Injector
@ Idle | IAC
@ Idle | Air
@ Idle |
| 1992-1993 | ES 300 | 3VZ-FE | | yes | | VAF |
| 1993-1995 | GS 300 | 2JZ-GE | 2 ms | 16-20% | 35 ms | VAF |
| 1990-1992 | LS 400 | 1UZ-FE | | yes | | VAF |
| 1993-1994 | LS 400 | 1UZ-FE | 3.2 ms | 26% | 40 ms | VAF |
| 1993-1995 | SC 300 | 2JZ-GE | 2 ms | 16-20% | 35 ms | VAF |
| 1992-1995 | SC 400 | 1UZ-FE | 3.4 ms | 28% | 35 ms | VAF |