DTC 5-1-2

5-1-2 SHORT TERM FUEL TRIM (INTEGRATOR), LOWER LIMIT

Condition(s) for DTC
^ The short term fuel trim value is a measure of degree to which the injection period in the basic MFI module program must be varied to ensure that the engine is supplied with the optimum air/fuel mixture. This optimum mixture corresponds to a value of Lambda = 1. If the engine is running rich, the short term fuel trim will reduce the injection period to restore the optimum mixture. A DTC will be recorded if the engine runs so rich that the short term fuel trim reaches its lower limit and remains there for 166 seconds.

Substitute value
^ None.

Fault source(s)
^ Heated Oxygen Sensor (HO2S) signal lead short-circuited to ground.
^ HO2S faulty.
^ Contact resistance and oxidation in Mass Air Flow (MAF) sensor connector.
^ MAF sensor faulty.

Fault symptom(s)
^ Fault may cause high fuel consumption and driving problems.

TESTING PROCEDURES





Checking HO2S signal
^ START and RUN engine up to working temperature.
^ Disconnect HO2S connector (2-pin, C/EE or 4-pin, C/EP).
^ Display DTC parameter list.
^ Read HO2S signal value.

Value should be 0.45-0.46 Volts.

If reading is correct:
- Check HO2S signal lead.
If reading is incorrect:
- Check lead between MFI A22 and HO2S connector terminal 2 (2-pin, C/EE) or terminal 4 (4-pin, C/EP) for short-circuit to supply.





Checking HO2S signal lead
^ Switch OFF ignition.
^ Connect HO2S.
^ Connect test box to MFI system and check ground terminals.





^ Plug MFI module into test box adapter.
^ Switch ON ignition.
^ WAIT approx. 1 minute until voltage stabilizes.
^ Connect voltmeter across #22 (MFI A22) and #23 (MFI A23) on test box.

Instrument should indicate 0.45-0.46 Volts.

If reading is correct:
- Check MAF sensor signal.
If reading is incorrect:
- Try with new HO2S.





Checking MAF sensor signal
^ Switch ON ignition.
^ Connect voltmeter across #13 (MFI A13) and #20 (MFI A20) on test box.

Instrument should indicate approx. 2.8 Volts.

If reading is correct:
- Check all signals to and from ICM.
If reading is incorrect:
- DTC is due to poor connection in MAF sensor connector or MFI module connector. Check connectors for contact resistance and oxidation.

^ Repeat above measurement after connectors have been checked.

If reading is still incorrect:
- Try with new MAF sensor.