Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
DTC P1128: MAP Sensor Signal Lower Than Expected
- Before you troubleshoot, record all freeze data and any on-board snapshot, and review the general troubleshooting information (see GENERAL TROUBLESHOOTING INFORMATION ).
- Before you begin, check for poor connections or a blockage at the intake air duct.
- Turn the ignition switch to ON (II).
- Check the MAP SENSOR in the DATA LIST with the HDS.
Is less than 54.1 kPa (16.0 in.Hg, 406 mmHg) or 1.61 V held for more than 5 seconds?
YES -Go to step 7.
NO -Go to step 3.
- Clear the DTC with the HDS.
- Start the engine. Hold the engine speed at 3,000 rpm without load (A/T in P or N, M/T in neutral) until the radiator fan comes on, then let it idle.
- Test-drive under these conditions:
- Engine coolant temperature (ECT SENSOR 1) above 158°F (70°C)
- Engine speed between 1,200-5,400 rpm
- A/T in D, M/T in 3rd
- Vehicle speed accelerated from 16-31 mph (25-50 km/h) under half throttle
- Monitor the OBD STATUS for DTC P1128 in the DTCs MENU with the HDS.
Does the screen indicate FAILED?
YES -Go to step 7.
NO -If the screen indicates PASSED, intermittent failure, the system is OK at this time. Check for poor connections or loose terminals at the MAP sensor and the ECM/PCM. If the screen indicates NOT COMPLETED, go to step 4 and recheck.
- Turn the ignition switch to LOCK (0).
- Replace the MAP sensor (see MAF SENSOR/IAT SENSOR REPLACEMENT ).
- Turn the ignition switch to ON (II).
- Reset the ECM/PCM with the HDS.
- Do the ECM/PCM idle learn procedure (see ECM/PCM IDLE LEARN PROCEDURE ).
- Start the engine. Hold the engine speed at 3,000 rpm without load (A/T in P or N, M/T in neutral) until the radiator fan comes on, then let it idle.
- Test-drive under these conditions:
- Engine coolant temperature (ECT SENSOR 1) above 158°F (70°C)
- Engine speed between 1,200-5,400 rpm
- A/T in D, M/T in 3rd
- Vehicle speed accelerated from 16-31 mph(25-50 km/h) under half throttle
- Check for Pending or Confirmed DTCs with the HDS.
Is DTC P1128 indicated?
YES -Check for poor connections or loose terminals at the MAP sensor and the ECM/PCM, then go to step 1.
NO -Go to step 15.
- Monitor the OBD STATUS for DTC P1128 in the DTCs MENU with the HDS.
Does the screen indicate PASSED?
YES -Troubleshooting is complete. If any other Pending or Confirmed DTCs were indicated in step 14, go to the INDICATED DTC'S TROUBLESHOOTING .
NO -If the screen indicates FAILED, check for poor connections or loose terminals at the MAP sensor and the ECM/PCM, then go to step 1. If the screen indicates NOT COMPLETED, go to step 12.
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.