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P1202

(Alternative Fuel) Gas Mass Sensor Circuit Low Frequency.

  • Cadillac
  • Powertrain
  • Manufacturer code
  • Medium severity

How this code is built

P
SystemPowertrain
1
Defined byThe manufacturer
2
AreaFuel and air metering (injector circuit)
02
FaultThe specific fault in that area

What it means

P1202 on a Cadillac means the engine control module has seen the alternative fuel gas mass sensor signal running at a lower frequency than expected. This applies to bi-fuel or dedicated gaseous fuel (such as CNG or LPG) setups, where the sensor measures the mass of gas going to the engine. The module sets the code when the sensor's frequency output is below its expected range for the operating conditions.

Symptoms

  • Malfunction indicator lamp on
  • Poor running or hesitation when on gas
  • Rich or lean running on alternative fuel
  • Reduced power
  • Car may refuse to run on or switch to gas, staying on petrol

Possible causes

  • Damaged wiring, poor connector contact or corrosion in the gas mass sensor circuit (signal, supply or ground)likely
  • Faulty alternative fuel gas mass sensor giving a low frequency outputlikely
  • Contaminated or blocked sensor or gas supply passage, for example from oil or debris in the gas systempossible
  • Low or unstable sensor supply voltage or poor groundpossible
  • Low gas pressure or restricted gas flow affecting the sensor readingpossible
  • Fault within the engine control modulerare

How to diagnose

  1. Read all stored codes and freeze frame data, noting fuel mode, engine speed and load when the code set
  2. Check for other gas system or fuel trim codes that point to a common cause
  3. Inspect the gas mass sensor wiring and connector for damage, corrosion, water and bent or loose pins
  4. Check the gas system for leaks, restriction and correct fuel pressure, following the procedure for the specific model and conversion
  5. Check the sensor supply voltage and ground at the connector against the manufacturer's specification
  6. View the sensor frequency signal live and compare it with the expected value at idle and under load
  7. Inspect the sensor and its passage for contamination, then test or substitute a known good sensor if the signal is still wrong
  8. Clear the code and road test on gas to confirm it does not return

How to fix

  • Repair or replace damaged wiring and clean or renew faulty connector terminals
  • Replace the alternative fuel gas mass sensor if its output is confirmed faulty
  • Clean out contamination and correct the cause of it in the gas system, such as filter or regulator problems
  • Restore correct sensor supply and ground by repairing the circuit
  • Correct gas pressure or flow faults found in the regulator, lines or filters
  • Repair or replace the engine control module only after all other causes are ruled out

Can I keep driving?

Medium severity

The car can usually be driven, often on petrol, but running on gas may be poor or unreliable. Get it checked soon, and stop if there is any smell of gas or the car runs badly.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.

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