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1997 Chevrolet Astro Van P0123 and P0118 Fault: Coolant & Throttle Sensor Issues, Fuel Pressure Diagnosis

Model: 97 Astro Van Fault Code: P0118 Posted: 2013-03-09 14:24

This is my first post on the forum. I'm currently working on a friend's 1997 Chevrolet Astro Van with a 4.3L Vortec V6 engine, automatic transmission, air conditioning, and 2-wheel drive. The issue began when the vehicle died unexpectedly while driving—cranking but failing to start repeatedly. After towing it home, I initially suspected a fuel pump failure and replaced the fuel pump without checking fuel pressure first. I also ensured the fuel filter was clean. Following the replacement, I noticed a significant coolant leak from the intake manifold. I repaired the intake gasket and replaced the 2-wire coolant temperature (ECT) sensor located at the thermostat housing. After reassembly, the vehicle would not start. I then bypassed the oil pressure sender with a jumper to get the fuel pump running—verified at 60+ psi at the Schrader valve. The engine started successfully on cold or high idle for about 30–60 seconds but then began dropping idle and exhibiting roughness, bucking, and popping through the intake—indicating a potential fuel delivery issue. With a fuel pressure gauge attached, I observed normal fluctuation with throttle input. Using an OBD2 scanner, I detected two DTCs: P0123 (Throttle Position Sensor Circuit High Voltage) and P0118 (Engine Coolant Temperature Sensor Circuit High Voltage). After clearing the codes, P0123 returns immediately upon restart. The vehicle starts smoothly for 30–40 seconds before idle drops and throttle response fails—mirroring a fuel-related symptom. Parts replaced: fuel pump, fuel filter, ECT sensor (2-wire), intake manifold gasket. Diagnostic checks performed: - Alternator charging output is adequate. - Voltage readings at the ECT and Throttle Position Sensor (TPS) show ~5V—this is within expected range for signal wires. - Continuity tests on all signal and ground wires from sensor to ECM were confirmed. - A dedicated ground wire was added between the ECT sensor and ECM connector pin 4 (white connector) to eliminate potential ground issues. - Resistance test of the ECT sensor in hot water shows proper function. I now suspect a faulty or misdiagnosed ECM, but I'm seeking expert input on possible root causes. Any guidance on testing procedures or additional diagnostics would be greatly appreciated.

Related fault codes
P0118P0123
Comments (3)
Anonymous 2013-03-09 15:01

DTC P0118 indicates an excessively high voltage signal from the Engine Coolant Temperature (ECT) sensor circuit. DTC P0123 points to a high voltage reading on the Throttle Position Sensor (TPS) circuit. I recommend performing a fuel pressure leak-down test. Also, verify the power and signal voltages at the sensors while the engine is running—especially the TPS signal wire (should be below 5V; ~1V at idle for TPS, ~1.5–2.0V for ECT at operating temperature). Check the fuel pump relay and fuse as well. Ensure your scanner can read live sensor data or provide freeze-frame information to better understand the fault timing.

Anonymous 2013-03-09 15:25

Important note: You've removed the jumper from the oil pressure sender—this may have masked a real issue. I suspect two primary problems: (1) high voltage on sensor circuits and (2) an underlying fuel delivery issue. Be aware that when the key is turned to 'ON', the ECM only supplies 12V to the fuel pump for about 3 seconds—this can make testing difficult, especially if you're relying on a jumper. This may explain why you needed to bypass the fuel pump circuit. Check for any poorly routed ignition wires—they can cause voltage leakage and induce high voltages in sensor circuits. Also verify for vacuum leaks, as these can affect engine idle and sensor readings. Do you have access to an OBD2 scanner? Can it provide real-time sensor data or freeze-frame logs during the fault occurrence?

Anonymous 2013-03-09 15:57

The ECT sensor is a two-wire device: yellow (5V reference) and black (ground). What is the voltage on the black wire? TPS has three wires: gray (5V), black (ground), blue (signal return). What are your readings on each? This helps confirm if signal voltages are within expected ranges. Clarifying for accuracy—this applies to a 1997 Chevrolet Astro Van with Vortec engine. Ensuring we're all aligned on the vehicle and sensor configurations.